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- \documentclass{article}
- \usepackage[scale=0.8]{geometry}
- \usepackage{hyperref}
- \usepackage{graphicx}
- \title{The Blocktree Cloud Orchestration Platform}
- \author{Matthew Carr}
- \begin{document}
- \maketitle
- \begin{abstract}
- This document is a proposal for a novel cloud platform called Blocktree.
- The system is described in terms of the actor model,
- where tasks and services are implemented as actors.
- The platform is responsible for orchestrating these actors on a set of native operating system processes.
- A service is provdied to actors which allows them access to a highly available distributed file system,
- which serves as the only source of persistent state for the system.
- High availability is achieved using the Raft consensus protocol to synchronize the state of files between processes.
- All data stored in the filesystem is secured with strong integrity and optional confidentiality protections.
- A network block device like interface allows for fast low-level read and write access to the encrypted data,
- with full support for client-side encryption.
- Well-known cryptographic primitives and constructions are employed to provide this protection,
- the system does not attempt to innovate in terms of cryptography.
- The system's trust model allows for mutual TLS authentication between all processes in the system,
- even those which are controlled by different owners.
- By integrating these ideas into a single platform,
- the system aims to advance the status quo in the security and reliability of software systems.
- \end{abstract}
- \section{Introduction}
- % The "Big" Picture.
- Blocktree is an attempt to extend the Unix philosophy that everything is a file
- to the entire distributed system that comprises modern IT infrastructure.
- The system is organized around a global distributed filesystem which defines security
- principals, resources, and their authorization attributes.
- This filesystem provides a language for access control that can be used to securely grant
- access to resources from different organizations, without the need to setup federation.
- The system provides an actor runtime for orchestrating services.
- Resources are represented by actors, and actors are grouped into operating system processes.
- Each process has its own credentials which authenticate it as a unique security principal,
- and which specify the filesystem path where the process is located.
- A process has authorization attributes which determine the set of processes that may communicate with it.
- Every connection between processes is established using mutual TLS authentication,
- which is accomplished without the need to trust any third-party certificate authorities.
- The cryptographic mechanisms which make this possible are described in detail in section 3.
- Messages addressed to actors in a different process are forwarded over these connections,
- while messages delivered to actors in the same process are delivered with zero-copying.
- % Self-certifying paths and the chain of trust.
- The single global Blocktree filesystem is partitioned into disjoint domains of authority.
- Each domain is controlled by a root principal.
- As is the case for all principals,
- a root principal is authenticated by a public-private key pair,
- and is identified by a hash of its public key.
- The domain of authority for a given absolute path is determined by its first component,
- which is the identifier of the root principal that controls the domain.
- Because there is no meaning to the directory "/",
- a directory consisting of only a single component equal to a root principal's identifier is
- referred to as the root principal's root directory.
- The root principal delegates its authority to write files to subordinate principals by issuing
- them certificates which specify the path that the authority of the subordinate is limited to.
- File data is signed for authenticity and a certificate chain is contained in its metadata.
- This certificate chain must lead back to the root principal
- and consist of certificates with correctly scoped authority in order for the file to be validated.
- Given the path of a file and the file's contents,
- this allows the file to be validated by anyone without the need to trust a third-party.
- Blocktree paths are called self-certifying for this reason.
- % Persistent state provided by the filesystem.
- One of the major challenges in distributed systems is managing persistent state.
- Blocktree solves this issue with its distributed filesystem.
- Files are broken into segments called sectors.
- The sector size of a file can be configured when it is created,
- but cannot be changed later.
- Reads and writes of individual sectors are guaranteed to be atomic.
- The sectors which comprise a file and its metadata are replicated by a set of processes running
- the sector service.
- This service is responsible for storing the sectors of files which are contained in the directory
- containing the process in which it is running.
- The actors providing the sector service in a given directory coordinate with one another using
- the Raft protocol to synchronize the state of the sectors they store.
- By partitioning the data in the filesystem based on directory,
- the system can scale beyond the capabilities of a single consensus cluster.
- Sectors can be integrity protected and verified without reading the entire file,
- because each file has a Merkle tree of sector hashes associated with it.
- Encryption can be optionally applied to sectors,
- and when it is key is managed by the system.
- The cryptographic mechanisms used to implement these protections are described in section 3.
- % Protocol contracts.
- One of the design goals of Blocktree is to facilitate the creation of composable distributed
- systems.
- A major challenge to building such systems is the difficulty in pinning down bugs when they
- inevitably occur.
- Research into session types (a.k.a. Behavioral Types) promises to bring the safety benefits
- of type checking to actor communication.
- Blocktree integrates a session typing system that allows protocol contracts to be defined that
- specify the communication patterns of a set of actors.
- This model allows the state space of the set of actors participating in a computation to be defined,
- and the state transitions which occur to be specified based on the types of received messages.
- These contracts are used to verify protocol adherence statically and dynamically.
- This system is implemented using compile time code generation,
- making it a zero-cost abstraction.
- This frees the developer from dealing with the numerous failure modes that can occur in a
- communication protocol.
- % Implementation language and project links.
- Blocktree is implemented in the Rust programming language.
- It is currently only tested on Linux.
- Running it on other Unix-like operating systems should be straight-forward,
- though FUSE support is required to mount the filesystem.
- Its source code is licensed under the Affero GNU Public License Version 3.
- It can be downloaded at the project homepage at \url{https://blocktree.systems}.
- Anyone interested in contributing to development is welcome to submit a pull request
- to \url{https://gogs.delease.com/Delease/Blocktree}.
- If you have larger changes or architectural suggestions,
- please submit an issue for discussion prior to spending time implementing your idea.
- % Outline of the rest of the paper.
- The remainder of this paper is structured as follows:
- \begin{itemize}
- \item Section 2 describes the actor runtime, service and task orchestration, and service
- discovery.
- \item Section 3 discusses the filesystem, its concurrency semantics and implementation.
- \item Section 4 details the cryptographic mechanisms used to secure communication between
- actor runtimes and to protect sector data.
- \item Section 5 is a set of examples describing ways that Blocktree can be used to build systems.
- \item Section 6 provides some concluding remarks.
- \end{itemize}
- \section{Actor Runtime}
- % Motivation for using the actor model.
- Building scalable fault tolerant systems requires us to distribute computation over
- multiple computers.
- Rather than switching to a different programming model when an application scales beyond the
- capacity of a single computer,
- it is beneficial in terms of programmer time and program simplicity to begin with a model that
- enables multi-computer scalability.
- Fundamentally, all communication over an IP network involves the exchange of messages,
- namely IP packets.
- So if we wish to build scalable fault-tolerant systems,
- it makes sense to choose a programming model built on message passing,
- as this will ensure low impedance with the underlying networking technology.
- % Overview of message passing interface.
- That is why Blocktree is built on the actor model
- and why its actor runtime is at the core of its architecture.
- The runtime can be used to spawn actors, register services, dispatch messages immediately,
- and schedule messages to be delivered in the future.
- Messages can be dispatched in two different ways: with \texttt{send} and \texttt{call}.
- A message is dispatched with the \texttt{send} method when no reply is required,
- and with \texttt{call} when exactly one is.
- The \texttt{Future} returned by \texttt{call} can be awaited to obtain the reply.
- If a timeout occurs while waiting for the reply,
- the \texttt{Future} completes with an error.
- The name \texttt{call} was chosen to bring to mind a remote procedure call,
- which is the primary use case this method was intended for.
- Awaiting replies to messages serves as a simple way to synchronize a distributed computation.
- % Scheduling messages for future delivery.
- Executing actions at some point in the future or at regular intervals are common tasks in computer
- systems.
- Blocktree facilitates this by allows messages to be scheduled for future delivery.
- The schedule may specify a one time delivery at a specific instant in time,
- or a repeating delivery with a given period.
- These scheduling modes can be combined so that you can specify an anchoring instant
- and a period whose multiples will be added to this instant to calculate each delivery time.
- For example, a message could be scheduled for delivery every morning at 3 AM.
- Messages scheduled in a runtime are persisted in the runtime's file.
- This ensures scheduled messages will be delivered even if the runtime is restarted.
- If a message has been delivered
- and the schedule is such that it will never be delivered again,
- it is removed from the runtime's file.
- If a message is scheduled for delivery at a single instant in time,
- and that delivery is missed,
- the message will be delivered as soon as possible.
- But, if a message is periodic,
- any messages which were missed due to a runtime not being active will never be sent.
- This is because the runtime only persists the message's schedule,
- not every delivery.
- This mechanism is intended for periodic tasks or delaying work to a later time.
- It is not for building hard realtime systems.
- % Description of virtual actor system.
- One of the challenges when building actor systems is supervising and managing actors' lifecycles.
- This is handled in Erlang through the use of supervision trees,
- but Blocktree takes a different approach inspired by Microsoft's Orleans framework.
- Orleans introduced the concept of virtual actors,
- which are purely logical entities that exist perpetually.
- In Orleans, one does not need to spawn actors nor worry about respawning them should they crash,
- the framework takes care of spawning an actor when a message is dispatched to it.
- This model also gives the framework the flexibility to deactivate actors when they are idle
- and to load balance actors across different computers.
- In Blocktree a similar system is used when messages are dispatched to services.
- The Blocktree runtime takes care of routing these messages to the appropriate actors,
- spawning them if needed.
- A service must be registered in a runtime before messages can be routed to it.
- The actors which are spawned based on this registration are called \emph{service providers} of the
- service.
- Services which directly use operating system resource,
- such as those that listen on network sockets,
- are often started immediately after registration so that they are available to external clients.
- % Message addressing modes.
- Messages can be addressed to services or specific actors.
- When addressing a specific actor,
- the message contains an \emph{actor name},
- which is a pair consisting of the path of the runtime hosting the actor and the \texttt{Uuid}
- identifying the specific actor in that runtime.
- When addressing a service,
- the message is dispatched using a \emph{service name},
- which contains the following fields:
- \begin{enumerate}
- \item \texttt{service}: The path identifying the receiving service.
- \item \texttt{scope}: A filesystem path used to specify the intended recipient.
- \item \texttt{rootward}: A boolean describing whether message delivery is attempted towards or
- away from the root of the filesystem tree. A value of
- \texttt{false} indicates that the message is intended for a runtime directly contained in the
- scope. A value of \texttt{true} indicates that the message is intended for a runtime contained
- in a parent directory of the scope and should be delivered to a runtime which has the requested
- service registered and is closest to the scope.
- \item \texttt{id}: An identifier for a specific service provider.
- \end{enumerate}
- The ID can be a \texttt{Uuid} or a \texttt{String}.
- It is treated as an opaque identifier by the runtime,
- but a service is free to associate additional meaning to it.
- Every message has a header containing the name of the sender and receiver.
- The receiver name can be an actor or service name,
- but the receiver name is always an actor name.
- For example, to open a file in the filesystem,
- a message is dispatched with \texttt{call} using the service name of the filesystem service.
- The reply contains the name of the file actor spawned by the filesystem service which owns the opened
- file.
- Messages are then dispatched to the file actor using its actor name to read and write to the file.
- % The runtime is implemented using tokio.
- The actor runtime is currently implemented using the Rust asynchronous runtime tokio.
- Actors are spawned as tasks in the tokio runtime,
- and multi-producer single consumer channels are used for message delivery.
- Because actors are just tasks,
- they can do anything a task can do,
- including awaiting other \texttt{Future}s.
- Because of this, there is no need for the actor runtime to support short-lived worker tasks,
- as any such use-case can be accomplished by awaiting a set of \texttt{Future}s.
- This allows the runtime to focus on providing support for services.
- Using tokio also means that we have access to a high performance multi-threaded runtime with
- evented IO.
- This asynchronous programming model ensures that resources are efficiently utilized,
- and is ideal for a system focused on orchestrating services which may be used by many clients.
- % Delivering messages over the network.
- Messages can be forwarded between actor runtimes using a secure transport layer called
- \texttt{bttp}.
- The transport is implemented using the QUIC protocol, which integrates TLS for security.
- A \texttt{bttp} client may connect anonymously or using credentials.
- If an anonymous connection is attempted,
- the client has no authorization attributes associated with it.
- Only runtimes which grant others the execute permission allow connections from such clients.
- If these permissions are not granted in the runtime's file,
- anonymous connections are rejected.
- When a client connects with credentials,
- mutual TLS authentication is performed as part of the connection handshake,
- which cryptographically verifies the credentials of each runtime.
- These credentials contain the filesystem paths where each runtime is located.
- This information is used to securely route messages between runtimes.
- The \texttt{bttp} server is always authenticated during the handshake,
- even when the client is connecting anonymously.
- Because QUIC supports the concurrent use of many different streams,
- it serves as an ideal transport for a message oriented system.
- \texttt{bttp} uses different streams for independent messages,
- ensuring that head of line blocking does not occur.
- Note that although data from separate streams can arrive in any order,
- the protocol does provide reliable in-order delivery of data in any given stream.
- The same stream is used for sending the reply to a message dispatched with \texttt{call}.
- Once a connection is established,
- messages may flow both directions (provided both runtimes have execute permissions for the other),
- regardless of which runtime is acting as the client or the server.
- % Delivering messages locally.
- When a message is sent between actors in the same runtime it is delivered into the queue of the recipient without any copying,
- while ensuring immutability (i.e. move semantics).
- This is possible thanks to the Rust ownership system,
- because the message sender gives ownership to the runtime when it dispatches the message,
- and the runtime gives ownership to the recipient when it delivers the message.
- % Security model based on filesystem permissions.
- A runtime is represented in the filesystem as a file.
- This file contains the authorization attributes which are associated with the runtime's security
- principal.
- The credentials used by the runtime specify the file, so other runtimes are able to locate it.
- The metadata of the file contains authorization attributes just like any other file
- (e.g. UID, GID, and mode bits).
- In order for a principal to be able to send a message to an actor in the runtime,
- it must have execute permissions for this file.
- Thus communication between runtimes can be controlled using simple filesystem permissions.
- Permissions checking is done during the \texttt{bttp} handshake.
- Note that it is possible for messages to be sent in one direction in a \texttt{bttp} connection
- but not in the other.
- In this situation replies are permitted but unsolicited messages are not.
- An important trade-off which was made when designing this model was that messages which are
- sent between actors in the same runtime are not subject to any authorization checks.
- This was done for two reasons: performance and security.
- By eliminating authorization checks messages can be more efficiently delivered between actors in the
- same process,
- which helps to reduce the performance penalty of the actor runtime over directly using threads.
- Security is enhanced by this decision because it forces the user to separate actors with different
- security requirements into different operating system processes,
- which ensures all of the process isolation machinery in the operating system will be used to
- isolate them.
- % Representing resources as actors.
- As in other actor systems, it is convenient to represent resources in Blocktree using actors.
- This allows the same security model used to control communication between actors to be used for
- controlling access to resources,
- and for resources to be shared by many actors.
- For instance, a Point-to-Point Protocol connection could be owned by an actor.
- This actor could forward traffic delivered to it in messages over this connection.
- The set of actors which are able to access the connection is controlled by setting the filesystem
- permissions on the file for the runtime executing the actor owning the connection.
- % Actor ownership.
- The concept of ownership in programming languages is very useful for ensuring that resources are
- properly freed when the type using them dies.
- Because actors are used for encapsulating resources in Blocktree,
- a similar system of ownership is employed for this reason.
- An actor is initially owned by the actor that spawned it.
- An actor can only have a single owner,
- but the owner can grant ownership to another actor.
- An actor is not allowed to own itself,
- though it may be owned by the runtime.
- When the owner of an actor returns,
- the actor is sent a message instructing it to return.
- If it does not return after a timeout,
- it is interrupted.
- This is the opposite of how supervision trees work in Erlang.
- Instead of the parent receiving a message when the child returns,
- the child receives a message when the parent returns.
- Service providers spawned by the runtime are owned by it.
- They continue running until the runtime chooses to reclaim their resources,
- which can happen because they are idle or the runtime is overloaded.
- Note that ownership is not limited to a single runtime,
- so distributed resources can be managed by owning actors in many different runtimes.
- % Message routing to services.
- A service is identified by a Blocktree path.
- Only one service implementation can be registered in a particular runtime,
- though this implementation may be used to spawn many actors as providers for the service,
- each associated with a different ID.
- The runtime spawns a new actor when it finds no service provider associated with the ID in the
- message it is delivering.
- Some services may only have one service provider in a given runtime,
- as is the case for the sector and filesystem services.
- The \texttt{scope} and \texttt{rootward} field in an actor name specify the set of runtimes to
- which a message may be delivered.
- They allow the sender to express their intended recipient,
- while still affording enough flexibility to the runtime to route messages as needed.
- If \texttt{rootward} is \texttt{false},
- the message is delivered to a service provider in a runtime that is directly contained in
- \texttt{scope}.
- If \texttt{rootward} is \texttt{true},
- the parent directories of scope are searched,
- working towards the root of the filesystem tree,
- and the message is delivered to the first provider of \texttt{service} which is found.
- When there are multiple service providers to which a given message could be delivered,
- the one to which it is actually delivered is unspecified,
- which allows the runtime to balance load.
- Delivery will occur to at most one recipient,
- even in the case that there are multiple potential recipients.
- In order to contact other runtimes and deliver messages to them,
- their network endpoint (IP address and UDP port) needs to be known.
- This is achieved by maintaining a file with a runtime's endpoint address in the same directory as
- the runtime.
- The runtime is granted write permissions on the file,
- and it is updated by \texttt{bttp} when it begins listening on a new endpoint.
- The port a \texttt{bttp} server uses to listen for unicast connections is uniformly
- randomly selected from the set of ports in the dynamic range (49152-65535) which are unused on the
- server's host.
- Use of a random port allows many different \texttt{bttp} servers to share a single IP address
- and makes Blocktree more resistent to censorship.
- The services which are allowed to be registered in a given runtime are specified in the runtime's
- file.
- The runtime reads this list and uses it to deny service registrations for unauthorized services.
- The list is also read by other runtime's when they're searching for service providers.
- % The sector and filesystem service.
- The filesystem is itself implemented as a service.
- A filesystem service provider can be passed messages to delete files, list directory contents,
- open files, or perform several other standard filesystem operations.
- When a file is opened,
- a new actor is spawned which owns the newly created file handle and its name is returned to the
- caller in a reply.
- Subsequent read and write messages are sent to this actor.
- The filesystem service does not persist any data itself,
- its job is to function as an integration layer,
- conglomerating sector data from many different sources into a single unified interface.
- The sector service is what is ultimately responsible for storing data,
- and thus maintaining the persistent state of the system.
- It stores sector data in the local filesystem of each computer on which it is registered.
- The details of how this is accomplished are deferred to the next section.
- % Runtime queries.
- While it is possible to resolve runtime paths to network endpoints when the filesystem is available,
- another mechanism is needed to allow the filesystem service providers to be discovered.
- This is accomplished by allowing runtimes to query one another to learn of other runtimes.
- Because queries are intended to facilitate message delivery,
- the query fields and their meanings mirror those used for addressing messages:
- \begin{enumerate}
- \item \texttt{service} The path of the service whose providers are sought.
- Only runtimes with this service registered will be returned.
- \item \texttt{scope} The filesystem path relative to which the query will be processed.
- \item \texttt{rootward} Indicates if the query should search for runtimes from \texttt{scope}
- toward the root.
- \end{enumerate}
- The semantics of \texttt{scope} and \texttt{rootward} in a query are identical to their use in an
- actor name.
- As long as at least one other runtime is known,
- a query can be issued to learn of more runtimes.
- A runtime which receives a query may not be able to answer it directly.
- If it cannot,
- it returns the endpoint of the next runtime to which the query should be sent.
- % Bootstrap discovery methods.
- In order to bootstrap the discovery processes,
- another mechanism is needed to find the first peer to query.
- There were several possibilities explored for doing this.
- One way is to use a blockchain to store the endpoints of the runtimes hosting the filesystem service
- in the root directory.
- As long as these runtimes can be located,
- then all others can be found using the filesystem.
- This idea may be worth revisiting in the future,
- but the author wanted to avoid the complexity of implementing a new proof of work blockchain.
- Instead, two independent mechanisms are used,
- one that can discover runtimes over the internet as long as their path is known,
- and another that can discover runtimes on the local network even when the discoverer does not know
- their paths.
- % Searching DNS for root principals.
- When the path to a runtime is known,
- DNS is used to resolve SRV records using a fully qualified domain name
- (FQDN) derived from the path's root principal identifier.
- The SRV records are resolved using the name \texttt{\_bttp.\_udp.<FQDN>},
- where \texttt{<FQDN>} is the FQDN derived from the root principal's identifier.
- One SRV record may be created for each of the filesystem service providers in the root
- directory.
- Each record contains the UDP port and hostname where a runtime is listening.
- Every runtime is configured with a search domain that is used as a suffix in the FQDN.
- The leading labels in the FQDN are computed by base32 encoding the binary representation of the
- root principal's identifier.
- If the encoded string is longer than 63 bytes (the limit for each label in a hostname),
- it is separated into the fewest number of labels possible,
- working from left to right along the string.
- A dot followed by the search domain is concatenated onto the end of this string to form the FQDN.
- This method has the advantages of being simple to implement
- and allowing runtimes to discover each other over the internet.
- Implementing this system would be facilitated by hosting DNS servers in actors in the same
- runtimes as the root sector service providers.
- Then, records could be dynamically created which point to these runtimes.
- These runtimes would also need to be configured with static IP addresses,
- and the NS records for the search domain would need to point to them.
- Of course it is also possible to build such a system without hosting DNS inside of Blocktree.
- The downside of using DNS is that it couples Blocktree with a centralized,
- albeit distributed, system.
- % Using link-local multicast datagrams to find runtimes.
- Because the previous mechanism requires knowledge of the root principal of a domain to perform
- discovery,
- it will not work if a runtime is first starting up with no credentials and so does not know its
- own root principal.
- This runtime needs a way to discover other runtimes so it can connect to the filesystem and sector
- services.
- This issue is solved by using link-local multicast addressing to discover the runtimes on the same
- network as the discoverer.
- When a \texttt{bttp} server starts listening for unicast traffic,
- it also listens for UDP datagrams on port 50142 at addresses 224.0.0.142 and FE02::142,
- if the IPv4 or IPv6 networking stack is available, respectively.
- If the host is attached to a dual-stack network,
- the server listens on both addresses.
- When a runtime is attempting to discover other runtimes,
- it sends out datagrams to these endpoints.
- Each \texttt{bttp} server replies with its unicast address and filesystem path
- (as specified in its credentials).
- If the server is available at both IPv4 and IPv6 unicast addresses,
- it is at the server's discretion which address to respond with,
- it may even respond with an IPv4 to an IPv4 datagram,
- and IPv6 address to an IPv6 datagram.
- Once a client has discovered the \texttt{bttp} servers on its network,
- it can route messages to them,
- such as the provisioning requests which are used to obtain new credentials.
- Provisioning is described in the Cryptography section.
- Note that port 50142 is in the dynamic range,
- so it does not need to registered with the Internet Assigned Names and Numbers Authority (IANA).
- Both addresses 224.0.0.142 and FE02::142 are currently unassigned.
- but they will need to be registered with IANA if Blocktree is widely adopted.
- % Security model for queries.
- To allow runtimes which are not permitted to execute the root directory to query for other runtimes,
- authorization logic which is specific to queries is needed.
- If a process is connected with credentials
- and the path in the credentials contains the scope of the query,
- the query is permitted.
- If a process is connected anonymously,
- its query will only be answered if the query scope
- and all of its parent directories,
- grant others the execute permission.
- Queries from authenticated processes can be authorized using only the information in the query,
- but anonymous queries require knowledge of filesystem permissions,
- some of which may not be known to the answering runtime.
- When authorizing an anonymous query,
- an answering runtime should check that that the execute permission is granted on all directories
- that it is responsible for storing.
- If all these checks pass, it should forward the querier to the next runtime as usual.
- % Overview of protocol contracts and runtime checking of protocol adherence.
- To facilitate the creation of composable systems,
- a protocol contract checking system based on session types has been designed.
- This system models a communication protocol as a directed graph representing state transitions
- based on types of received messages.
- The protocol author defines the states that the actors participating in the protocol can be in using
- Rust traits.
- These traits define handler methods for each message type the actor is expected to handle in that
- state.
- A top-level trait which represents the entire protocol is defined that contains the types of the
- initial state of every actor in the protocol.
- A macro is used to generate the message handling loop for the each of the parties to the protocol,
- as well as enums to represent all possible states that the parties can be in and the messages that
- they exchange.
- The generated code is responsible for ensuring that errors are generated when a message of an
- unexpected type is received,
- eliminating the need for ad-hoc error handling code to be written by application developers.
- % Example of a protocol contract.
- Let's explore how this system can be used to build a simple pub-sub communications protocol.
- In this protocol,
- there will be a server which handles \texttt{Sub} messages by remembering the names of the actors
- who sent them.
- It will handle \texttt{Pub} messages by forwarding them to all of the subscribed actors.
- The state-transition graph for the system is shown in figure \ref{fig:pubsub}.
- \begin{figure}
- \begin{center}
- \includegraphics[scale=0.6]{PubSubStateGraph.pdf}
- \end{center}
- \caption{The state-transition graph for a simple pub-sub protocol.}
- \label{fig:pubsub}
- \end{figure}
- The solid edges in the graph indicate state transitions and are labeled with the message type
- which triggered the transition.
- The dashed edges indicate message delivery and are labeled with the type of the message delivered.
- Although \texttt{Runtime} is not the state of any actor in the system,
- it is included in the graph as the sender of the \texttt{Activate} and \texttt{Pub} messages.
- \texttt{Activate} is delivered by the runtime to pass a reference to the runtime and provide the
- actor's \texttt{Uuid}.
- \texttt{Pub} messages are dispatched by actors outside the graph and are routed to actors in the
- \texttt{Listening} state by the runtime.
- Note that the runtime itself doesn't have any notion of the state of any actor,
- it just delivers messaging using the rules described previously.
- Only an actor can tell whether a message is expected or not given its current state.
- Each of the actor states are modeled by Rust traits.
- \begin{verbatim}
- pub struct ClientInit {
- type AfterActivate: Subed;
- type Fut: Future<Output = Result<Self::AfterActivate>>;
- fn handle_activate(self, msg: Activate) -> Self::Fut;
- }
- pub struct Subed {
- type AfterPub: Subed;
- type Fut: Future<Output = Result<Self::AfterPub>>;
- fn handle_pub(self, msg: Envelope<Pub>) -> Self::Fut;
- }
- pub struct ServerInit {
- type AfterActivate: Listening;
- type Fut: Future<Output = Result<Self::AfterActivate>>;
- fn handle_activate(self, msg: Activate) -> Self::Fut;
- }
- pub struct Listening {
- type AfterSub: Listening;
- type SubFut: Future<Output = Result<Self::AfterSub>>;
- fn handle_sub(self, msg: Envelope<Sub>) -> Self::SubFut;
- type AfterPub: Listening;
- type PubFut: Future<Output = Result<Self::AfterPub>>;
- fn handle_pub(self, msg: Envelope<Pub>) -> Self::PubFut;
- }
- \end{verbatim}
- The definition of \texttt{Activate} is as follows:
- \begin{verbatim}
- pub struct Activate {
- rt: &'static Runtime,
- act_id: Uuid,
- }
- \end{verbatim}
- The \texttt{Envelope} type is a wrapper around a message which contains information about who sent
- it and a method that can be used to send a reply.
- In general a new actor state, represented by a new type, can be returned by a messaging handling
- method.
- The protocol itself is also represented by a trait:
- \begin{verbatim}
- pub trait PubSubProtocol {
- type Server: ServerInit;
- type Client: ClientInit;
- }
- \end{verbatim}
- By modeling this protocol independently of any implementation of it,
- we allow for many different interoperable implementations to be created.
- We can also isolate bugs in these implementations because unexpected or malformed messages are
- checked for by the generated code.
- % Implementing actors in languages other than Rust.
- Today the actor runtime only supports executing actors implemented in Rust.
- A WebAssembly (Wasm) plugin system is planned to allow any language which can compile to Wasm to be
- used to implement an actor.
- This work is blocked pending the standardization of the WebAssembly Component Model,
- which promises to provide an interface definition language which will allow type safe actors to be
- defined in many different languages.
- % Running containers using actors.
- Blocktree allows containers to be run by encapsulating them using a supervising actor.
- This actor is responsible for starting the container and managing the container's kernel namespace.
- Logically, it owns any kernel resources created by the container, including all spawned operating
- system processes.
- When the actor halts,
- all of these resources are destroyed.
- All network communication to the container is controlled by the supervising actor.
- The supervisor can be configured to bind container ports to host ports,
- as is commonly done today,
- but it can also be used to encapsulate traffic to and from the container in Blocktree messages.
- These messages are routed to other actors based on the configuration of the supervisor.
- This essentially creates a VPN for containers,
- ensuring that regardless of well secured their communication is,
- they will be safe to communicate over any network.
- This network encapsulation system could be used in other actors as well,
- allowing a lightweight and secure VPN system to built.
- % Web GUI used for managing the system.
- Any modern computer system must include a GUI,
- it is required by users.
- For this reason Blocktree includes a web-based GUI called \texttt{btconsole} that can
- monitor the system, provision runtimes, and configure access control.
- \texttt{btconsole} is itself implemented as an actor in the runtime,
- and so has access to the same facilities as any other actor.
- \section{Filesystem}
- % The division of responsibilities between the sector and filesystem services.
- The responsibility for serving data in Blocktree is shared between the filesystem and sector
- services.
- Most actors will access the filesystem through the filesystem service,
- which provides a high-level interface that takes care of the cryptographic operations necessary to
- read and write files.
- The filesystem service relies on the sector service for actually persisting data.
- The individual sectors which make up a file are read from and written to the sector service,
- which stores them in the local filesystem of the computer on which it is running.
- A sector is the atomic unit of data storage
- and the sector service only supports reading and writing entire sectors at once.
- File actors spawned by the filesystem service buffer reads and writes until there is enough
- data to fill a sector.
- Because cryptographic operations are only performed on full sectors,
- the cost of providing these protections is amortized over the size of the sector.
- Thus there is tradeoff between latency and throughput when selecting the sector size of a file:
- a smaller sector size means less latency while a larger one enables more throughput.
- % Types of sectors: metadata, integrity, and data.
- A file has a single metadata sector, a Merkle sector, and zero or more data sectors.
- The sector size of a file can be specified when it is created,
- but cannot be changed later.
- Every data sector contains the ciphertext of the number of bytes equal to the sector size,
- but the metadata and Merkle sectors contain a variable amount of data.
- The metadata sector contains all of the filesystem metadata associated with the file.
- In addition to the usual metadata present in any Unix filesystem (the contents of the \texttt{stat} struct),
- cryptographic information necessary to verify and decrypt the contents of the file are also stored.
- The Merkle sector of a file contains a Merkle tree over the data sectors of a file.
- The hash function used by this tree can be configured at file creation,
- but cannot be changed after the fact.
- % How sectors are identified.
- When sector service providers are contained in the same directory they connect to each other to form
- a consensus cluster.
- This cluster is identified by a \texttt{u64} called the cluster's \emph{generation}.
- Every file is identified by a pair of \texttt{u64}, its generation and its inode.
- The sectors within a file are identified by an enum which specifies which type they are,
- and in the case of data sectors, their 0-based index.
- \begin{verbatim}
- pub enum SectorKind {
- Meta,
- Merkle,
- Data(u64),
- }
- \end{verbatim}
- The byte offset in the plaintext of the file at which each data sector begins can be calculated by
- multiplying the sector's index by the sector size of the file.
- The \texttt{SectorId} type is used to identify a sector.
- \begin{verbatim}
- pub enum SectorId {
- generation: u64,
- inode: u64,
- sector: SectorKind,
- }
- \end{verbatim}
- % How the sector service stores data.
- The sector service persists sectors in a directory in its local filesystem,
- with each sector is stored in a different file.
- The scheme used to name these files involves security considerations,
- and is described in the next section.
- When a sector is updated,
- a new local file is created with a different name containing the new contents.
- Rather than deleting the old sector file,
- it is overwritten by the creation of a hardlink to the new file,
- and the name that used to create the new file is unlinked.
- This method ensures that the sector file is updated in one atomic operation
- and is used by other Unix programs.
- The sector service also uses the local filesystem to persist the replicated log it uses for Raft.
- This file serves as a journal of sector operations.
- % Types of messages handled by the sector service.
- Communication with the sector service is done by passing it messages of type \texttt{SectorMsg}.
- \begin{verbatim}
- pub struct SectorMsg {
- id: SectorId,
- op: SectorOperation,
- }
- pub enum SectorOperation {
- Read,
- Write(WriteOperation),
- }
- pub enum WriteOperation {
- Meta(Box<FileMeta>),
- Data {
- meta: Box<FileMeta>,
- contents: Vec<u8>,
- }
- }
- \end{verbatim}
- Here \texttt{FileMeta} is the type used to store metadata for files.
- Note that updated metadata is required to be sent when a sector's contents are modified.
- % Scaling horizontally: using Raft to create consensus cluster. Additional replication methods.
- A generation of sector service providers uses the Raft protocol to synchronize the state of the
- sectors it stores.
- The message passing interface of the runtime enables this implementation
- and the sector service's requirements were important considerations in designing this interface.
- The system currently replicates all data to each of the service providers in the cluster.
- Additional replication methods are planned for future implementation
- (e.g. erasure encoding and distribution via consistent hashing),
- which allow for different tradeoffs between data durability and storage utilization.
- % Scaling vertically: how different generations are stitched together.
- The creation of a new generation of the sector service is accomplished with several steps.
- First, a new directory is created in which the generation will be located.
- Next, one or more processes are credentialed for this directory,
- using a procedure which is described in the next section.
- The credentialing process produces files for each of the processes stored in the new directory.
- The sector service provider in each of the processes uses the filesystem service
- (which connects to the parent generation of the sector service)
- to find the other runtimes hosting the sector service in the directory and messages them to
- establish a fully-connected cluster.
- Finally, the service provider which is elected leader contacts the generation in the root directory
- and requests a new generation number.
- Once this number is known it is stored in the superblock for the generation,
- which is the file identified by the new generation number and inode 2.
- The superblock is not contained in any directory and cannot be accessed outside the sector service.
- The superblock also keeps track of the next inode to assign to a new file.
- % Authorization logic of the sector service.
- To prevent malicious actors from writing invalid data,
- the sector service must cryptographically verify all write messages.
- The process it uses to do this involves several steps:
- \begin{enumerate}
- \item The certificate chain in the metadata that was sent in the write message is validated.
- It is considered valid if it ends with a certificate signed by the root principal
- and the paths in the certificates are correctly nested,
- indicating valid delegation of write authority at every step.
- \item Using the last public key in the certificate chain,
- the signature in the metadata is validated.
- This signature covers all of the fields in the metadata.
- \item The new sector contents in the write message are hashed using the digest function configured
- for the file and the resulting hash is used to update the file's Merkle tree in its Merkle
- sector.
- \item The root of the Merkle tree is compared with the integrity value in the file's metadata.
- The write message is considered valid if and only if there is a match.
- \end{enumerate}
- This same logic is used by file actors to verify the data they read from the sector service.
- Only once a write message is validated is it shared with the sector service provider's peers in
- its generation.
- Although the data in a file is encrypted,
- it is still beneficial for security to prevent unauthorized principal's from gaining access to a
- file's ciphertext.
- To prevent this, a sector service provider checks a file's metadata to verify that the requesting
- principal actually has a readcap (to be defined in the next section) for the file.
- This ensures that only principals that are authorized to read a file can gain access to the file's
- ciphertext, metadata, and Merkle tree.
- % File actors are responsible for cryptographic operations. Client-side encryption.
- The sector service is relied upon by the filesystem service to read and write sectors.
- Filesystem service providers communicate with the sector service to open files and perform
- filesystem operations.
- These providers spawn file actors that are responsible for verifying and decrypting the information
- contained in sectors and providing it to other actors.
- They use the credentials of the runtime they are hosted in to decrypt sector data using
- information contained in file metadata.
- File actors are also responsible for encrypting and integrity protecting data written to files.
- In order for a file actor to produce a signature over the root of the file's Merkle tree,
- it maintains a copy of the tree in memory.
- This copy is read from the sector service when the file is opened.
- While this does mean duplicating data between the sector and filesystem services,
- this design was chosen to reduce the network traffic between the two services,
- as the entire Merkle tree does not need to be transmitted on every write.
- Encapsulating all cryptographic operations in the filesystem service and file actors allows the
- computer storing data to be different from the computer encrypting it.
- This approach allows client-side encryption to be done on more capable computers
- and low powered devices to delegate this task to a storage server.
- % Prevention of resource leaks through ownership.
- A major advantage of using file actors to access file data is that they can be accessed over the
- network from a different runtime as easily as they can be from the same runtime.
- One complication arising from this approach is that file actors must not outlive the actor which
- caused them to be spawned.
- This is handled in the filesystem service by making the actor who opened the file the owner of the
- file actor.
- When a file actor receives notification that its owner returned,
- it flushes any buffered data in its cache and returns,
- ensuring that a resource leak does not occur.
- % Encrypted metadata. Extended attributes in metadata. Cache control.
- Some of the information stored in metadata needs to be kept in plaintext to allow the sector
- service to verify and decrypt the file
- but most of it is encrypted using the same key as the file's contents.
- The file's authorization attributes, its size, and its access times are all encrypted.
- The table storing the file's extended attributes (EAs) is also encrypted.
- Cache control information is included in this area as well.
- It specifies the number of seconds, as a u32, that a file may be cached.
- The filesystem service uses this information to evict sectors from its cache when they have been
- cached for longer than this threshold,
- causing them to be reloaded from the sector service.
- % Authorization logic of the filesystem service.
- The filesystem service uses an \texttt{Authorizer} type to make authorization decisions.
- It passes this type the authorization attributes of the principal accessing the file, the
- attributes of the file, and the type of access (read, write, or execute).
- The \texttt{Authorizer} returns a boolean indicating if access is permitted or denied.
- These access control checks are performed for every message processed by the filesystem service,
- including opening a file.
- A file actor only responds to messages sent from its owner,
- which ensures that it can avoid the overhead of performing access control checks as these were
- carried out by the filesystem service when it was created.
- The file actor is configured when it is spawned to allow read only, write only, or read write
- access to a file,
- depending on what type of access was requested by the actor opening the file.
- % Streaming replication.
- Often when building distributed systems it is convenient to alert any interested party that an event
- has occurred.
- To facilitate this pattern,
- the sector service allows actors to subscribe for notification of writes to a file.
- The sector service maintains a list of actors which are currently subscribed
- and when it commits a write to its local storage,
- it sends each of them a notification message identifying the sector written
- (but not the written data).
- By using different files to represent different events,
- a simple notification system can be built.
- Because the contents of a directory may be distributed over many different generations,
- this system does not support the recursive monitoring of directories.
- Although this system lacks the power of \texttt{inotify} in the Linux kernel,
- it does provides some of its benefits without incurring much or a performance overhead
- or implementation complexity.
- For example, this system can be used to implement streaming replication.
- This is done by subscribing to writes on all the files that are to be replicated,
- then reading new sectors as soon as notifications are received.
- These sectors can then be written into replica files in a different directory.
- This ensures that the contents of the replicas will be updated in near real-time.
- % Peer-to-peer distribution of sector data.
- Because of the strong integrity protection afforded to sectors,
- it is possible for peer-to-peer distribution of sector data to be done securely.
- Implementing this mechanism is planned as a future enhancement to the system.
- The idea is to base the design on bit torrent,
- where the sector service responsible for a file acts as a tracker for that file,
- and the file actors accessing the file communicate with one another directly using the information
- provided by the sector service.
- This could allow the system to scale to a much larger number of concurrent reads by reducing
- the load on the sector service.
- % The FUSE daemon.
- Being able to access the filesystem from actors allows a programmer to implement new applications
- using Blocktree,
- but there is an entire world of existing applications which only know how to access the local
- filesystem.
- To allow these applications access to Blocktree,
- a FUSE daemon called \texttt{btfuse} is included which allows a Blocktree directory to be mounted
- to a directory in the local filesystem.
- This daemon can directly access the sector files in a local directory,
- or it can connect over the network to filesystem or sector service provider.
- This FUSE daemon could be included in a system's initrd to allow it to mount its root filesystem
- from Blocktree,
- opening up many interesting possibilities for hosting machine images in Blocktree.
- A planned future enhancement is to develop a Blocktree filesystem driver which actually runs in
- kernel space.
- This would reduce the overhead associated with context switching from user space, to kernel space,
- and back to user space, for every filesystem interaction,
- making the system more practical to use for a root filesystem.
- \section{Cryptography}
- This section describes the cryptographic mechanisms used to integrity and confidentiality protect
- files.
- These mechanisms are based on well-established cryptographic constructions.
- % Integrity protection.
- File integrity is protected by a digital signature over its metadata.
- The metadata contains the integrity field which contains the root node of a Merkle tree over
- the file's contents.
- This allows any sector in the file to be verified with a number of hash function invocations that
- is logarithmic in the size of the file.
- It also allows the sectors of a file to be verified in any order,
- enabling random access.
- The hash function used in the Merkle tree can be configured when the file is created.
- Currently, SHA-256 is the default, and SHA-512 is supported.
- A file's metadata also contains a certificate chain,
- and this chain is used to authenticate the signature over the metadata.
- In Blocktree, the certificate chain is referred to as a \emph{writecap}
- because it grants the capability to write to files.
- The certificates in a valid writecap are ordered by their paths,
- the initial certificate contains the longest path,
- the path in each subsequent certificate must be a prefix of the one preceding it,
- and the final certificate must be signed by the root principal.
- These rules ensure that there is a valid delegation of write authority at every
- link in the chain,
- and that the authority is ultimately derived from the root principal specified by the absolute path
- of the file.
- By including all the information necessary to verify the integrity of a file in its metadata,
- it is possible for a requestor who only knows the path of a file to verify that the contents of the
- file are authentic.
- % Confidentiality protecting files with readcaps. Single pubkey operation to read a dir tree.
- Confidentiality protection of files is optional but when it is enabled,
- a file's sectors are individually encrypted using a symmetric cipher.
- The key to this cipher is randomly generated when a file is created.
- A different IV is generated for each sector by hashing the index of the sector with a
- randomly generated IV for the entire file.
- A file's key and IV are encrypted using the public keys of the principals to whom read access is
- to be allowed.
- The resulting ciphertext is referred to as a \emph{readcap}, as it grants the capability to read the
- file.
- These readcaps are stored in a table in the file's metadata.
- Each entry in the table is identified by a byte string that is derived from the public key of the
- principal who owns the entry's readcap.
- The byte string is computed by calculating an HMAC of the the principal's public key.
- The HMAC is keyed with a randomly generated salt that is stored in the file's metadata.
- An identifier for the hash function that was used in the HMAC is included in the byte string so
- that the HMAC can be recomputed later.
- When the filesystem service accesses the file,
- it recomputes the HMAC using the salt, its public key, and the hash function specified in each entry
- of the table.
- It can then identify the entry which contains its readcap,
- or that such an entry does not exist.
- This mechanism was designed to prevent offline correlation attacks on file metadata,
- as metadata is stored in plaintext in local filesystems.
- The file key and IV are also encrypted using the keys of the file's parents.
- Note that there may be multiple parents of a file because it may be hard linked to several
- directories.
- Each of the resulting ciphertexts is stored in another table in the file's metadata.
- The entries in this table are identified by an HMAC of the parent's generation and inode numbers,
- where the HMAC is keyed using the file's salt.
- By encrypting a file's key and IV using the key and IV of its parents,
- it is possible to traverse a directly tree using only a single public key decryption.
- The file where this traversal begins must contain a readcap owned by the accessing principal,
- but all subsequent accesses can be performed by decrypting the key and IV of a child using the
- key and IV of a parent.
- Not only does this allow traversals to use more efficient symmetric key cryptography,
- but it also means that it suffices to grant a readcap on a single directory in order to grant
- access to the entire tree rooted at that directory.
- % File key rotation and readcap revocation.
- Because it is not possible to change the key used by a file after it is created,
- a file must be copied in order to rotate the key used to encrypt it.
- Similarly, revoking a readcap is accomplished by creating a copy of the file
- and adding all the readcaps from the original's metadata except for the one being revoked.
- While it is certainly possible to remove a readcap from the metadata table,
- this is not supported because the readcap holder may have used custom software to save the file's
- key and IV while it had access to them,
- so data written to the same file after revocation could potentially be decrypted by it.
- By forcing the user to create a new file,
- they are forced to re-encrypt the data using a fresh key and IV.
- % Obfuscating sector files stored in the local filesystem.
- From an attacker's perspective,
- not every file in your domain is equally interesting.
- They may be particularly interested in reading your root directory,
- or they may have identified the inode of a file containing kompromat.
- To make offline identification of which files sectors in the local filesystem belong to,
- an obfuscation mechanism is used.
- This works by generating a random salt for each generation of the sector service,
- and storing it in the generation's superblock.
- It is hashed along with the inode and the sector ID to produce the file name of the sector file
- in the local filesystem.
- These files are arranged into different subdirectories according to the value of the first two
- digits in the hex encoding of the resulting hash,
- the same way git organizes object files.
- This simple method makes it more difficult for an attacker to identify the files each sector belongs
- to
- while still allowing the sector service efficient access.
- % Credential stores.
- Processes need a way to securely store their credentials.
- They accomplish this by using a credential store,
- which is a type that implementor the trait \texttt{CredStore}.
- A credential store provides methods for using a process's credentials to encrypt, decrypt,
- sign, and verify data,
- but it does not allow them to be exported.
- A credential store also provides a method for generating new root credentials.
- Because root credentials represent the root of trust for an entire domain,
- it must be possible to securely back them up from one credential store to another.
- Root credentials can also be used to perform cryptographic operations without exporting them.
- A password is set when the root credentials are generated,
- and this same password must be provided to use, export, and import them.
- When root credentials are exported from a credential store they are confidentiality protected
- using multiple layers of encryption.
- The outer most layer is encryption by a symmetric key cipher whose key is derived from the
- password.
- a public key of the receiving credential store must also be provided when root credentials are
- exported.
- This public key is used to perform the inner encryption of the root credentials,
- ensuring that only the intended credential store is able to import them.
- Currently there are two \texttt{CredStore} implementors in Blocktree,
- one which is used for testing and one which is more secure.
- The first is called \texttt{FileCredStore},
- and it uses a file in the local filesystem to store credentials.
- A symmetric cipher is used to protect the root credentials, if they are stored,
- but it relies on the security of the underlying filesystem to protect the process credentials.
- For this reason it is not recommended for production use.
- The other credential store is called \texttt{TpmCredStore},
- and it uses a Trusted Platform Module (TPM) 2.0 on the local machine to store credentials.
- The TPM is used to generate the process's credentials in such a way that they can never be
- exported from the TPM (this is a feature of TPM 2.0).
- A randomly generated cookie is needed to use these credentials.
- The cookie is stored in a file in the local filesystem which its permissions set to prevent
- others from accessing it.
- Thus this type also relies on the security of the local filesystem.
- But, an attacker would need to steal the TPM and this cookie in order to steal a process's
- credentials.
- % Manual provisioning via the command line.
- The term provisioning is used in Blocktree to refer to the process of acquiring credentials.
- A command line tool call \texttt{btprovision} is provided for provisioning credential stores.
- This tool can be used to generate new process or root credentials, create a certificate request
- using them, issue a new certificate, and finally to import the new certificate chain.
- When setting up a new domain,
- \texttt{btprovision} can create a new sector storage directory in the local filesystem
- and write the new process's files to it.
- It is also capable of connecting to the filesystem service if it is already running.
- % Automatic provisioning.
- While manual provisioning is necessary to bootstrap a domain,
- an automatic method is needed to make this process more ergonomic.
- When a runtime starts it checks its configured credential store to find the certificate chain to
- use for authenticating to other runtimes.
- If no such chain is stored,
- the runtime can choose to request a certificate from the filesystem service.
- This is done by dispatching a message with \texttt{call} to the filesystem service without
- specifying a scope.
- Because the message specifies no path, there is no root directory to begin discovery at.
- So, the runtime resorts to using link-local discovery to find other runtimes.
- Once one is discovered,
- the runtime connects to it anonymously
- and sends it a certificate request.
- This request includes a copy of the runtime's public key and, optional, a path where the
- runtime would like to be located.
- This path is purely advisory,
- the filesystem service is free to place the runtime in any directory it sees fit.
- The filesystem service creates a new process file containing the public key and marks it as
- pending.
- The reply to the runtime contains the path of the file created for it.
- The operators of the domain can then use the web GUI or \texttt{btprovision} to view the request
- and approve it at their discretion.
- Assuming an operator approves the request,
- it uses its credentials and the public key in the new process's file to issue a certificate
- and then stores it in the file.
- Authorization attributes (e.g. UID and GID) are also assigned to the process and written into its
- file.
- Note that a process's file is normally not writeable by the process itself,
- so as to prevent it from setting its own authorization attributes.
- Once these data have been written to the process file,
- the runtime can read them to retrieve its new certificate chain.
- It stores this chain in its credential store for later use.
- The runtime can avoid polling its file for changes if it subscribes to write notifications.
- The runtime must close the anonymous connections it made
- and reconnect using the new certificate chain.
- Once new connections are established,
- it can read and write files using the authorization attributes specified in its file.
- Note that this procedure only works when the runtime is on the same LAN as another runtime.
- % The generation of new root credentials and the creation of a new domain.
- The procedure for creating a new domain is straight-forward,
- and all the steps can be performed using \texttt{btprovision}.
- \begin{enumerate}
- \item Generate the root credentials for the new domain.
- \item Generate the credentials for the first runtime.
- \item Create a certificate request using the runtime credentials.
- \item Approve the request using the root credentials.
- \item Import the new certificate into the credential store of the first runtime.
- \end{enumerate}
- The first runtime is configured to host the sector and filesystem services,
- so that subsequent runtimes will have access to the filesystem.
- After that, additional runtime on the same LAN can be provisioned using the automatic process.
- % Setting up user based access control.
- Up till now the focus has been on authentication and authorization of processes,
- but it bears discussing how user based access control can be accomplished with Blocktree.
- Because credentials are locked to the device on which they're created,
- a user will have at least as many principals as they have devices.
- But, all of these principals can be configured to have the same authorization attributes (UID, GID),
- giving them the same permissions.
- It makes sense to keep the files for all of the provisioned runtimes associated with a user in one
- place
- and the natural place is in the user's home directory.
- Although every one of the user's processes needs to be provisioned,
- this is not a huge limitation because a single runtime can host many different actors,
- implementing many different applications.
- Managing the users in a domain is facilitated by putting their home directories in a single user
- directory for the domain.
- Runtimes hosting the sector service on storage servers could then be provisioned in this directory
- to provide the sector and filesystem services for the users' home directories.
- It would be at the administrators discretion whether or not to enable client-side encryption.
- If they wanted to,
- the principal of at least one of a user's runtimes would need to be issued a readcap for the
- user's home directory.
- This runtime could then directly access the sector service in the domain's user directory.
- By moving encryption onto the user's computer,
- load can be shed from the storage servers.
- Note that this setup does require all of the user's runtimes to be able to communicate with the
- runtime whose principal was issued the readcap.
- % Example of how these mechanisms allow data to be shared.
- To illustrate how these mechanisms can be used to facilitate collaboration between enterprises,
- consider a situation where two companies wish to partner to the development of a product.
- To facilitate their collaboration,
- they wish to have a way to securely exchange data with each other.
- One of the companies is selected to host the data
- and accepts the cost and responsibility of serving it.
- The host company creates a directory which will be used to store all of the data created during
- development.
- The other company will connect to the filesystem service in the host company's domain to access
- data in the shared directory.
- Each of the principals in the other company which wish to connect request to be credentialed in the
- shared directory.
- The hosting company manually reviews these requests and approves them,
- assigning each of the principals authorization attributes appropriate for its domain.
- This may involve issuing UID and GID values to each of the principals, or perhaps SELinux contexts.
- The actually set of attributes supported is determined by the \texttt{Authorization} type used by
- by the filesystem service in the host company's domain.
- Once the principals have their credentials,
- they can dispatch messages to the filesystem service using the shared directory as the scope and
- setting the rootward field to true.
- This allows actors authenticating with the credentials of these principals to perform all filesystem
- operations authorized by the hosting company.
- This situation gives the hosting company a lot of control over the data.
- If the other company wishes to protect its investment in the R\&D effort,
- it should subscribe to write events on the shared directory and the files in it so that it can
- copy new sectors out of the host company's domain as soon as they are written.
- Note that although it is not possible to directly subscribe to writes on the contents of a
- directory, by monitoring a directory for changes,
- one can begin monitoring files as soon as they are created.
- \section{Examples}
- This section contains examples of systems that could be built using Blocktree.
- The hope is to illustrate how this platform can be used to implement existing applications more
- easily and to make it possible to implement systems which are currently out of reach.
- \subsection{A distributed AI execution environment.}
- Neural networks are just vector-valued functions with vector inputs,
- albeit very complicated ones with potentially billions of parameters.
- But, just like any other computation,
- these functions can be conceptualized as computational graphs.
- Imagine that you have a set of computers equipped AI accelerator hardware
- and you have a neural network that is too large to be processed by any one of them.
- By partitioning the graph into small enough subgraphs,
- we can break the network down into pieces which can be processed by each of the accelerators.
- The full network can be stitched together by passing messages between each of these pieces.
- Let us consider how this could be accomplished with Blocktree.
- We begin by provisioning a runtime on each of the accelerator machines,
- each of which will have a new accelerator service registered.
- Messages will be sent to the accelerator service describing the computational graph to execute,
- as well as the name of the actor to which the output is to be sent.
- When such a message is received by an accelerator service provider,
- it spawns an actor which compiles its subgraph to a kernel for its accelerator
- and remembers the name of the actor to send its output to.
- An orchestrator service will be responsible for partitioning the graph and sending these messages.
- Ownership of the actors spawned by the accelerator service is given to the orchestrator service,
- ensuring that they will all be stopped when the orchestrator returns.
- When one of the spawned actors stops,
- it unloads the kernel from the accelerator's memory and returns it to its initial state.
- Note that the orchestrator actor must have execute permissions on each of the accelerator runtimes
- in order to send messages to them.
- The orchestrator dispatches messages to the accelerator service in reverse order of the flow of data
- in the computational graph,
- so that it can tell each service provider where its output should be sent.
- The actors responsible for the last layer in the computational graph send their output to the
- orchestrator.
- To begin the computation,
- the actors which are responsible for input are given the filesystem path of the input data.
- The orchestrator learns of the completion of the computation once it receives the output from
- final layer.
- It can then save these results to the file system and return.
- Because inference and training can both be modeled by computational graphs,
- this same procedure can be used for both.
- \subsection{A decentralized social media network.}
- One of the original motivations for designing Blocktree was to create a platform for a social
- network that puts users in fully in control of their data.
- In the opinion of the author,
- the only way to actually accomplish this is for users to host the data themselves.
- One might think it is possible to use client-side encryption to solve the privacy issue,
- but this does not solve the full problem.
- While it is true that good client-side encryption will prevent the service provider from reading
- the user's data,
- the user could still loose everything if the service provider goes out of business or simply
- decides to stop offering its service.
- Similarly, putting data in a federated system, as has been proposed by the Mastodon developers,
- also puts the user at risk of loosing their data if the operator of the server they use decides to
- shut it down.
- To have real control the user must host the data themselves.
- Then they decide how its encrypted, how its served, and to whom.
- Let us explore how Blocktree can be used to build a social media platform which provides this
- control.
- To participate in this network each user will need to setup their own domain by generating new root
- credentials
- and provisioning at least one runtime to host the social media service.
- A technical user could do this on their own hardware by reading the Blocktree documentation,
- but a non-technical user might choose to purchase a new router with Blocktree pre-installed.
- By connecting this router directly to their WAN,
- the user ensures that the services running on it will always have direct internet access.
- The user can access the \texttt{btconsole} web GUI via the router's WiFi interface to generate their
- root credentials and provision new runtimes on their network.
- A basic function of any social network is keeping track of a user's contacts.
- This would be handled by maintaining the contacts as files in a well-known directory in the user's
- domain.
- Each file in the directory would be named using the user defined nickname for the contact
- and its contents would include the root principal of the contact as well as any additional user
- defined attributes,
- such as address or telephone number.
- The root principal would be used to discover runtimes controlled by the contact
- so that messages can be sent to the social media service running in them.
- When a user adds a new contact,
- a connection message would be sent to it,
- which the contact could choose to accept or reject.
- If accepted,
- the contact would create an entry in its contacts directory for the user.
- The contact's social media service would then accept future direct messages from the user.
- When the user sends a direct message to the contact,
- its runtime discovers runtimes controlled by the contact and delivers the message.
- Once delivered the contact's social media service stores the message in a directory for the user's
- correspondence,
- sort of like an mbox directory but where messages are sorted into directories based on sender
- instead of receiver.
- Note that this procedure only works if a contact's root principal can be resolved using the
- search domain configured in the user's runtime.
- We can ensure this is the case by configuring the runtime to use a search domain that operates
- a Dynamic DNS (DDNS) service
- and by arranging with this service to create the correct records to resolve the root principal.
- The author intends to operate such a service to facilitate the use of Blocktree by home users,
- but a more long-term solution is to implement a blockchain for resolving root principals.
- Only then would the system be fully decentralized.
- Making public posts is accomplished by creating files in a directory with the HTML contents of the
- post.
- This file, the directory containing it, and all parents of it,
- would be configured to allow others to read, and in the case of directories, execute them.
- At least one runtime with the filesystem service registered would need to have the execute
- permission granted to others to allow anyone to access these files.
- When someone wanted to view the posts of another user,
- they would use the filesystem service to read these files from the well-known posts directory.
- Of course user's would not be using a file manager to interact with this social network,
- they would use their browsers as they do now.
- This web interface would be served by the social media service in their domain.
- A normal user who has a Blocktree enabled router would just type in a special hostname into their
- browser to open this interface.
- Because the router provides DNS services to their network,
- it can generate the appropriate records to ensure this name resolves to the address where the social
- media service is listening.
- The social media service would be responsible for sending message to other user's domains to
- get their posts,
- and to read the filesystem to display the user's direct messages.
- All this file data would be used to populate the web interface.
- It is not hard to see how the same system could be used to serve any type of media: text, images,
- video, immersive 3D worlds.
- All of these can be stored in files in the filesystem,
- and so all of them are accessible to Blocktree actors.
- One issue that must be addressed with this design is how it will scale to a large number of users
- accessing data at once.
- In other words,
- what happens if the user goes viral?
- Currently, the way to solve this would be to add more computers to the user's network which run
- the sector and filesystem services.
- This is not ideal as it means the user would need to buy more hardware to serve their dank memes.
- A better solution would be implement peer-to-peer distribution of sector data in the filesystem
- service.
- This would reduce the load on the user's computers and allow their follows to share the posted
- data with each other.
- This work is planned as a future enhancement.
- \subsection{A smart lock.}
- The access control language provided by Blocktree's filesystem can be used for more than just
- authorizing access to data.
- To illustrate this point,
- consider a smart lock installed on the front door of a company's office building.
- When the company first got the lock they used NFC to configure the lock
- and connect it to their WiFi network.
- The lock then used link-local runtime discovery to perform automatic provisioning.
- An IT administrator accessed \texttt{btconsole} to approve the provisioning request
- and position the lock in a specific directory in the company's domain.
- Permission to actuate the lock is granted if a principal has execute permission on the lock's file.
- To verify the physical presence of an employee,
- NFC is used for the authentication handshake.
- When an employee presses their NFC device, for instance their phone, to the lock,
- it generates a nonce and transmits it to the device.
- The device then signs the nonce using the credentials it used during provisioning in the company's
- domain.
- It transmits this signature to the lock along with the path to the principal's file in the domain.
- The lock then reads this file to obtain the principal's authorization attributes and its public key.
- It uses the public key to validate the signature presented by the device.
- If this is successful,
- it then checks the authorization attributes of the principal against the authorization attributes on
- its own file.
- If execute permissions are granted, the lock actuates, allowing the employee access.
- The administrators of the company's domain create a group specifically for controlling physical
- access to the building.
- All employees with physical access permission are added to this group,
- and the group is granted execute permission on the lock,
- rather than individual users.
- \subsection{A traditional three-tier web application.}
- While it is hoped that Blocktree will enable interesting and novel applications,
- it can also be used to build the kind of web applications that are common today.
- Suppose that we wish to build a three-tier web application.
- Let us explore how Blocktree could help.
- First, let us consider which database to use.
- It would be desirable to use a traditional SQL database,
- preferably one which is open source and not owned by a large corporation with dubious motivations.
- These constraints lead us to choose Postgres,
- but Postgres was not designed to run on Blocktree.
- However, Postgres does have a container image available on docker hub,
- we can create a service to run this container image in our domain.
- But Postgres stores all of its data in the local filesystem of the machine it runs on.
- How can we ensure this does not become a single point of failure?
- First, we should create a directory in our domain to hold the Postgres cluster directory.
- Then we should procure at least three servers for our storage cluster
- and provision runtimes hosted on each of them in this directory.
- The sector service is registered on each of the runtimes,
- so all the data stored in the directory will be replicated on each of the server.
- Now, the Postgres service should be register in one and only one of these runtimes,
- as Postgres requires exclusive access to its database cluster.
- \texttt{btfuse} will be used to mount the Postgres directory to a path in the local filesystem
- and the Postgres container will be configured to access it.
- We now have to decide how other parts of the system are going to communicate with Postgres.
- We could have the Postgres service setup port forwarding for the container,
- so that ordinary network connection can be used to talk to it.
- But we will have to setup TLS if we want this to be secure.
- The alternative is to use Blocktree as a VPN and proxy network communications in messages.
- This is accomplished by registering a proxy service in the same runtime as the Postgres service
- and configuring it to allow traffic it receives to pass to the Postgres container on TCP port 5432.
- In a separate directory,
- a collection runtimes are provisioned which will host the webapp service.
- This service will use axum to serve the static assets to our site,
- including the Wasm modules which make up our frontend,
- as well as our site's backend.
- In order to do this,
- it will need to connect to the Postgres database.
- This is accomplished by registering the proxy service in each of the runtimes hosting the
- webapp service.
- The proxy service is configured to listen on TCP 127.0.0.1:5432 and forwards all traffic
- to the proxy service in the Postgres directory.
- The webapp can then use the \texttt{tokio-postgres} crate to establish a TCP connection to
- 127.0.0.1:5432
- and it will end up talking to the containerized Postgres instance.
- Although the data in our database is stored redundantly,
- we do still have a single point of failure in our system,
- namely the Postgres container.
- To handle this we can implement a failover service.
- It will work by calling the Postgres service with heartbeat messages.
- If too many of these timeout,
- we assume the service is dead and start a new instance one of the other runtimes in the Postgres
- directory.
- This new instance will have access to all the same data the old,
- including its journal file.
- Assuming it can complete any in progress transactions,
- the new service will come up after a brief delay
- and the system will recover.
- \subsection{A realtime geo-spacial environment.}
- % Motivation
- If we are to believe science fiction,
- then the natural evolution of computer interaction is the development
- of a persistent virtual world that we use to communicate, conduct business, and
- enjoy our leisure.
- This kind of system has been a dream for a long time,
- but as it has grown closer to becoming a reality,
- the popular consciousness has shifted against it.
- People are rightly horrified by the idea of giving control over their virtual worlds to the same
- social media company which has an established track record for causing societal harm.
- But this technology does not need to be dystopian.
- If an open system can be built, which actually works,
- it can prevent the market from accepting a closed system designed to lock in user attention
- and monetize them relentlessly.
- This is the future,
- it is only a question of who will own it.
- % Coordinates
- Let us explore how Blocktree could be used to build such a system.
- The world we are going to render will be a planet with a roughly spherical surface and a
- configurable radius $\rho$.
- $\rho$ is a \texttt{u32} value whose units are meters.
- We will use latitude ($\phi$) and longitude ($\lambda$) in radians to describe the locations of
- points on the surface.
- Both $\phi$ and $\lambda$ will take \texttt{f64} values.
- The elevation of a point will be given by $h$,
- which is the deviation from $\rho$.
- $h$ is measured in meters and takes values in \texttt{i32}.
- So, the distance from the center of the planet to the point ($\phi$, $\lambda$, $h$) is
- $\rho + h$.
- % Directory organization. Quadtrees.
- The data describing how to render a planet consists of its terrain mesh, terrain textures, and
- the objects on its surface.
- This could represent a very large amount of data for a planet with realistic terrain populated by
- many structures.
- To facilitate sharding the information in a planet over many different servers,
- the planet is broken into disjoint regions,
- each of which is stored in its own directory.
- A single top-level directory represents the entire planet,
- and contains a manifest describing it.
- This manifest specifies the planet's name, its radius, its rotational period,
- the size of its regions in MB, as well as any
- other global attributes.
- This top-level directory also contains the texture for the sky box to render the view of
- space from the planet.
- In the future it may be interesting to explore the creation of more dynamic environments surrounding
- the planet,
- but a simple sky box has the advantage of being efficient.
- The data in a planet is recursively broken into the fewest number of regions such that the
- amount of data in each regions is less than a configured threshold.
- When a regions grows too large it is broken into four new regions by cutting it along the
- centerline parallel to the $\phi$ axis, and the one parallel to the $\lambda$ axis.
- In other words, it is divided in half north to south and east to west.
- The four new regions are stored in four subdirectories of the original region's directory
- named 0, 1, 2, and 3.
- The data in the old region is then moved into the appropriate directory based on its location.
- Thus the directory tree of a planet essentially forms a quadtree,
- albeit one which is built up progressively.
- % Region data files.
- In the leaf directories of this tree the actual data for a region are stored in two files,
- one which describes the terrain and the other which describes objects.
- It is expected that the terrain will rarely be modified,
- but that the objects may change regularly.
- The terrain file contains the mesh vertices in the region as well as its textures.
- It is organized as an R-tree to allow for efficient spacial queries based on player location.
- The region's objects file is also organized as an R-tree.
- It contains all of the graphical data for the objects to be rendered in the region,
- such as meshes, textures, and shaders.
- % Plots.
- The creation of a shared virtual world must involve players collaboratively building persistent
- structures.
- This is allowed in a controlled way by defining plot objects.
- A plot is like a symbolic link,
- it points to a file whose contents contain the data used to render the plot.
- This mechanisms allows the owner of the planet to delegate a specific area on the surface
- to another player by creating a plot defining that area and pointing it to a file owned by the
- player.
- The other player can then write meshes, textures, and shaders into this file to describe the
- contents of the plot.
- If the other player wishes to collaborate with others on the construction,
- they can grant write access on the file to a third party.
- This is not unlike the ownership of land in the real world.
- % LOD files in interior directories.
- To facilitate the viewing of the planet from many distances,
- each interior node in the planet's directory tree contains a reduced level of detail (LOD) version
- of the terrain contained in it.
- For example, the top-level directory contains the lowest LOD mesh and textures for the terrain.
- This LOD would be suitable for rendering the planet as a globe on a shelf,
- or as it would appear from a high orbit.
- By traversing the directory tree,
- the LOD can be increased as the player travels closer to the surface.
- This system assist with rendering an animation where the player appears to approach and land upon
- the planet's surface.
- % Sharding planet data.
- By dividing the planet's data into different leaf directories,
- it becomes possible to provision computers running the sector service in each of them.
- This divides the storage and bandwidth requirements for serving the planet over this set of
- servers.
- In addition to serving these data,
- another service is needed to keep track of player positions and execute game logic.
- Game clients address their messages using the directory of the region their player is located
- in, and set \texttt{rootward} to true.
- These messages are delivered to the closest game server to the region the player is in,
- which may be located in the region's directory or higher up the tree.
- When a player transitions from one region to the next,
- its game client begins addressing messages using the path of the next region as the scope.
- \section{Conclusion}
- % Blocktree serves as the basis for building distributed Unix.
- There have been many attempts to create a distributed Unix over the years.
- Time has shown that this is a very hard problem,
- but time has not diminished its importance.
- IT systems are more complex now than ever,
- with many layers of abstraction which have built up over time.
- We have suffered greatly from systems which were never designed to be secure on the hostile internet
- that exists today.
- Security has been bolted onto these systems (HTTPS, STARTTLS, DNSSEC)
- in a backwards compatible way,
- which results in weakened protections for these systems.
- What's worse,
- the entire trust model of the web relies on the ludicrous idea that there is a distinguished group
- of certificate authorities who have the power to secure our communications.
- We need to take a different approach.
- Data should be certified by its path,
- it must always be transported between processes in an authenticated manner,
- and user code should never have to care how this is accomplished!
- Time will tell whether the programming model of Blocktree is comprehensible and useful for
- developers,
- but the goal is to create the kind of easy to extend computing environment which allowed Unix to
- be successful.
- % The system enables individuals to self-host the services they rely on.
- These days, the typical internet user stores all of their important data in the cloud with
- third-party service providers.
- They do this because of the convenience of being able to access this information from anywhere,
- and because of the perceived safety in having a large internet company look after it for them.
- This convenience comes at the price of putting users at the mercy of these companies.
- Take email for example,
- a service which is universally used for account recovery and password reset.
- If a service provided decided to stop providing a user access to their email,
- the user would be effectively cut off from any website which sends login verification emails.
- This is not a hypothetical situation,
- such an incident has occurred (TODO: INSERT CITATION FROM LVL1).
- There is no technical reason for things to be this way.
- Blocktree allows users to host their own services in their own domain.
- If we can make setting up an email or VOIP server as simple as clicking a button in a web GUI,
- their will be no convenience advantage to cloud services.
- One challenge for self-hosting data is ensuring it is protected from loss when hardware inevitably
- fails.
- The data redundancy in Blocktree's sector layer ensures that the loss of any one storage
- device will not cause data loss.
- Streaming replication can also be used to maintain additional redundant copies.
- If more users begin hosting their own services,
- the internet will become more distributed,
- which will make it more resistent to disruption and centralized control.
- % Benefits to businesses.
- Cloud computing has also driven changes to the way businesses acquire computing resources.
- It is common for startups to rent all of their computing resources from one large cloud
- provider.
- There are compelling economic and technical reasons to do this,
- but as a firm grows they often experience growing pains as their cloud bills also grow.
- If the firm has not developed their software with a multi-cloud, or hybrid approach in mind,
- they may face the prospect of major changes in order to bring their application on-prem or to a
- rival cloud.
- By developing their application on Blocktree,
- businesses have a single platform to target which can run on rented computers in the cloud just as
- easily servers in their own data center.
- This ensures the choice to rent or buy can be made on a purely economic basis.
- Blocktree is not the only system that provides this flexibility.
- The portability of containers is one of the reasons they have become so popular.
- Containers have their place and will most likely be used for years to come,
- but they are a lower level abstraction which requires the developer to the problems that Blocktree
- handles.
- % Blocktree advances the status quo in secure computing.
- Ransomware attacks and data breaches are embarrassingly common these days.
- There are many reasons for this,
- from the reliance on passwords for authentication, to the complexity of the software supply chain,
- but it is clear that as IT professionals we need to do more to keep the systems under our
- protection safe.
- Blocktree helps to do this by solving many of the difficult problems involved with securing
- communication on a hostile network.
- It takes a true zero-trust approach,
- ensuring that all communications between processes is authenticated using public key cryptography.
- Data at rest is also secured with encryption and integrity protection.
- No security system can prevent all attacks,
- but by putting these mechanisms together in an easy to use platform,
- we can advance the status quo of secure computing.
- % Composability leads to emergent benefits.
- When Unix was first developed in the 1970's, its authors could not have foreseen the applications
- that would be enabled by their system.
- Although there have been many different kinds of Unices over the years,
- the core programming model, built around the filesystem, has remained since the beginning.
- It is a testament to the importance of this abstraction that it has persisted for so long.
- No designer can foresee all the ways that their abstractions will be used,
- but they can try to build them in such a way that as much choice is left to the user as possible.
- By making the actor model, and messaging passing, the core of Blocktree,
- it is hoped that low overhead communication between distributed components can be achieved.
- By using this system to provide a global distributed filesystem,
- it is hoped that the interoperable sharing of data can be achieved.
- And by using protocol contracts to constrain actor communication,
- it is hoped that the structure and safety of type theory can bring order to distributed
- computation.
- While it is possible to see some of the applications that can be built from these abstractions,
- it seems likely that their composability and the creativity of developers will enable systems that
- cannot be foreseen.
- \end{document}
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