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\title{Blocktree \\
\large A platform for distributed computing.}
\author{Matthew Carr}
%\date{May 28, 2022}
\begin{document}
\maketitle
\begin{abstract}
Blocktree is a platform which aims to make developing reliable distributed systems easy and safe. 
It does this by defining a format for data stored in the system, a mechanism for replicating
that data, and a programming interface for accessing it. It aims to solve the difficult problems
of cryptographic key management, consensus, and global addressing so that user code can focus on
solving problems germane to their application.
\end{abstract}

\section{Introduction}
The online services that users currently have access to are incredible. They handle all
the details of backing up user data and implementing access controls to facilitate
safe sharing. However, because these are closed systems, users are forced to trust that
the operators are benevolent, and they lack any real way of ensuring that the access
controls they prescribe will actually be enforced. There have been several systems proposed
as alternatives to the conventional model \cite{blockstack, filecoin}, but these systems
suffer from their own shortcomings which Blocktree aims to address.

The idea behind blocktree is to organize a user's computing devices and data into a single
tree, called a blocktree. The user is said to own the blocktree, and they wield sovereign authority
over it.
The artifacts granting them this authority are their private keys, one for use in an encryption
scheme and the other in a signing scheme.
Measures for managing these keys and delegating their authority are important design considerations
of the system.
The owners of blocktrees are encouraged to collaborate with each other to replicate data by
means of a cryptocurrency known as blockcoin.
The blockchain implementing this cryptocurrency is the source of global state for the system,
and allows for the creation of a name resolution mechanism.

A blocktree consists of four different types of blocks: files, directories, servers and processes.
Each block has a path corresponding to its location in the tree.
A server is responsible for storing the files and directories that are children of the directory
it's contained in.
When multiple servers are contained in the same directory, they form a cluster, and run
the Raft consensus protocol \cite{raft} to ensure consistency of the data they store.
A process is either the child of a server, or the child of the process which started it.
Processes operate under the actor model and exchange messages that are addressed using paths.
System calls are implemented using this same messaging facility.

The first component of a fully qualified path is the fingerprint of the blocktree owner's public
signing key, allowing a block to be globally specified.
The contents of directories are managed by the system,
and they may contain links to files, other directories and servers.
In addition to its payload of data, each file and directory has system managed metadata.
These metadata are used to implement cryptographic access control mechanism which ensure
that only authorized users can read and optionally write to the block.
Access control for a given security principal is set using a hash of the principal's public signing
key.

This paper is intended to be a short introduction to the ideas of blocktree. A book is planned which
will specify the system in greater detail. In keeping with the agile software methodology, this
book is being written concurrently with an open source implementation of the system.
The remainder of this paper is organized as follows:
\begin{enumerate}
\item A description of the operations of a single blocktree.
\item The definition of a blockchain which provides global state and links individual blocktrees
together.
\item The programming interface for interacting with blocktrees and sending messages.
\item An exploration of applications that could be written using this platform.
\item Conclusion.
\end{enumerate}

\section{An Individual Blocktree}
Files and directories are collectively known as data blocks. Data blocks have three components:
\begin{enumerate}
  \item Their body, which is a sequence of bytes. This is managed by user code in the case of files
    and by the system in the case of directories.
  \item Their metadata, which is managed by the system.
  \item The log of events which have been committed, or are in the process of being committed.
    This is managed by the system.
\end{enumerate}

The body of a data block and its metadata are both covered by integrity protection.
The log is not directory covered, but the events in it are.
The body of the block may be optionally encrypted to provide it with confidentiality protection.

Confidentiality of the body is achieved by encrypting it using a symmetric cipher using a random
key.
This random key is known as the block key.
When a principal is to be given access to a data block, it's public encryption key is used to
encrypt the block key.
The resulting ciphertext is referred to as a read capability, or readcap, for short.
It is stored in the metadata of the block in a dictionary under the key which is computed by hashing
the principal's public signing key.
We say that the principal has been \emph{issued} a readcap for the block.
When the principal issued the readcap wishes to read the block it looks for a hash of its public
signing key in the block's metadata, and if it find a value, uses it's private encryption key
to decrypt the block key.

If a block is not the root, then its parent's block key is used to encrypt its block key and the
result is stored in the block's metadata.
This enables anyone with a readcap for a block to read all blocks which are descended from it.
This also enables the contents of any block to be decrypted using only a single private key
decryption operation.
The root always contains a readcap for the owner, ensuring they can grant a readcap to any block
in their blocktree.

Integrity protection is provided using a digital signature scheme. In order to change the
contents of a block a data structure called a write capability, or writecap
\footnote{The names readcap and writecap were taken from the Tahoe Least-Authority Filesystem
\cite{tahoe}. The access control mechanism described in the Tahoe system heavily influenced the
design of Blocktree.},
is used.
A writecap is a certificate chain and it contains the following data:
\begin{itemize}
\item The path the writecap can be used under.
\item The principal that the writecap was issued to.
\item The timestamp when the writecap expires.
\item The public key of the principal who issued the writecap.
\item A digital signature produced by the private key corresponding to the public key above.
\item Optionally, the next certificate in the chain.
\end{itemize}
The last item is only excluded in the case of a self-signed writecap, i.e. one that was signed by
the same principal it was issued to. A writecap is considered valid for use in a block if all
of the following conditions are met:
\begin{itemize}
\item The signature on every certificate in the chain is valid.
\item The signing principal matches the principal the next certificate was issued to for every
certificate in the chain.
\item The path in every certificate is contained in the path of the next certificate for each
certificate.
\item The path of the block is contained in the path of every certificate in the chain.
\item The current timestamp is strictly less than the expiration of all the certificates.
\item The principal corresponding to the public key used to sign the last certificate,
is the owner of the blocktree.
\end{itemize}
The intuition behind these rules is that a writecap is only valid if there is a chain of trust
that leads back to the owner of the blocktree. The owner may delegate their trust to any number
of intermediaries by issuing them writecaps. These writecaps are scoped based on the path
specified when they are issued. These intermediaries can then delegate this trust as well.
A block is considered valid if it contains a valid writecap, it was signed using the key
corresponding to the first certificate's public key, and this signature is valid. Note that because
the first component of the path is the fingerprint
\footnote{By \emph{fingerprint} I mean the base64url encoding of a hash of the owner's public
signing key.}
of the owners public signing key, anyone who receives a block and knowns the block's path can verify
that the block has not been altered.

Blocks are used for more than just organizing data, they also organize computation. A program
participating in a blocktree is referred to as a server. Multiple servers may be run on
a single computer. Every server is contained in a directory in the blocktree.
A server is responsible for the storage of
the directory where it is attached and all of the blocks that are recursively contained within it.
If there is a child server attached to a subdirectory contained in the directory the server is
responsible for, then the child server is responsible for the subdirectory.
In this way data storage can be delegated, allowing the system to scale. When more than one
server is attached to the same directory they form a cluster.
Each server in the cluster contains a copy of the data that the cluster is responsible for. They
maintain consistency of this data by running the Raft \cite{raft} consensus protocol.

When a new blocktree is created a server generates a key pair to serve as the root keys.
It is imperative for the security of the system that the root private key is protected, and it
is highly recommended that it be stored in a Trusted Platform Module (TPM) \cite{tpm} and that
the TPM
be configured to disallow unauthenticated use of this key. The server then generates its own key pair
and uses the root private key to issue itself a writecap for the root of the tree. Once it has
this writecap, it creates the root block and generates a block key for it. A readcap is added to
this block for the root public key and the server's public key. Additional
cryptographic operations are performed using the server's key pair, and only when a new writecap
needs to be created for an addition root server is the root private key used.

When a new server comes online and wishes to join the blocktree, it generates its own key pair.
The public key of this
server then needs to be transmitted to another server that's already part of the user's blocktree. The
mechanism used will depend on the nature of the device on which the new server is running.
For example, a phone could scan a QR code which contains
the IP address of the user's root server, and then transmit its public key to that internet host.
In order for the new server to be added to the user's blocktree, it needs to be issued a writecap
and a readcap must be added to the directory where it will be attached.
This could be accomplished
by providing a user interface on the server which received the public key from the new server.
This interface would show the requests that have been received from servers attempting
to join the blocktree. The user can then choose to approve or deny the request, and can specify
the path where the new server will attach. If the user chooses to approve the request, then the writecap
is signed using the server's key and transmitted to then new server.

The ability to cope with key compromise is an important design consideration in any real-world
cryptosystem. In blocktree the compromise of a server key is handled by re-keying every block under
the directory where the server was attached. Specifically, this means that a new block key is generated for
each block, and the readcap for the compromised server is removed. This ensures that new writes to
these blocks will not be visible to the holder of the compromised key. To ensure that writes
will not be allowed, the root directory contains a revocation list containing the public keys
which have been revoked by the tree. These only need to be maintained until their writecaps
expire, after which time the list can be cleaned. Note that if the root private key is compromised or lost,
then the blocktree must be abandoned, there is no recovery. This is real security, the artifact
which grants control over the blocktree is the root private key which is why storing the root
private key in multiple secure cryptographic co-processors is so important.

servers in a blocktree communicate with one another by sending blocktree messages. These messages
are used for implementing consensus and distributed locking,
as well as notifying parent servers when a 
child has written new data. The later mechanism allows the parent to replicate the data stored in
a child for redundancy. User code is also able to initiate the sending of messages. Messages are
addressed using blocktree paths. When a server receives a message that is not addressed to it,
but is addressed to its blocktree, it forwards it to the closest server to the recipient that it
is connected to. In order to enable efficient low-latency message transfers, servers maintain open
TCP connections to the other servers in their cluster, and the cluster leader maintains a connection to
its parent. Diffie-Hellman key exchange is used to exchange a key for use in an AEAD cipher, and
once this cipher context is established, the two servers mutually authenticate each other using their
respective key pairs. When a server comes online, it uses the global blocktree (described later)
to find the other servers in its cluster. If it is not part of a cluster, or this information is
not stored in the global blocktree, then it instead looks up the
IP address of a root server and connects to it. The root server may direct the server to connect to
one of root's children, and this process repeats until the new server is connected to its parent.

A concept that has proven to be very useful in the world of filesystems is the symbolic link.
This is a short file that contains the path to another file, and is interpreted by most programs
as being a "link" to that file. Blocktree supports a similar system, where a block can be
marked as a symbolic link and a blocktree path placed in its body. This also provides us with
a convenient way of storing readcaps for data that a server would otherwise not have access to.
For instance a symbolic link could be created which points to a block in another user's blocktree.
The other user only knows the public key of the owner of our blocktree, so they issue
a readcap to it. But the root servers can open this readcap and extract
the block key. This key can then be encapsulated using the public key of the server which
requires access, and placed in the symbolic link. When the server needs to read the data
in the block, it opens the readcap in the symbolic link, follows the link to the block (how
that actually happens will be discussed below) and decrypts its contents.

While the consistency of individual blocks can be maintained using Raft, a distributed locking
mechanism is employed to enable transactions which span multiple blocks. This is
accomplished by exploiting the hierarchical arrangement of servers in the tree. In order to describe
this, its first helpful to define a new term. The \emph{servertree} of a blocktree is tree obtained
from the blocktree by collapsing all the blocks that a server (or cluster of servers) is responsible
for into a single logical block representing the server itself. Thus we can talk about a server having
a parent, and by this we mean its parent in the servertree. In terms of the blocktree, the parent
of a server is the first server encountered when the path back to the root is traversed.
Now, distributed locking works as follows:
\begin{itemize}
\item A server sends a request to lock a block to the current consensus leader in its cluster.
  If the server is not part of a cluster, then it is the leader. This request contains a timestamp
  for when the lock expires.
\item If the leader is responsible for the block then it moves on to the next step. Otherwise
  it contacts its parent and forwards the lock request and this step is repeated for the parent.
\item The responsible server checks to see if there is already a lock for this block. If there is
  then the request fails. Otherwise the request succeeds and a lock is placed on the block. A
  message indicating the result is then passed back up the tree ending at the original server. This
  message includes the principal of the server enforcing the lock.
\item Once the locking server is done making its updates it sends a message directly to the server
  enforcing the lock, causing it to be removed.
\end{itemize}
Locking a block locks the subtree rooted at that block. Thus no writes to any path contained in
the path of the locked block will be allowed, unless they come from locking server. If the locking
server does not send the message
unlocking the block before the lock expires, then the modifications which had been performed by
it are dropped and the block reverts to its prior state. Since the locking server is the leader
of the consensus cluster that is responsible for the block, this guarantees that
writes from other servers will not be accepted.

\section{Connecting Blocktrees}
In order to allow servers to access blocks in other blocktrees, a global ledger of events is used.
This ledger is implemented using a proof of work (PoW) blockchain and a corresponding 
cryptocurrency known as blockcoin. Servers mine chain blocks (not to be confused with the tree 
blocks we've been discussing up till now) in the same way they do in other PoW blockchain
systems such as Bitcoin \cite{bitcoin}. The server which manages to mine the next chain block receives
a reward,
which is the sum of the fees for each event in the chain and a variable amount of newly minted
blockcoin. The amount of new blockcoin created by a chain block is directly proportional to the
amount of data storage events contained in the chain block. Thus the total amount of blockcoin
in circulation has a direct relationship to the amount of data stored in the system, reflecting
the fact that blockcoin exists to provide an accounting mechanism for data.

When a server writes data to a tree block, and it wishes this block to be globally accessible or
replicated for redundancy,
it produces what are called fragments. Fragments are the output symbols of the RaptorQ code
\cite{raptorq}. This algorithm is an example of an Erasure Code, which is a class of fountain codes
which have the property that only $m$ out of $n$ (where $m < n$) symbols are needed to reconstruct
the original data. Such a code ensures that even if some of the fragments are lost, as long as $m$
remain, the original data can be recovered.

Once these fragments have been computed an event is created for each one and published to the
blockchain. This event indicates to other servers that this server wishes to store a fragment and
states the amount of blockcoin it will pay for each of the fragment's maintenance payments. When
another servers wishes to accept the offer, it directly contacts the first server, which then sends 
it the fragment and publishes an event stating that the fragment is stored with the second 
server. This event includes the path of the block the fragment was computed from, the fragment's 
ID (the sequence number from the erasure code), and the principal of the server which stored it.
Thus any other server in the network can use the information contained in this event to
determine the set of servers which contain the fragments of any given path.

In order for the server which stored a fragment to receive its next payment, it has to pass
a time-bound challenge-response protocol initiated by the server that owns the fragment.
The owning server select a leaf in the Merkle tree of the fragment and sends the index of
this leaf to the storing server. The storing server then walks the path from this leaf back to
the root of the Merkle tree, and updates a hash value using the data in each node it traverses.
It sends this result back to the owning server which verifies that this value matches its
own computation. If it does, then the owning server signs a message indicating that the challenge
passed and that the storing server should be paid. The storing server receives this message and uses
it to construct an event, which it signs and publishes to the blocktree. This event causes
the blockcoin amount specified to be transferred from the owning server's to the storing
server's account.

The fact that payments occur over time provides a simple incentive for servers to be honest and
store the data they agree to. In banking terms, the storing server views the fragment as an
asset, it is a loan of its disk space which provides a series of payments over time.
On the other hand the owning server views the fragment as a liability, it requires payments to
be made over time. In order for a blocktree owner to remain solvent, it must balance its
liabilities with its assets, incentivizing it to store data for others so that its own data
will be stored.

In order for servers to be able to contact other servers, a mechanism is required for associating
an internet protocol (IP) address with a principal. This is done by having servers publish events
to the blockchain when their IP address changes. This event includes their new IP address,
their public key, and a digital signature computed using their private key. Other servers can
then verify this signature to ensure that an attacker cannot bind the wrong
IP address to a principal in order to receive messages meant for it.

While this event ledger is useful for appending new 
events, and ensuring previous events
cannot be changed, another data structure is required to enable efficient queries.
In particular, it's important to be able to quickly perform the
following queries:
\begin{itemize}
\item Find the set of servers storing the fragments for a given path.
\item Find the IP address of a server or owner given a principal.
\item Find the public key associated with a principal.
\end{itemize}
To enable these queries a special blocktree is maintained by each server in the network: the global
blocktree. This tree does not support the usual writing and locking semantics of local blocktrees.
In functional programming terms, it can be thought of as a left fold over all of the events in the
blockchain starting from the empty state.
The above queries are facilitated by the following blocks:
\begin{itemize}
\item \emph{/global/fragments}: this block contains a hashtable where the key is a path and the
value is the list of servers storing the fragments for the block at that path.
\item \emph{/global/principals}: contains a hashtable where the key is the a principal and the value
is the tuple containing the public key of that principal, its current IP address, and its current
blockcoin balance.
\end{itemize}
To compute the entries in these tree blocks, the servers in the network iterate over all the chain blocks, updating
their local copy of each tree block appropriately. The reader may recognize this as an event 
sourced architecture. Currently only these two tree blocks are known to be needed, but if new events are
added to the system it can be easily extended to enable queries that have yet to be envisioned.

\section{Programming Environment}
Enabling an excellent developer experience is one of the primary goals of this system (the others being security
and performance). servers execute user code that has been compiled into WebAssembly modules \cite{wasm}. Such code
running on a blocktree server is referred to as a process. A process 
executes in a sandbox that isolates it from other processes, as well as the security critical operations of the server
itself. The sandbox provides the code with an extension of the WebAssembly System Interface (WASI),
with extra functions to send and receive blocktree messages.
The standard POSIX filesystem APIs are used to interact with the contents of blocktrees.
For instance, a file descriptor for a block can be obtained by calling path\_open.
Additional non-POSIX functionality is implemented by adding
messages which are handled by the server. This functionality includes the following:
\begin{itemize}
\item Distributed Locking
\item Messaging
\item Supervision Trees
\item Protocol Contracts (Session Types)
\end{itemize}
This additional functionality is described later in this section.

% Package Publishing
These modules are distributed in packages stored in a blocktree. A package contains one or more
Wasm modules and a TOML manifest file which describes the package.
This manifest defines the package's user-friendly name as well as the list
of permissions it requires.
This list of permissions is used to determine which APIs the package has access to.
These artifacts are then
placed in a zip file and stored in a blocktree file. This ensures the integrity of the code in the
package, as all blocks in a blocktree are integrity protected. 

When a package is installed it's given a directory under which it can store data that is shared between all servers
in a blocktree. The path of this block is formed by prefixing the path the package was published at
with the string ``/apps". When a package is runs on a server, it is confined to a block contained
in the server's directory. It is only allowed read and write blocks in this block, but to allow it to
access shared data, a symbolic link is created to the package's shared directory in ``/apps". 

% Privacy Safe vs Unsafe
Packages are broken into two categories: those that are privacy safe and those that are not.
A package is privacy unsafe if it requests any permissions which allow it to send data
outside of the blocktree it's part of. Thus requesting the ability to
open a TCP socket would cause a package to be privacy unsafe. Similarly, the creation of a protocol
handler for HTTP would also be privacy unsafe. Privacy unsafe packages can limit the scope of their
unsafety by
imposing limits on the unsafe APIs that they request. For instance a package which needs to send
blocktree message back to the blocktree it was published in can request the messaging permission
for a path in that tree. Similarly, a package which only wants to open a TCP socket listening on the
local network, can limit the scope of its requested permission.

% Protocol Contracts
In order to make it easy to write packages which interface with existing systems, most notably
those using HTTP, packages are able to define protocols using a contract language and then register
call backs for these protocols. This works by providing a system call which the package can
supply a protocol contract to. This contract is then compiled into a state machine by the server
and a handle is returned to the package. This handle is then used to register callbacks for different
parts of the protocol. This ensures that the protocol is handled by the server itself and that
protocols can be shared between many different packages as a library. For instance, the HTTP protocol
would be compiled to a particularly simply state machine, with only one state: the listening state.
This state would expose a hook that where a callback can be registered to handle a request.
This state also defines the record format used to pass the request information to the callback
and the record format of the return value that is expected in order to produce the response.
More complicated (stateful) protocols would have more states, each defining their own request and
response records, as well as hooks. One nice thing about this setup is that it will enable
optimizations where the state machine and the user callbacks can be compiled into a program
which can be safely run in the server itself, or even in a SmartNIC. This would require that
the callbacks only use an approved set of APIs, but could enable much higher performance.

% Supervision Trees
Processes can arrange themselves into supervision trees, in the same way that Erlang
processes are arranged \cite{armstrong}. In this scheme, when a child package crashes, or the server its
running on dies (which is detected by other servers), then the process receives a message.
In the
simplest case this can be used to implement a logging system, where crashes and server
deaths are recorded. More interestingly, this can be used to integrate with a control
plane. For instance, if a blocktree were running in AWS, when a message is received indicating
that a server has died, a new EC2 instance could be started to replace it. The reliability of Erlang
and other system employing the Actor Model have shown the robustness of this approach.
Processes can form this relationship after they've started running, provided that both of the processes 
have permission to send messages to each other. Processes, with the appropriate permissions, can also
spawn other processes on descendent servers. This can be used to implement map-reduce workloads, where
a process spawns mapping jobs on descendent servers containing the data of interest, and it processes
their messages to compute the final reduction. Due to the tiny size of most programs, this is a much
more efficient approach than moving the data.

\section{Potential Applications}
In order to explore how blocktree can be used, the design of several hypothetical systems
is discussed. It's important to note that blocktree does not try to force all computation
to be local to a user's device, but it tries to enable this for applications where it
is possible.

\subsection{Contacts and Mail}
The first application we'll consider is one which manages a user's contacts. This would expose
the usual create, read, update and delete operations, allowing a user to input the name
of a person they know and associate
that name with their public key. Once the principal of a person is known, then their public
key can be looked up in the global blocktree. This principal needs to be communicated to the
user via some out-of-band method. They could receive it in an email, a text message, or embedded
in a QR code. Of course this out-of-band communication needs to be authenticated, otherwise
it would be easy to fool the user into associating an attacker's key for the person.

The user now has a way of associating a blocktree with the name of this person. However, the
root public key of this blocktree is not enough to establish secure communications, because
the root private key is not available to every server in the person's blocktree. In particular
it would be inadvisable for the root private key to be stored on a user's mobile device. To
address this mailbox directories are created.

For each contact two directories are created: the inbox and the outbox. The user creates a readcap
for another user's root key and adds it to their outbox. The inbox for the other user is a symbolic
link to the user's outbox in the blocktree of the other user. Thus each user
can write messages into their own blocktree at a location where the other party knows how
to find them. But, in order for a server to read these messages it requires a its own readcap.
Only the root
servers can issue this readcap as only they have access to the root key. Once permission has been
granted to a server, a root server can use the root key to decrypt the readcap issued to it, and then
encrypt it using the public key of the server. The resulting readcap is then stored in the metadata
of the inbox.

In addition to being able to check the inbox for mail, a blocktree message is sent to the receiving
blocktree when mail is sent. This message may contain the entire contents of the mail, if the
contents are short enough. But if the mail contains a lot of data, for instance a video, then
the message just serves as a notification that new mail is available.

\subsection{Social Network}
Building a social network on top of the contacts app is fairly straight-forward. Once
a contacts entry has been created for a person, most interactions between the user and that
person can be implemented by sending mail. For example, when the
user sends a direct message to a person mail is placed in their outbox
and a blocktree message is sent to the root cluster of their blocktree. If the user wishes to 
send a message to a group of people, mail is sent to each one.

This same mechanism can be used to implement status updates. When a user updates their status,
they send mail to each of their "friends". The social networking app running on each of the contacts'
devices will then display the latest status update from the user as their current status. This
setup allows the user to "unfriend" anyone by simply omitting them from the list of recipients
of these updates. To allow comments and likes on the status update to be visible to everyone
that it was shared with, the block created in the outbox of each of the friends
is a symbolic link to a single status update block. This symbolic link contains the
readcap for the status update block.

Comments and likes on status updates are implemented by sending mail to the user who posted the
update. When one of the user's servers receives this mail, it then updates the block containing
the status update with the new comment, or increments the like counter.
It then sends mail to the people this status update was shared with, notifying them
that the data has changed.

\subsection{An e-commerce website.}
The previous two examples show how to build decentralized versions of existing web applications,
but blocktree also excels at building traditional web applications. Imagine an e-commerce website
with multiple warehouses, all of whose inventory is to be visible to customers of the site. Part
of the design consideration for the site is that the warehouses need to be able to update their
inventory even when their internet service goes down, and that these updates need to be visible
on the website once connectivity is restored.

To accomplish this a designer could create a directory for each warehouse. This directory would have
servers attached to it that are physically located at each warehouse. The inventory of the warehouse
is then maintained in the warehouse's directory, satisfying the first requirement. Now, in order to
enable efficient queries of the overall available inventory, the data from each of the warehouses
needs to be merged. This is accomplished by creating another directory containing the merged data.
In event sourcing terms this is called a read-model. This directory will have another cluster of
servers attached which will act as web servers. These servers will subscribe to events published by the
warehouse clusters, events indicating changing levels of inventory, and digest this information into
a format that can be efficiently queried. These servers will also publish events to the warehouses
when orders are placed, cancelled or amended.

When a warehouse goes offline, its previous inventory counts are
still recorded in the read-model, so queries can still be answered using the cluster's best
knowledge of the warehouse. Once the warehouse comes back online, then the events that were recorded by the
warehouse cluster while it was offline can be replayed to the web server cluster, and their read-model
can be brought up to date.

One of the advantages of this approach is that the cluster of web servers need not be physically
close to each other. In fact they could be spread over several data centers, even in different
cloud providers. This allows for greater fault tolerance and reliability. Of course, running a
consensus cluster over a larger area means more latency and thus reduced performance, but if
the workload is read-heavy, and writes can be handled mostly in the warehouse clusters, then this
tradeoff may be worthwhile.

I hope this example shows that having a standard format for data and the federation of servers,
can provide designers with much greater flexibility, even if they do not care about decentralization
or their user's privacy.

\subsection{A Metaverse}
As a final example I'd like to consider a platform for recording spacial information. The key insight
that enables this is very general: blocktree enables the creation of distributed tree-like data structures.
For instance, its straight forward to imagine creating a distributed hashtable implemented as a red-black tree.
This impedance match between efficient query structures and the design of blocktree is one
of the reasons why I believe it is so useful. The particular data structure germane to building the metaverse
is the Binary Space Partition (BSP) tree.

I don't see the metaverse as being one world, but rather many. Each world would be hosted by its own blocktree.
The world will have a directory in the blocktree, that directory will be the root of a BSP. read
access to the world is controlled by adding readcaps to this directory. If the world is to be
publicly accessible, then then its block can be stored in plaintext.

Thinking of worlds
like ours (those that can be reasonably approximated as the surface of a sphere) we can impose the usual latitude
and longitude coordinate system. We can then define parcels in the world by specifying the polygonal boundary as
a set of these coordinates (more precisely, the parcel is the convex hull of this set of points). These parcels
are then recorded as blocks in the world's directory, whose path is determined by its virtual location using
the BSP algorithm. If a parcel
is owned by the same user who owns the world, then the data contained in the parcel is stored in
the same blocktree. However, if a parcel is to be owned by another user, then a symbolic link is created
pointing to a block in the owner's blocktree. They can then write whatever data they want into this block, defining
the contents of their parcel. Collaboration on a single parcel is accomplished by issuing a read and writecap to
another user.

It's easy to imagine that one world would be more important than the rest and that the creation of a metaverse 
representation of the Earth will be an important undertaking. The hierarchical nature of permissions in blocktree
make such a shared world possible. National blocktrees could be given ownership of their virtual territory,
this would then
be delegated down to the state and municipal levels. Finally municipalities would delegate individual parcels
to their actual owners. Owners could then use these as they see fit, including leasing them to third parties.
The ending date of such a lease would be enforced technically by the writecap issued to the lessee; when it
expires so too does the lease.

\section{Conclusion}
In this paper I have given the outline of a decentralized method of organizing information, computation,
and trust in a way that I hope will be useful and easy to use. The use of cryptographic primitives for
implementing access control were discussed, as well as methods of protecting private keys. A blockchain
and corresponding cryptocurrency was proposed as a means of incentivizing the distribution of data.
Erasure coding was used to ensure that distributed data is resilient to the loss of servers and can be
reconstructed efficiently. A programming environment based on WASM and WASI was proposed as a way
of providing an interface to this data. APIs for defining protocol contracts and efficient web servers
were indicated. APIs for constructing supervision trees were mentioned as a means for building reliable
systems.

When Sir Tim Berners-Lee invented HTTP he could not have anticipated the applications
that his inventions would bring about. It has been said that the key to the success of his system is
that it made networking programming so easy that anyone could do it. I don't know what the future will
bring, but I hope that this system, or one like it, will enable anyone to fearlessly build distributed
systems.
One thing is certain however, it is a moral imperative that we provide users with viable alternatives
to online services which harvest their data and weaponize it against them. Only then will the web
become the place it was meant to be.

\printbibliography

\end{document}