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< p class = "no_toc" > This document describes the Atlas network, a peer-to-peer content-addressed
storage system whose chunks’ hashes are announced on a public blockchain. Atlas
allows users and developers to < strong > permanently store< / strong > chunks of data that are
< strong > replicated across every peer.< / strong > As long as at least one Atlas peer is online,
all chunks are available to clients.< / p >
< p > This document is aimed at developers and technical users. The following
concepts are discussed:< / p >
< ul id = "markdown-toc" >
< li > < a href = "#architecture" id = "markdown-toc-architecture" > Architecture< / a > < / li >
< li > < a href = "#motivation" id = "markdown-toc-motivation" > Motivation< / a > < / li >
< li > < a href = "#feature-comparison" id = "markdown-toc-feature-comparison" > Feature Comparison< / a > < / li >
< / ul >
< p > The reader of this document is expected to be familiar with the < a href = "blockstack_naming_service.md" > Blockstack
Naming Service< / a > (BNS), as well as Blockstack’s
storage system < a href = "https://github.com/blockstack/gaia" > Gaia< / a > . We advise the reader
to familiarize themselves with both systems before approaching this document.< / p >
< h2 id = "architecture" > Architecture< / h2 >
< p > Atlas is designed to integrate with BNS in order to allow users to
store name state off-chain, encoded as a DNS zone file.
The overwhelmingly-common use-cases in Blockstack are:< / p >
< ul >
< li > Storing a name’s routing information for its owners’ < a href = "https://github.com/blockstack/gaia" > Gaia< / a >
datastores.< / li >
< li > Storing BNS subdomain transactions and associated state.< / li >
< / ul >
< p > Atlas is a middleware system in Blockstack. Most developers do not
interact with it directly. BNS clients like the
< a href = "https://github.com/blockstack/blockstack-browser" > Blockstack Browser< / a >
automatically generate zone files for the names they register, and automatically
propagate them to the Atlas network. BNS API endpoints, including our
< a href = "https://core.blockstack.org" > public endpoint< / a > and the
< a href = "https://github.com/blockstack/blockstack.js" > blockstack.js< / a > library,
will automatically fetch zone files from Atlas when they need to look
up data in Gaia (such as profiles and app data).< / p >
< div class = "highlighter-rouge" > < pre class = "highlight" > < code > +--------------+ +---------------+ +----------------+
clients | Blockstack | | blockstack.js | | BNS API module |
| Browser | | | | |
+--------------+ +---------------+ +----------------+
^ ^ ^ ^ ^ ^
| | | | | |
| | | | | |
V | V | V |
+----------+ | +----------+ | +----------+ |
Gaia | Gaia hub | | | Gaia hub | | | Gaia hub | |
+----------+ | +----------+ | +----------+ |
| | |
| | |
V V V
+---------------------------------------------------------------+
Atlas | Atlas Peer Network |
+----------+------+----------+-----+----------+------+----------+
BNS | BNS node | | BNS node | | BNS node | | BNS node |
+----------+ +----------+ +----------+ +----------+
^ ^ ^ ^
| (indexing | | |
| blockchain) | | |
+---------------------------------------------------------------+
Blockchain | Blockchain Peer Network |
+---------------------------------------------------------------+
Figure 1: Location of Atlas in the Blockstack architecture. Each BNS node
implements an Atlas peer. An Atlas peer treats a name state value in BNS as
the hash of a DNS zone file. Atlas peers exchange zone files with one another
until they each have a full replica of all known zone files. Clients can look
up zone files for names using the name's stat value as a zone file hash. Clients
can broadcast zone files to the network if they match a previously-announced
hash. In practice, zone files store URLs to a name owner's Gaia hubs, thereby
allowing Blockstack apps to read and write data in Gaia.
< / code > < / pre >
< / div >
< p > Nevertheless, Atlas is a general-purpose content-addressed storage
system that advanced developers can use to < strong > host data in an immutable
and durable manner.< / strong > Beyond its default use-case in Blockstack,
Atlas is ideal for tasks like:< / p >
< ul >
< li > Announcing PGP public keys under a human-readable name< / li >
< li > Storing package hashes for a software release< / li >
< li > Securely deploying shell scripts to remote VMs< / li >
< li > Binding human-readable names to Tor .onion addresses
(< a href = "https://github.com/blockstack-packages/blockstack-tor" > example< / a > )< / li >
< / ul >
< h2 id = "motivation" > Motivation< / h2 >
< p > Atlas was designed to augment BNS. BNS allows each name to store a small
amount of state—on the order of 20 bytes. The size is so small because the
state must be recorded to a public blockchain, where the cost per byte is
high and the blockchain protocol limits the size of transactions.< / p >
< p > To compensate for this, we developed an off-chain storage system allows BNS
names to bind and store a large amount of state to each name in a way that
< em > preserves the security properties of having written that state to the
blockchain< / em > . Instead of storing 20 bytes of data on the blockchain, a BNS name
owner would store the < em > cryptograhpic hash< / em > of its state, and then store the actual state
Atlas. This decouples the name’s state size from the blockchain.< / p >
< p > The reference implementation of Atlas currently allows up to 40kb of state to be
bound to a BNS name, instead of a measly 20 bytes. The 40kb of data is
replicated to each BNS node, where it is stored forever.< / p >
< h2 id = "feature-comparison" > Feature Comparison< / h2 >
< p > Atlas is not the only peer-to-peer content-addressible chunk store in existance. The following
feature table describes Atlas in relation to other popular chunk stores.< / p >
< table >
< thead >
< tr >
< th > < strong > Features< / strong > < / th >
< th > Atlas< / th >
< th > BitTorrent< / th >
< th > < a href = "https://datproject.org/" > DAT< / a > < / th >
< th > < a href = "https://ipfs.io" > IPFS< / a > < / th >
< th > < a href = "https://github.com/ethersphere/swarm" > Swarm< / a > < / th >
< / tr >
< / thead >
< tbody >
< tr >
< td > Each peer stores all chunks< / td >
< td > X< / td >
< td > X< / td >
< td > < / td >
< td > < / td >
< td > < / td >
< / tr >
< tr >
< td > Replicas are permanent [1]< / td >
< td > X< / td >
< td > X< / td >
< td > X< / td >
< td > < / td >
< td > < / td >
< / tr >
< tr >
< td > Replicas are free< / td >
< td > < / td >
< td > X< / td >
< td > X< / td >
< td > X< / td >
< td > < / td >
< / tr >
< tr >
< td > Sybil-resistant chunk discovery< / td >
< td > X< / td >
< td > X< / td >
< td > < / td >
< td > < / td >
< td > X< / td >
< / tr >
< tr >
< td > Sybil-resistant peer discovery< / td >
< td > X< / td >
< td > < / td >
< td > < / td >
< td > < / td >
< td > < / td >
< / tr >
< tr >
< td > Fixed chunk size< / td >
< td > X< / td >
< td > < / td >
< td > X< / td >
< td > X< / td >
< td > X< / td >
< / tr >
< / tbody >
< / table >
< p > [1] Here, “permanent” means that once a peer has data, they will never evict it
as part of the protocol.< / p >
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