Fossil

<title>The Fossil Sync Protocol</title>

<p>This document describes the wire protocol used to synchronize
content between two Fossil repositories.</p>

<h2>1.0 Overview</h2>

<p>The global state of a fossil repository consists of an unordered
collection of artifacts.  Each artifact is identified by a cryptographic
hash of its content, expressed as a lower-case hexadecimal string.
Synchronization is the process of sharing artifacts between
servers so that all servers have copies of all artifacts.  Because
artifacts are unordered, the order in which artifacts are received
at a server is inconsequential.  It is assumed that the hash names
of artifacts are unique - that every artifact has a different hash.
To a first approximation, synchronization proceeds by sharing lists
hash values for available artifacts, then sharing the content of artifacts
whose names are missing from one side or the other of the connection.  
In practice, a repository might contain millions of artifacts.  The list of
hash names for this many artifacts can be large.  So optimizations are
employed that usually reduce the number of hashes that need to be
shared to a few hundred.</p>

<p>Each repository also has local state.  The local state determines
the web-page formatting preferences, authorized users, ticket formats,
and similar information that varies from one repository to another.
The local state is not using transferred during a sync.  Except,
some local state is transferred during a [/help?cmd=clone|clone]

or the artifact delta is the first <i>size</i> bytes of the
x-fossil content that immediately follow the newline that
terminates the file card.
</p>

<p>The first argument of a file card is the ID of the artifact that
is being transferred.  The artifact ID is the lower-case hexadecimal
representation of the name hash for the artifact.
The last argument of the file card is the number of bytes of
payload that immediately follow the file card.  If the file
card has only two arguments, that means the payload is the
complete content of the artifact.  If the file card has three
arguments, then the payload is a delta and second argument is
the ID of another artifact that is the source of the delta.</p>

<blockquote>
<b>cfile</b> <i>artifact-id usize csize</i> <b>\n</b> <i>content</i><br>
<b>cfile</b> <i>artifact-id delta-artifact-id usize csize</i> <b>\n</b> <i>content</i><br>
</blockquote>

<p>The first argument of the cfile card is the ID of the artifact that
is being transferred.  The artifact ID is the lower-case hexadecimal
representation of the name hash for the artifact.  The second argument of
the cfile card is the original size in bytes of the artifact.  The last
argument of the cfile card is the number of compressed bytes of payload
that immediately follow the cfile card.  If the cfile card has only
three arguments, that means the payload is the complete content of the
artifact.  If the cfile card has four arguments, then the payload is a
delta and the second argument is the ID of another artifact that is the
source of the delta and the third argument is the original size of the

<blockquote>
<b>uvfile</b> <i>name mtime hash size flags</i> <b>\n</b> <i>content</i>
</blockquote>

<p>The <i>name</i> field is the name of the unversioned file.  The
<i>mtime</i> is the last modification time of the file in seconds
since 1970.  The <i>hash</i> field is the hash of the content
for the unversioned file, or "<b>-</b>" for deleted content.
The <i>size</i> field is the (uncompressed) size of the content
in bytes.  The <i>flags</i> field is an integer which is interpreted
as an array of bits.  The 0x0004 bit of <i>flags</i> indicates that
the <i>content</i> is to be omitted.  The content might be omitted if
it is too large to transmit, or if the sender merely wants to update the
modification time of the file without changing the files content.

<blockquote>
<b>uvigot</b> <i>name mtime hash size</i>
</blockquote>

<p>The <i>name</i> argument is the name of an unversioned file.
The <i>mtime</i> is the last modification time of the unversioned file
in seconds since 1970.
The <i>hash</i> is the SHA1 or SHA3-256 hash of the unversioned file 
content, or "<b>-</b>" if the file has been deleted.
The <i>size</i> is the uncompressed size of the file in bytes.

<p>When the server sees a "pragma uv-hash" card for which the hash
does not match, it sends uvigot cards for every unversioned file that it
holds.  The client will use this information to figure out which
unversioned files need to be synchronized.
The server might also send a uvigot card when it receives a uvgimme card