[/artifact/28987096ac | here].

<a name="cluster"></a>
<h2>2.0 Clusters</h2>

A cluster is an artifact that declares the existence of other artifacts.
Clusters are used during repository synchronization to help
reduce network traffic.  Clusters are also used to record aliases
for artifact names, so that if the artifact hash algorithm changes,
artifacts can still be looked up using the older hash algorithm.

Clusters follow a syntax that is very similar to manifests.
A cluster is a line-oriented text file.  Newline characters
(ASCII 0x0a) separate the artifact into cards.  Each card begins with a single
character "card type".  Zero or more arguments may follow
the card type.  All arguments are separated from each other
and from the card-type character by a single space
character.  There is no surplus white space between arguments
and no leading or trailing whitespace except for the newline
character that acts as the card separator.
All cards of a cluster occur in strict sorted lexicographical order.
No card may be duplicated.
The cluster may not contain additional text or data beyond
what is described here.
Unlike manifests, clusters are never PGP signed.

Allowed cards in the cluster are as follows:

<blockquote>
<b>M</b> <i>artifact-id</i> ?<i>alias</i><br />
<b>Z</b> <i>checksum</i>
</blockquote>

A cluster contains one or more "M" cards followed by a single "Z"
card.  Each M card has at least on argument which is the artifact ID of
another artifact in the repository.  If the M card has a second argument,
the second argument is an alias for the artifact name.  
The Z card works exactly like
the Z card of a manifest.  The argument to the Z card is the
lower-case hexadecimal representation of the MD5 checksum of all
prior cards in the cluster.  The Z-card is required.

An example cluster from Fossil can be seen
[/artifact/d03dbdd73a2a8 | here].

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</tr>
<tr>
<td><b>M</b> <i>uuid</i> ?<i>alias</i>?</td>
<td>&nbsp;</td>
<td align=center><b>1+</b></td>
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