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<sect1 id="cluster">
<title>Defining &slony1; Clusters</title>
<indexterm>
 <primary>cluster</primary>
</indexterm>

<para>A &slony1; cluster is the basic grouping of database instances
in which replication takes place.  It consists of a set of &postgres;
database instances in which is defined a namespace specific to that
cluster.</para>

<para>Each database instance in which replication is to take place is
identified by a node number.</para>

<para>For a simple install, it may be reasonable for the origin to be
node #1, and for the subscriber to be node #2.</para>

<para>Some planning should be done, in more complex cases, to ensure
that the numbering system is kept sane, lest the administrators be
driven insane.  The node numbers should be chosen to somehow
correspond to the shape of the environment, as opposed to (say) the
order in which nodes were initialized.</para>

<para>It may be that in version 1.1, nodes will also have a
<quote>name</quote> attribute, so that they may be given more mnemonic names.
In that case, the node numbers might remain somewhat cryptic; it will
be the node name that is used to organize the cluster.</para>
</sect1>

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