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updated documentation
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chenson42 committed Apr 8, 2013
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</section>


<section id="purge">
<title>Database Purging</title>
<para>
Purging is the act of cleaning up captured data that is no longer needed in SymmetricDS's runtime tables.
Data is purged through
delete statements by the <emphasis>Purge Job</emphasis>. Only data that has been successfully synchronized will be purged. Purged tables include:
<itemizedlist>
<listitem>
<xref linkend="table_data" xrefstyle="table"/>
</listitem>
<listitem>
<xref linkend="table_data_event" xrefstyle="table"/>
</listitem>
<listitem>
<xref linkend="table_outgoing_batch" xrefstyle="table"/>
</listitem>
<listitem>
<xref linkend="table_incoming_batch" xrefstyle="table"/>
</listitem>
<listitem>
<xref linkend="table_data_gap" xrefstyle="table"/>
</listitem>
<listitem>
<xref linkend="table_node_host_stats" xrefstyle="table"/>
</listitem>
<listitem>
<xref linkend="table_node_host_channel_stats" xrefstyle="table"/>
</listitem>
<listitem>
<xref linkend="table_node_host_job_stats" xrefstyle="table"/>
</listitem>
</itemizedlist>
The purge job is enabled by the <literal>start.purge.job</literal> SymmetricDS property. The timing of the three purge jobs (incoming, outgoing, and data gaps) is controlled
by a cron expression as specified by the following properties: <literal>job.purge.outgoing.cron</literal>, <literal>job.purge.incoming.cron</literal>,
and <literal>job.purge.datagaps.cron</literal>. The default is <literal>0 0 0 * * *</literal>, or once per day at midnight.
</para>

<important>
<para>
SymmetricDS utilizes Spring's CRON support, which includes seconds as the first parameter. This differs from the typical Unix-based
implementation, where the first parameter is usually minutes. For example, <literal>*/15 * * * * *</literal> means every 15 seconds, not every 15 minutes.
See <ulink url="http://static.springsource.org/spring/docs/3.0.x/javadoc-api/org/springframework/scheduling/support/CronSequenceGenerator.html">Spring's documentation</ulink>
for more details.
</para>
</important>

<para>
Two retention period properties
indicate how much history SymmetricDS will retain before purging. The <literal>purge.retention.minutes</literal> property indicates the period
of history to keep for synchronization tables. The default value is 5 days.
The <literal>statistic.retention.minutes</literal> property
indicates the period of history to keep for statistics. The default value is also 5 days.
</para>
<para>
The purge properties should be adjusted according to how much data is flowing through the system and the amount of storage space the database has.
For an initial deployment it is recommended that the purge properties be kept at the defaults, since it is often helpful to be able to look at
the captured data in order to triage problems and profile the synchronization patterns. When scaling up to more nodes, it is recomended that the
purge parameters be scaled back to 24 hours or less.
</para>
</section>





</chapter>

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