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HBASE-6524 Hooks for hbase tracing; add documentation as an appendix 

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1388337 13f79535-47bb-0310-9956-ffa450edef68
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Michael Stack 2012-09-21 06:23:03 +00:00
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@ -3583,6 +3583,67 @@ Comparator class used for Bloom filter keys, a UTF>8 encoded string stored usi
</section>
</appendix>
<appendix xml:id="tracing" ><title>Enabling Dapper-like Tracing in HBase</title>
<para><link xlink:href="https://issues.apache.org/jira/browse/HBASE-6449">HBASE-6449</link> added support
for tracing requests through HBase, using the open source tracing library,
<link xlink:href="http://github.com/cloudera/htrace">HTrace</link>. Setting up tracing is quite simple,
however it currently requires some very minor changes to your client code (it would not be very difficult to remove this requirement).
</para>
<section xml:id="tracing.spanreceivers"><title>SpanReceivers</title>
<para>The tracing system works by collecting information in structs called ‘Spans’.
It is up to you to choose how you want to receive this information by implementing the
<classname>SpanReceiver</classname> interface, which defines one method:
<programlisting>public void receiveSpan(Span span);</programlisting>
This method serves as a callback whenever a span is completed. HTrace allows you to use
as many SpanReceivers as you want so you can easily send trace information to multiple destinations.
</para>
<para>Configure what SpanReceivers you’d like to use by putting a comma separated list of the
fully-qualified class name of classes implementing <classname>SpanReceiver</classname> in
<filename>hbase-site.xml</filename> property: <varname>hbase.trace.spanreceiver.classes</varname>.
</para>
<para>HBase includes a <classname>HBaseLocalFileSpanReceiver</classname> that writes all span
information to local files in a JSON-based format. The <classname>HBaseLocalFileSpanReceiver</classname>
looks in <filename>hbase-site.xml</filename> for a <varname>hbase.trace.spanreceiver.localfilespanreceiver.filename</varname>
property with a value describing the name of the file to which nodes should write their span information.
</para>
<para>If you do not want to use the included <classname>HBaseLocalFileSpanReceiver</classname>,
you are encouraged to write your own receiver (take a look at <classname>HBaseLocalFileSpanReceiver</classname>
for an example). If you think others would benefit from your receiver, file a JIRA or send a pull request to
<link xlink:href="http://github.com/cloudera/htrace">HTrace</link>.
</para>
</section>
<section xml:id="tracing.client.modifications">
<title>Client Modifications</title>
<para>Currently, you must turn on tracing in your client code. To do this, you simply turn on tracing for
requests you think are interesting, and turn it off when the request is done.
</para>
<para>For example, if you wanted to trace all of your get operations, you change this:
<programlisting>HTable table = new HTable(...);
Get get = new Get(...);</programlisting>
into:
<programlisting>Span getSpan = Trace.startSpan(“doing get”, Sampler.ALWAYS);
try {
HTable table = new HTable(...);
Get get = new Get(...);
...
} finally {
getSpan.stop();
}</programlisting>
If you wanted to trace half of your ‘get’ operations, you would pass in:
<programlisting>new ProbabilitySampler(0.5)</programlisting> in lieu of <varname>Sampler.ALWAYS</varname> to <classname>Trace.startSpan()</classname>.
See the HTrace <filename>README</filename> for more information on Samplers.
</para>
</section>
</appendix>
<index xml:id="book_index">
<title>Index</title>
</index>