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<title>Accumulo Isolation</title>
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<h1>Apache Accumulo Documentation : Isolation</h1>

<h3>Scanning</h3>

<p>Accumulo supports the ability to present an isolated view of rows when scanning. There are three possible ways that a row could change in accumulo :
<ul>
 <li>a mutation applied to a table
 <li>iterators executed as part of a minor or major compaction
 <li>bulk import of new files
</ul>
Isolation guarantees that either all or none of the changes made by these
operations on a row are seen. Use the <code>IsolatedScanner</code> to obtain an
isolated view of an accumulo table. When using the regular scanner it is
possible to see a non isolated view of a row. For example if a mutation
modifies three columns, it is possible that you will only see two of those
modifications. With the isolated scanner either all three of the changes are
seen or none. For an example of this try running the
<code>InterferenceTest</code> example.

<p>At this time there is no client side isolation support for the
<code>BatchScanner</code>. You may consider using the
<code>WholeRowIterator</code> with the  <code>BatchScanner</code> to achieve
isolation though. This drawback of doing this is that entire rows are read into
memory on the server side. If a row is too big, it may crash a tablet server.
The <code>IsolatedScanner</code> buffers rows on the client side so a large row will not crash a tablet server.

<h3>Iterators</h3>
<p>See the user manual's chapter on iterator design for more information on considerations when Isolation is enabled.