2011-09-13 17:44:24 +00:00
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Read-Your-Writes Consistency</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Getting Started with Replicated Berkeley DB Applications" />
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<link rel="up" href="addfeatures.html" title="Chapter 5. Additional Features" />
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<link rel="prev" href="autoinit.html" title="Stop Auto-Initialization" />
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<link rel="next" href="c2ctransfer.html" title="Client to Client Transfer" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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2012-11-14 21:35:20 +00:00
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<p>Library Version 11.2.5.3</p>
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2011-09-13 17:44:24 +00:00
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Read-Your-Writes Consistency</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="autoinit.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 5. Additional Features</th>
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<td width="20%" align="right"> <a accesskey="n" href="c2ctransfer.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="rywc"></a>Read-Your-Writes Consistency</h2>
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</div>
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</div>
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</div>
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<p>
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In a distributed system, the changes made at the master are not
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always instantaneously available at every replica, although
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they eventually will be. In general, replicas not directly
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involved in contributing to the acknowledgement of a
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transaction commit will lag behind other replicas because they
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do not synchronize their commits with the master.
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</p>
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<p>
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For this reason, you might want to make use of the
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read-your-writes consistency feature. This feature allows you
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to ensure that a replica is at least current enough to have the
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changes made by a specific transaction. Because transactions
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are applied serially, by ensuring a replica has a specific
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commit applied to it, you know that all transaction commits
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occurring prior to the specified transaction have also been
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applied to the replica.
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</p>
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<p>
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You determine whether a transaction has been applied to a replica by
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generating a <span class="emphasis"><em>commit token</em></span> at the master.
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You then transfer this commit token to the replica, where it is
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used to determine whether the replica is consistent enough
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relative to the master.
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</p>
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<p>
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For example, suppose the you have a web application
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where a replication group is implemented within a load
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balanced web server group. Each request to the web server
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consists of an update operation followed by read operations
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(say, from the same client), The read operations naturally
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expect to see the data from the updates executed by the
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same request. However, the read operations might have been
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routed to a replica that did not execute the update.
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</p>
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<p>
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In such a case, the update request would generate a commit
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token, which would be resubmitted by the browser, along with
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subsequent read requests. The read request could be directed at
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any one of the available web servers by a load balancer. The
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replica which services the read request would use that commit
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token to determine whether it can service the read
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operation. If the replica is current enough, it can
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immediately execute the transaction and satisfy the request.
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</p>
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<p>
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What action the replica takes if it is not consistent enough to
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service the read request is up to you as the application
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developer. You can do anything from blocking while you wait
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for the transaction to be applied locally, to rejecting the
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read request outright.
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</p>
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<p>
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For more information, see the
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<code class="literal">Read your writes consistency</code> section in
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the <code class="literal">Berkeley DB Replication</code> chapter of the
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<em class="citetitle">Berkeley DB Programmer's Reference Guide</em>.
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</p>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="autoinit.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="addfeatures.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="c2ctransfer.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Stop Auto-Initialization </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Client to Client Transfer</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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