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<h2 class="title" style="clear: both"><a id="rywc"></a>Read-Your-Writes Consistency</h2>
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<p>
In a distributed system, the changes made at the master are not
always instantaneously available at every replica, although
they eventually will be. In general, replicas not directly
involved in contributing to the acknowledgement of a
transaction commit will lag behind other replicas because they
do not synchronize their commits with the master.
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<p>
For this reason, you might want to make use of the
read-your-writes consistency feature. This feature allows you
to ensure that a replica is at least current enough to have the
changes made by a specific transaction. Because transactions
are applied serially, by ensuring a replica has a specific
commit applied to it, you know that all transaction commits
occurring prior to the specified transaction have also been
applied to the replica.
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<p>
You determine whether a transaction has been applied to a replica by
generating a <span class="emphasis"><em>commit token</em></span> at the master.
You then transfer this commit token to the replica, where it is
used to determine whether the replica is consistent enough
relative to the master.
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<p>
For example, suppose the you have a web application
where a replication group is implemented within a load
balanced web server group. Each request to the web server
consists of an update operation followed by read operations
(say, from the same client), The read operations naturally
expect to see the data from the updates executed by the
same request. However, the read operations might have been
routed to a replica that did not execute the update.
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<p>
In such a case, the update request would generate a commit
token, which would be resubmitted by the browser, along with
subsequent read requests. The read request could be directed at
any one of the available web servers by a load balancer. The
replica which services the read request would use that commit
token to determine whether it can service the read
operation. If the replica is current enough, it can
immediately execute the transaction and satisfy the request.
</p>
<p>
What action the replica takes if it is not consistent enough to
service the read request is up to you as the application
developer. You can do anything from blocking while you wait
for the transaction to be applied locally, to rejecting the
read request outright.
</p>
<p>
For more information, see the
<code class="literal">Read your writes consistency</code> section in
the <code class="literal">Berkeley DB Replication</code> chapter of the
<em class="citetitle">Berkeley DB Programmer's Reference Guide</em>.
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