2011-09-13 17:44:24 +00:00
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<title>Processing Loop</title>
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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="fwrkmasterreplica.html" title="Chapter 4. Replica versus Master Processes" />
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<link rel="prev" href="fwrkmasterreplica.html" title="Chapter 4. Replica versus Master Processes" />
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<link rel="next" href="exampledoloop.html" title="Example Processing Loop" />
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</head>
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<body>
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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">Processing Loop</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="fwrkmasterreplica.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 4. Replica versus Master Processes</th>
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<td width="20%" align="right"> <a accesskey="n" href="exampledoloop.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="processingloop"></a>Processing Loop</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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Typically the central part of any replication application
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is some sort of a continuous loop that constantly
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checks the state of the environment (whether it is a
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replica or a master), opens and/or closes the
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databases as is necessary, and performs other useful
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work. A loop such as this one must of necessity
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take special care to know whether it is operating
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on a master or a replica environment because all of its
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activities are dependent upon that state.
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</p>
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<p>
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The flow of activities through the loop will
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generally be as follows:
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</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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<p>
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Check whether the environment has
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changed state. If it has, you
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might want to reopen your
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database handles, especially if
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you opened your replica's
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database handles as read-only.
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In this case, you might need to
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reopen them as read-write.
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However, if you always open your
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database handles as read-write,
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then it is not automatically necessary to
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reopen the databases due to a
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state change. Instead, you
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could check for a
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<span>
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<code class="literal">DB_REP_HANDLE_DEAD</code>
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return code
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</span>
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when you use your
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database handle(s). If you see
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this, then you need to reopen
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your database handle(s).
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</p>
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</li>
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<li>
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<p>
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If the databases are closed,
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create new database handles,
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configure the handle as is
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appropriate, and then open the
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databases. Note that handle
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configuration will be different,
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depending on whether the handle
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is opened as a replica or a
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master. At a minimum, the master
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should be opened with database
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creation privileges, whereas the
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replica does not need to be. You
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must also open the master such
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that its databases are
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read-write. You
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<span class="emphasis"><em>can</em></span> open
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replicas with read-only
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databases, so long as you are
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prepared to close and then reopen
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the handle in the event the
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client becomes a master.
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</p>
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<p>
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Also, note that if the local
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environment
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is a replica, then it is possible
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that databases do not currently
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exist. In this case, the database
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open attempts will fail. Your
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code will have to take this
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corner case into account
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(described below).
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</p>
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</li>
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<li>
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<p>
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Once the databases are opened,
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check to see if the local
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environment is a
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master. If it is, do whatever it is
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a master should do for your
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application.
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</p>
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<p>
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Remember that the code for your
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master should include some way
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for you to tell the master
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to exit gracefully.
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</p>
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</li>
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<li>
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<p>
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If the local environment is not a
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master, then do whatever it is
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your replica environments should do.
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Again, like the code for your
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master environments, you should provide
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a way for your replicas to exit
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the processing loop gracefully.
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</p>
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</li>
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</ol>
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</div>
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<p>
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The following code fragment illustrates
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these points (note that we fill out this
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fragment with a working example
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next in this chapter):
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</p>
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<pre class="programlisting">/* loop to manage replication activities */
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Db *dbp;
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int ret;
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APP_DATA *app_data;
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u_int32_t flags;
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dbp = NULL;
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ret = 0;
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/*
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* Remember that for this to work, an APP_DATA struct would have first
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* had to been set to the environment handle's app_private data
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* member. (dbenv is presumably declared and opened in another part of
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* the code.)
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*/
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app_data = dbenv->get_app_private();
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/*
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* Infinite loop. We exit depending on how the master and replica code
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* is written.
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*/
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for (;;) {
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/* If dbp is not opened, we need to open it. */
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if (dbp == 0) {
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/*
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* Create the handle and then configure it. Before you open
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* it, you have to decide what open flags to use:
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*/
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dbp = new Db(&dbenv, 0);
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/*
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* Now you can open your database handle.
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*
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* One thing to watch out for is a case where the databases
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* you are trying to open do not yet exist. This can happen
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* for replicas where the databases are being opened
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* read-only. If this happens, ENOENT is returned by the
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* open() call.
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*/
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try {
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dbp->open(NULL, DATABASE, NULL, DB_BTREE,
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app_data->is_master ? DB_CREATE | DB_AUTO_COMMIT :
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DB_AUTO_COMMIT, 0);
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} catch(DbException dbe) {
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if (dbe.get_errno() == ENOENT) {
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cout << "No stock db available yet - retrying." << endl;
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try {
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dbp->close(0);
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} catch (DbException dbe2) {
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cout << "Unexpected error closing after failed" <<
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" open, message: " << dbe2.what() << endl;
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dbp = NULL;
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goto err;
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}
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dbp = NULL;
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sleep(SLEEPTIME);
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continue;
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} else {
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dbenv.err(ret, "DB->open");
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throw dbe;
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}
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}
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}
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/*
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* Now that the databases have been opened, continue with general
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* processing, depending on whether we are a master or a replica.
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*/
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if (app_data->is_master) {
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/*
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* Do master stuff here. Don't forget to include a way to
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* gracefully exit the loop. */
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*/
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} else {
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/*
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* Do replica stuff here. As is the case with the master
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* code, be sure to include a way to gracefully exit the
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* loop.
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*/
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}
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} </pre>
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</div>
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<hr />
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<table width="100%" summary="Navigation footer">
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<td width="40%" align="left"><a accesskey="p" href="fwrkmasterreplica.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="fwrkmasterreplica.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="exampledoloop.html">Next</a></td>
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<td width="40%" align="left" valign="top">Chapter 4. Replica versus Master Processes </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"> Example Processing Loop</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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