385 lines
14 KiB
HTML
385 lines
14 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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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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<html xmlns="http://www.w3.org/1999/xhtml">
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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>Adding the Replication Framework to
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simple_txn
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</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 V1.62.4" />
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<link rel="home" href="index.html" title="Getting Started with Replicated Berkeley DB Applications" />
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<link rel="up" href="repapp.html" title="Chapter 3. The DB Replication Framework" />
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<link rel="previous" href="repapp.html" title="Chapter 3. The DB Replication Framework" />
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<link rel="next" href="fwrkpermmessage.html" title="Permanent Message Handling" />
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</head>
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<body>
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<div class="navheader">
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Adding the Replication Framework to
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simple_txn
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</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="repapp.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 3. The DB Replication Framework</th>
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<td width="20%" align="right"> <a accesskey="n" href="fwrkpermmessage.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="repmgr_init_example_c"></a>Adding the Replication Framework to
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<span>simple_txn</span>
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</h2>
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</div>
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</div>
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<div></div>
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</div>
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<p>
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We now use the methods described above to add partial
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support to the
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<tt class="literal">simple_txn</tt>
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example that we presented in
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<a href="txnapp.html">Transactional Application</a>.
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That is, in this section we will:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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Enhance our command line options to
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accept information of interest to a
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replicated application.
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</p>
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</li>
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<li>
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<p>
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Configure our environment handle to use
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replication and the replication framework.
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</p>
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</li>
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<li>
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<p>
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Minimally configure the replication framework.
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</p>
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</li>
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<li>
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<p>
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Start replication.
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</p>
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</li>
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</ul>
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</div>
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<p>
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Note that when we are done with this section, we will be
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only partially ready to run the application. Some critical
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pieces will be missing; specifically, we will not yet be
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handling the differences between a master and a
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replica. (We do that in the next chapter).
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</p>
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<p>
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Also, note that in the following code fragments, additions
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and changes to the code are marked in <b class="userinput"><tt>bold</tt></b>.
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</p>
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<p>
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To begin, we copy the
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<tt class="literal">simple_txn</tt>
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code to a new file called
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<tt class="literal">rep_mgr.c</tt>.
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<span>
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We then make the corresponding change to the program name.
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</span>
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</p>
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<pre class="programlisting">/*
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<b class="userinput"><tt>* File: rep_mgr.c</tt></b>
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*/
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#include <stdlib.h>
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#include <string.h>
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#ifndef _WIN32
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#include <unistd.h>
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#endif
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#include <db.h>
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#ifdef _WIN32
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extern int getopt(int, char * const *, const char *);
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#endif
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#define CACHESIZE (10 * 1024 * 1024)
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#define DATABASE "quote.db"
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const char *progname = <b class="userinput"><tt>"rep_mgr";</tt></b>
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int create_env(const char *, DB_ENV **);
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int env_init(DB_ENV *, const char *);
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int doloop (DB_ENV *);
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int print_stocks(DBC *); </pre>
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<p>
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Next we update our usage function. The application will continue to
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accept the <tt class="literal">-h</tt> parameter so that we can identify
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the environment home directory used by this application. However,
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we also add the
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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<tt class="literal">-m</tt> parameter which allows us to
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identify the host and port used by this application to
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listen for replication messages.
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</p>
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</li>
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<li>
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<p>
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<tt class="literal">-o</tt> parameter which allows us to
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specify other replicas.
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</p>
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</li>
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<li>
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<p>
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<tt class="literal">-n</tt> parameter which allows us to
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identify the number of sites in this replication
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group.
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</p>
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</li>
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<li>
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<p>
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<tt class="literal">-p</tt> option, which is used to identify
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this replica's priority (recall that the priority is
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used as a tie breaker for elections)
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</p>
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</li>
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</ul>
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</div>
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<pre class="programlisting">/* Usage function */
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static void
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usage()
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{
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fprintf(stderr, "usage: %s ", progname);
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fprintf(stderr, "[-h home] <b class="userinput"><tt>-m host:port [-o host:port]</tt></b>\n");
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<b class="userinput"><tt>fprintf(stderr, "\t\t[-p priority]\n");</tt></b>
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fprintf(stderr, "where:\n");
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fprintf(stderr, "\t-h identifies the environment home directory\n");
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<b class="userinput"><tt>fprintf(stderr, "\t-o identifies another site participating in ");
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fprintf(stderr, "this replication group\n");
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fprintf(stderr, "\t-m identifies the host and port used by this ");
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fprintf(stderr, "site (required).\n");
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fprintf(stderr, "\t-n identifies the number of sites in this ");
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fprintf(stderr, "replication group (required).\n");
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fprintf(stderr, "\t-p identifies the election priority used by ");
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fprintf(stderr, "this replica.\n");</tt></b>
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exit(EXIT_FAILURE);
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} </pre>
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<p>
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Now we can begin working on our <tt class="literal">main()</tt> function.
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We begin by adding a couple of variables that we will use to
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collect TCP/IP host and port information.
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<span>
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We also declare a couple
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of flags that we use to make sure some required information is
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provided to this application.
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</span>
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</p>
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<pre class="programlisting">int
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main(int argc, char *argv[])
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{
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extern char *optarg;
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DB_ENV *dbenv;
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const char *home;
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char ch, <b class="userinput"><tt>*host, *portstr</tt></b>;
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int ret, <b class="userinput"><tt>local_is_set, totalsites</tt></b>;
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<b class="userinput"><tt>u_int32_t port</tt></b>;
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dbenv = NULL;
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ret = <b class="userinput"><tt>local_is_set = totalsites = </tt></b>0;
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home = NULL; </pre>
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<p>
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At this time we can create our environment handle and configure it
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exactly as we did for <tt class="literal">simple_txn</tt>.
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The only thing that we will do differently here is that we will set a priority,
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arbitrarily picked to be 100, so that we can be sure the environment has
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a priority other than 0 (the default value). This ensures that the
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environment can become a master via an election.
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</p>
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<pre class="programlisting"> if ((ret = create_env(progname, &dbenv)) != 0)
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goto err;
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/* Default priority is 100 */
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<b class="userinput"><tt>dbenv->rep_set_priority(dbenv, 100);</tt></b> </pre>
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<p>
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Now we collect our command line arguments. As we do so, we will
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configure host and port information as required, and we will
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configure the application's election priority if necessary.
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</p>
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<pre class="programlisting"> /* Collect the command line options */
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while ((ch = getopt(argc, argv, "h:<b class="userinput"><tt>m:o:p:</tt></b>")) != EOF)
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switch (ch) {
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case 'h':
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home = optarg;
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break;
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<b class="userinput"><tt>/* Set the host and port used by this environment */
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case 'm':
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host = strtok(optarg, ":");
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if ((portstr = strtok(NULL, ":")) == NULL) {
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fprintf(stderr, "Bad host specification.\n");
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goto err;
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}
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port = (unsigned short)atoi(portstr);
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if (dbenv->repmgr_set_local_site(dbenv, host, port, 0) != 0) {
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fprintf(stderr,
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"Could not set local address %s.\n", host);
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goto err;
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}
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local_is_set = 1;
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break;
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/* Identify another site in the replication group */
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case 'o':
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host = strtok(optarg, ":");
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if ((portstr = strtok(NULL, ":")) == NULL) {
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fprintf(stderr, "Bad host specification.\n");
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goto err;
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}
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port = (unsigned short)atoi(portstr);
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if (dbenv->repmgr_add_remote_site(dbenv, host, port,
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NULL, 0) != 0) {
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fprintf(stderr,
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"Could not add site %s.\n", host);
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goto err;
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}
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break;
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/* Set the number of sites in this replication group */
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case 'n':
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totalsites = atoi(optarg);
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if ((ret = dbenv->rep_set_nsites(dbenv, totalsites)) != 0)
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dbenv->err(dbenv, ret, "set_nsites");
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break;
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/* Set this replica's election priority */
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case 'p':
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dbenv->rep_set_priority(dbenv, atoi(optarg));
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break; </tt></b>
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case '?':
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default:
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usage();
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}
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/* Error check command line. */
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if (home == NULL <b class="userinput"><tt>|| !local_is_set || !totalsites</tt></b>)
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usage(); </pre>
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<p>
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Having done that, we can call <tt class="function">env_init()</tt>, which we use to
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open our environment handle. Note that this function changes
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slightly for this update (see below).
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</p>
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<pre class="programlisting"> if ((ret = env_init(dbenv, home)) != 0)
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goto err; </pre>
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<p>
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Finally, we start replication before we go into the
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<tt class="function">doloop()</tt> function (where we perform all our
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database access).
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</p>
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<pre class="programlisting">
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<b class="userinput"><tt>if ((ret = dbenv->repmgr_start(dbenv, 3, DB_REP_ELECTION)) != 0)
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goto err; </tt></b>
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if ((ret = doloop(dbenv)) != 0) {
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dbenv->err(dbenv, ret, "Application failed");
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goto err;
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}
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err: if (dbenv != NULL)
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(void)dbenv->close(dbenv, 0);
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return (ret);
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} </pre>
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<p>
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<span>Beyond that, the rest of our application remains the same for
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now, with the exception of the <tt class="function">env_init()</tt>
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function, which we use to actually open our environment handle.
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The flags </span>
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we use to open the environment are slightly different for a
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replicated application than they are for a non-replicated
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application. Namely, replication requires the
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<span>
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<tt class="literal">DB_INIT_REP</tt> flag.
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</span>
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</p>
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<p>
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Also, because we are using the replication framework, we must prepare
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our environment for threaded usage. For this reason, we also
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need the <tt class="literal">DB_THREAD</tt> flag.
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</p>
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<pre class="programlisting"> int
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env_init(DB_ENV *dbenv, const char *home)
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{
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u_int32_t flags;
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int ret;
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(void)dbenv->set_cachesize(dbenv, 0, CACHESIZE, 0);
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(void)dbenv->set_flags(dbenv, DB_TXN_NOSYNC, 1);
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flags = DB_CREATE |
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DB_INIT_LOCK |
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DB_INIT_LOG |
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DB_INIT_MPOOL |
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DB_INIT_TXN |
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<b class="userinput"><tt>DB_INIT_REP |
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DB_THREAD |</tt></b>
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DB_RECOVER;
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if ((ret = dbenv->open(dbenv, home, flags, 0)) != 0)
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dbenv->err(dbenv, ret, "can't open environment");
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return (ret);
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}</pre>
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<p>
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This completes our replication updates for the moment. We are not as
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yet ready to actually run this program; there remains a few
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critical pieces left to add to it. However, the work that we
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performed in this section represents a solid foundation for the
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remainder of our replication work.
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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="repapp.html">Prev</a> </td>
|
||
<td width="20%" align="center">
|
||
<a accesskey="u" href="repapp.html">Up</a>
|
||
</td>
|
||
<td width="40%" align="right"> <a accesskey="n" href="fwrkpermmessage.html">Next</a></td>
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||
</tr>
|
||
<tr>
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<td width="40%" align="left" valign="top">Chapter 3. The DB Replication Framework </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"> Permanent Message Handling</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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