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Editing: rep_init_code.html

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>Starting and Stopping Replication</title>
    <link rel="stylesheet" href="gettingStarted.css" type="text/css" />
    <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
    <link rel="start" href="index.html" title="Getting Started with Replicated Berkeley DB Applications" />
    <link rel="up" href="repapp.html" title="Chapter 3. The DB Replication Manager" />
    <link rel="prev" href="repapp.html" title="Chapter 3. The DB Replication Manager" />
    <link rel="next" href="repmgr_init_example_c.html" title="Adding the Replication Manager to ex_rep_gsg_simple" />
  </head>
  <body>
    <div xmlns="" class="navheader">
      <div class="libver">
        <p>Library Version 11.2.5.3</p>
      </div>
      <table width="100%" summary="Navigation header">
        <tr>
          <th colspan="3" align="center">
        Starting and Stopping Replication
    </th>
        </tr>
        <tr>
          <td width="20%" align="left"><a accesskey="p" href="repapp.html">Prev</a> </td>
          <th width="60%" align="center">Chapter 3. The DB Replication Manager</th>
          <td width="20%" align="right"> <a accesskey="n" href="repmgr_init_example_c.html">Next</a></td>
        </tr>
      </table>
      <hr />
    </div>
    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both"><a id="rep_init_code"></a>
        Starting and Stopping Replication
    </h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <dl>
          <dt>
            <span class="sect2">
              <a href="rep_init_code.html#election_flags">Managing Election Policies</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="rep_init_code.html#thread_count">Selecting the Number of Threads</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>
        As described above, you introduce replication to an application by
        starting with a transactional application, performing some basic
        replication configuration, and then starting replication using 
        <span><code class="methodname">DB_ENV-&gt;repmgr_start()</code>.</span>
        
        
    </p>
      <p>
        You stop replication by closing your environment
        cleanly in the same way you would for any DB application.
    </p>
      <p>
        For example, the following code fragment initializes, then
        stops and starts replication. Note that other replication
        activities are omitted for brevity.
    </p>
      <pre class="programlisting">#include &lt;db.h&gt;

/* Use a 10mb cache */
#define CACHESIZE   (10 * 1024 * 1024)

...

DB_ENV *dbenv;            /* Environment handle. */
    DB_SITE *dbsite;          /* Replication manager site handle. */
    const char *progname;     /* Program name. */
    const char *envHome;      /* Environment home directory. */
    const char *listen_host;  /* A TCP/IP hostname. */
    const char *other_host;   /* A TCP/IP hostname. */
    int ret;                  /* Error return code. */
    int is_group_creator;     /* A flag */
    u_int16 listen_port;      /* A TCP/IP port. */
    u_int16 other_port;       /* A TCP/IP port. */

/* Initialize variables */
    dbenv = NULL;
    progname = "example_replication";
    envHome = "ENVIRONMENT_HOME";
    listen_host = "mymachine.sleepycat.com";
    listen_port = 5001;
    other_host = "anothermachine.sleepycat.com";
    other_port = 4555;
    ret = 0;
    is_group_creator = 1; /* This is usually set via a command line 
                             argument or some other external 
                             configuration mechanism. */

/* Create the environment handle */
    if ((ret = db_env_create(&amp;dbenv, 0)) != 0 ) {
        fprintf(stderr, "Error creating environment handle: %s\n",
            db_strerror(ret));
        goto err;
    }

/* 
     * Configure the environment handle. Here we configure 
     * asynchronous transactional commits for performance reasons.
     */
    dbenv-&gt;set_errfile(dbenv, stderr);
    dbenv-&gt;set_errpfx(dbenv, progname);
    (void)dbenv-&gt;set_cachesize(dbenv, 0, CACHESIZE, 0);
    (void)dbenv-&gt;set_flags(dbenv, DB_TXN_NOSYNC, 1);

/*
     * Configure the local address. This is the local hostname and 
     * port that this replication environment will use to receive 
     * incoming replication messages. Note that this can be 
     * performed only once for the replication environment. 
     * It is required.

* First: Create a DB_SITE handle to identify the site's 
     * host/port network address.
     */
     if ((ret = dbenv-&gt;repmgr_site(dbenv, listen_host, listen_port,
                                   &amp;dbsite;, 0)) != 0) {
        fprintf(stderr, "Could not set local address (%d).\n", ret);
        goto err;
     }

/*
      * Second: Configure this site as the local site within the
      * replication group.
      */
      dbsite-&gt;set_config(dbsite, DB_LOCAL_SITE, 1);

/*
     * Third: Set DB_GROUP_CREATOR if applicable. This can be done
     * only for the local site. It should also only be peformed
     * for one and only one site in a replication group, so 
     * typically this is driven by an externally-supplied
     * configuration option. 
     *
     * DB_GROUP_CREATOR only has meaning if you are starting the
     * very first site for the very first time in a replication
     * group. It is otherwise ignored.
     */
     if (is_group_creator)
        dbsite-&gt;set_config(dbsite, DB_GROUP_CREATOR, 1);

/*
     * Having configured the local site, we can immediately
     * deallocate the DB_SITE handle.
     */
    if ((ret = dbsite-&gt;close(dbsite)) != 0) {
            dbenv-&gt;err(dbenv, ret, "DB_SITE-&gt;close");
            goto err;
    }

/*
     * Set this replication environment's priority. This is used 
     * for elections.
     *
     * Set this number to a positive integer, or 0 if you do not want 
     * this site to be able to become a master.
     */
    dbenv-&gt;rep_set_priority(dbenv, 100);

/*
     * Configure a bootstrap helper. This information is used only 
     * if the site currently exists, and the local site has never 
     * been started before. Otherwise, this configuration 
     * information is ignored.
     * 
     */
    if (!is_group_creator) {
        if ((ret = dbenv-&gt;repmgr_site(dbenv, other_host, other_port,
          &amp;dbsite, 0)) != 0) {
            dbenv-&gt;err(dbenv, ret, "Could not add site %s:%d\n",
              other_host, other_port);
            goto err;
        }

dbsite-&gt;set_config(dbsite, DB_BOOTSTRAP_HELPER, 1);
        if ((ret = dbsite-&gt;close(dbsite)) != 0) {
            dbenv-&gt;err(dbenv, ret, "DB_SITE-&gt;close");
            goto err;
        }

/*
         * Having configured the bootstrap helper site, we can 
         * immediately deallocate the DB_SITE handle.
         */
        if ((ret = dbsite-&gt;close(dbsite)) != 0) {
                dbenv-&gt;err(dbenv, ret, "DB_SITE-&gt;close");
                goto err;
        }
    }

/* Open the environment handle. Note that we add DB_THREAD and
     * DB_INIT_REP to the list of flags. These are required.
     */
    if ((ret = dbenv-&gt;open(dbenv, home, DB_CREATE | DB_RECOVER |
                                        DB_INIT_LOCK | DB_INIT_LOG |
                                        DB_INIT_MPOOL | DB_INIT_TXN  |
                                        DB_THREAD | DB_INIT_REP,
                                        0)) != 0) {
        goto err;
    }

/* Start the replication manager such that it uses 3 threads. */
    if ((ret = dbenv-&gt;repmgr_start(dbenv, 3, DB_REP_ELECTION)) != 0)
        goto err;

/* Sleep to give ourselves time to find a master */
    sleep(5);

/*
     **********************************************************
     *** All other application code goes here, including  *****
     *** database opens                                   *****
     **********************************************************
     */

err: /* 
      * Make sure all your database and dbsite handles are closed 
      *  (omitted from this example). 
      */

/* Close the environment */
    if (dbenv != NULL)
        (void)dbenv-&gt;close(dbenv, 0);

/* All done */
    return (ret); </pre>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="election_flags"></a>Managing Election Policies</h3>
            </div>
          </div>
        </div>
        <p>
                Before continuing, it is worth taking a look at the

<span>
                startup election flags accepted by 
                <span><code class="methodname">DB_ENV-&gt;repgmr_start()</code>.</span>
                
                These flags control how your replication application will
                behave when it first starts up.
                </span>

</p>
        <p>
                    In the previous example, we specified 
                        <code class="literal">DB_REP_ELECTION</code>
                        
                    when we started replication. This causes the
                    application to try to find a master upon startup. If it
                    cannot, it calls for an election. In the event an
                    election is held, the environment receiving the most number of
                    votes will become the master.
            </p>
        <p>
                    There's some important points to make here:
            </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                                    This 
                                    <span>flag</span> 
                                     
                                    only requires that other
                                    environments in the replication group
                                    participate in the vote. There is no
                                    requirement that
                                    <span class="emphasis"><em>all</em></span> such
                                    environments participate. In other
                                    words, if an environment
                                    starts up, it can call for an
                                    election, and select a master, even 
                                    if all other environment have not yet
                                    joined the  replication group.
                            </p>
            </li>
            <li>
              <p>
                                    It only requires a simple majority of
                                    participating environments to elect a master. 
                                    This is always true of elections held using the Replication Manager.
                            </p>
            </li>
            <li>
              <p>
                                    As always, the environment participating in the election with the most
                                    up-to-date log files is selected as
                                    master. If an environment with more recent log files
                                    has not yet joined the replication
                                    group, it may not become the master.
                            </p>
            </li>
          </ul>
        </div>
        <p>
                    Any one of these points may be enough to cause a
                    less-than-optimum environment to be selected as master.
                    Therefore, to give you a better degree of control over
                    which environment becomes a master at application startup,
                    the Replication Manager offers the following start-up
                                    <span>flags:</span> 
                                     
            </p>
        <div class="informaltable">
          <table border="1" width="80%">
            <colgroup>
              <col />
              <col />
            </colgroup>
            <thead>
              <tr>
                <th>Flag</th>
                <th>Description</th>
              </tr>
            </thead>
            <tbody>
              <tr>
                <td>
                  <code class="literal">DB_REP_MASTER</code>
                </td>
                <td>
                   <p>
                   The application starts up and declares the environment to be a master
                   without calling for an election. It is an error for more
                   than one environment to start up using this flag, or for
                   an environment
                   to use this flag when a master already exists. 
                   </p>
                   <p>
                           Note that no replication group should
                           <span class="emphasis"><em>ever</em></span> operate with more than
                           one master.
                   </p>
                   <p>
                           In the event that a environment attempts to become a
                           master when a master already exists, the
                           replication code will resolve the problem by
                           holding an election. Note, however, that there
                           is always a possibility of data loss in the face
                           of duplicate masters, because once a master is
                           selected, the environment that loses the election will
                           have to roll back any transactions committed
                           until it is in sync with the "real" master.
                   </p>

</td>
              </tr>
              <tr>
                <td>
                  <code class="literal">DB_REP_CLIENT</code>
                </td>
                <td>
                          <p>
                                  The application starts up and declares
                                  the environment to be a replica without calling for
                                  an election. Note that the environment
                                  can still become a master if a subsequent
                                  application starts up, calls for an
                                  election, and this environment is elected
                                  master.
                          </p>
                  </td>
              </tr>
              <tr>
                <td>
                  <code class="literal">DB_REP_ELECTION</code>
                </td>
                <td>
                          <p>
                          As described above, the application starts up,
                          looks for a master, and if one is not found calls
                          for an election.
                          </p>
                  </td>
              </tr>
            </tbody>
          </table>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="thread_count"></a>Selecting the Number of Threads</h3>
            </div>
          </div>
        </div>
        <p>
                    Under the hood, the Replication Manager is threaded and you can
                    control the number of threads used to process messages received from
                    other replicas. The threads that the Replication Manager uses are:
            </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                                    Incoming message thread. This thread
                                    receives messages from the site's
                                    socket and passes those messages to
                                    message processing threads (see below)
                                    for handling.
                            </p>
            </li>
            <li>
              <p>
                                    Outgoing message thread. Outgoing
                                    messages are sent from whatever thread
                                    performed a write to the database(s).
                                    That is, the thread that called, for
                                    example, 
                                    <code class="methodname">DB-&gt;put()</code>
                                    
                                    
                                    is the thread that writes replication messages
                                    about that fact to the socket. 
                            </p>
              <p>
                                    Note that if this write activity would
                                    cause the thread to be blocked due to
                                    some condition on the socket, the Replication Manager
                                    will hand the outgoing message to the
                                    incoming message thread, and it will
                                    then write the message to the socket.
                                    This prevents your database write
                                    threads from blocking due to abnormal
                                    network I/O conditions.
                            </p>
            </li>
            <li>
              <p>
                                    Message processing threads are
                                    responsible for parsing and then
                                    responding to incoming replication
                                    messages. Typically, a response will
                                    include write activity to your
                                    database(s), so these threads can be
                                    busy performing disk I/O.
                            </p>
            </li>
          </ul>
        </div>
        <p>
                    Of these threads, the only ones that you have any
                    configuration control over are the message processing
                    threads. In this case, you can determine how many
                    of these threads you want to run.
            </p>
        <p>
                    It is always a bit of an art to decide on a thread count,
                    but the short answer is you probably do not need more
                    than three threads here, and it is likely that one will
                    suffice. That said, the best thing to do is set your
                    thread count to a fairly low number and then increase
                    it if it appears that your application will benefit
                    from the additional threads.
            </p>
      </div>
    </div>
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