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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<th colspan="3" align="center">Adding the Replication Manager to
<span>ex_rep_gsg_simple</span>
</th>
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<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>
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="repmgr_init_example_c"></a>Adding the Replication Manager to
<span>ex_rep_gsg_simple</span>
</h2>
</div>
</div>
</div>
<p>
We now use the methods described above to add partial
support to the
<span>ex_rep_gsg_simple</span>
example that we presented in
<a class="xref" href="txnapp.html" title="Chapter 2. Transactional Application">Transactional Application</a>.
That is, in this section we will:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
Enhance our command line options to
accept information of interest to a
replicated application.
</p>
</li>
<li>
<p>
Configure our environment handle to use
replication and the Replication Manager.
</p>
</li>
<li>
<p>
Minimally configure the Replication Manager.
</p>
</li>
<li>
<p>
Start replication.
</p>
</li>
</ul>
</div>
<p>
Note that when we are done with this section, we will be
only partially ready to run the application. Some critical
pieces will be missing; specifically, we will not yet be
handling the differences between a master and a
replica. (We do that in the next chapter).
</p>
<p>
Also, note that in the following code fragments, additions
and changes to the code are marked in <strong class="userinput"><code>bold</code></strong>.
</p>
<p>
To begin, we copy the
<span>ex_rep_gsg_simple</span>
code to a new file called
<code class="literal">ex_rep_gsg_repmgr.c</code>.
<span>
We then make the corresponding change to the program name.
</span>
</p>
<pre class="programlisting">/*
<strong class="userinput"><code>* File: ex_rep_gsg_repmgr.c</code></strong>
*/
#include <stdlib.h>
#include <string.h>
#ifndef _WIN32
#include <unistd.h>
#endif
#include <db.h>
#ifdef _WIN32
extern int getopt(int, char * const *, const char *);
#endif
#define CACHESIZE (10 * 1024 * 1024)
#define DATABASE "quote.db"
const char *progname = <strong class="userinput"><code>"ex_rep_gsg_repmgr";</code></strong>
int create_env(const char *, DB_ENV **);
int env_init(DB_ENV *, const char *);
int doloop (DB_ENV *);
int print_stocks(DBC *); </pre>
<p>
Next we update our usage function. The application will continue to
accept the <code class="literal">-h</code> parameter so that we can identify
the environment home directory used by this application. However,
we also add the
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="literal">-l</code> parameter which allows us to
identify the host and port used by this application to
listen for replication messages.
</p>
</li>
<li>
<p>
<code class="literal">-r</code> parameter which allows us to
specify other replicas.
</p>
</li>
<li>
<p>
<code class="literal">-n</code> parameter which allows us to
identify the number of sites in this replication
group.
</p>
</li>
<li>
<p>
<code class="literal">-p</code> option, which is used to identify
this replica's priority (recall that the priority is
used as a tie breaker for elections)
</p>
</li>
</ul>
</div>
<pre class="programlisting">/* Usage function */
static void
usage()
{
fprintf(stderr, "usage: %s ", progname);
fprintf(stderr, "-h home <strong class="userinput"><code>-l host:port -n nsites</code></strong>\n");
<strong class="userinput"><code>fprintf(stderr, "\t\t[-r host:port][-p priority]\n");
fprintf(stderr, "where:\n");
fprintf(stderr, "\t-h identifies the environment home directory ");
fprintf(stderr, "(required).\n");
fprintf(stderr, "\t-l identifies the host and port used by this ");
fprintf(stderr, "site (required).\n");
fprintf(stderr, "\t-n identifies the number of sites in this ");
fprintf(stderr, "replication group (required).\n");
fprintf(stderr, "\t-r identifies another site participating in ");
fprintf(stderr, "this replication group\n");
fprintf(stderr, "\t-p identifies the election priority used by ");
fprintf(stderr, "this replica.\n");</code></strong>
exit(EXIT_FAILURE);
} </pre>
<p>
Now we can begin working on our <code class="literal">main()</code> function.
We begin by adding a couple of variables that we will use to
collect TCP/IP host and port information.
<span>
We also declare a couple
of flags that we use to make sure some required information is
provided to this application.
</span>
</p>
<pre class="programlisting">int
main(int argc, char *argv[])
{
extern char *optarg;
DB_ENV *dbenv;
const char *home;
char ch, <strong class="userinput"><code>*host, *portstr</code></strong>;
int ret, <strong class="userinput"><code>local_is_set, totalsites</code></strong>;
<strong class="userinput"><code>u_int32_t port</code></strong>;
dbenv = NULL;
ret = <strong class="userinput"><code>local_is_set = totalsites = </code></strong>0;
home = NULL; </pre>
<p>
At this time we can create our environment handle and configure it
exactly as we did for <code class="literal">simple_txn</code>.
The only thing that we will do differently here is that we will set a priority,
arbitrarily picked to be 100, so that we can be sure the environment has
a priority other than 0 (the default value). This ensures that the
environment can become a master via an election.
</p>
<pre class="programlisting"> if ((ret = create_env(progname, &dbenv)) != 0)
goto err;
/* Default priority is 100 */
<strong class="userinput"><code>dbenv->rep_set_priority(dbenv, 100);</code></strong> </pre>
<p>
Now we collect our command line arguments. As we do so, we will
configure host and port information as required, and we will
configure the application's election priority if necessary.
</p>
<pre class="programlisting"> /* Collect the command line options */
while ((ch = getopt(argc, argv, "h:<strong class="userinput"><code>l:n:p:r:</code></strong>")) != EOF)
switch (ch) {
case 'h':
home = optarg;
break;
<strong class="userinput"><code>/* Set the host and port used by this environment */
case 'l':
host = strtok(optarg, ":");
if ((portstr = strtok(NULL, ":")) == NULL) {
fprintf(stderr, "Bad host specification.\n");
goto err;
}
port = (unsigned short)atoi(portstr);
if (dbenv->repmgr_set_local_site(dbenv, host, port, 0)
!= 0) {
fprintf(stderr,
"Could not set local address %s.\n", host);
goto err;
}
local_is_set = 1;
break;
/* Set the number of sites in this replication group */
case 'n':
totalsites = atoi(optarg);
if ((ret = dbenv->rep_set_nsites(dbenv, totalsites))
!= 0)
dbenv->err(dbenv, ret, "set_nsites");
break;
/* Set this replica's election priority */
case 'p':
dbenv->rep_set_priority(dbenv, atoi(optarg));
break;
/* Identify another site in the replication group */
case 'r':
host = strtok(optarg, ":");
if ((portstr = strtok(NULL, ":")) == NULL) {
fprintf(stderr, "Bad host specification.\n");
goto err;
}
port = (unsigned short)atoi(portstr);
if (dbenv->repmgr_add_remote_site(dbenv, host, port,
NULL, 0) != 0) {
fprintf(stderr,
"Could not add site %s.\n", host);
goto err;
}
break;</code></strong>
case '?':
default:
usage();
}
/* Error check command line. */
if (home == NULL <strong class="userinput"><code>|| !local_is_set || !totalsites</code></strong>)
usage(); </pre>
<p>
Having done that, we can call <code class="function">env_init()</code>, which we use to
open our environment handle. Note that this function changes
slightly for this update (see below).
</p>
<pre class="programlisting"> if ((ret = env_init(dbenv, home)) != 0)
goto err; </pre>
<p>
Finally, we start replication before we go into the
<code class="function">doloop()</code> function (where we perform all our
database access).
</p>
<pre class="programlisting">
<strong class="userinput"><code>if ((ret = dbenv->repmgr_start(dbenv, 3, DB_REP_ELECTION)) != 0)
goto err; </code></strong>
if ((ret = doloop(dbenv)) != 0) {
dbenv->err(dbenv, ret, "Application failed");
goto err;
}
err: if (dbenv != NULL)
(void)dbenv->close(dbenv, 0);
return (ret);
} </pre>
<p>
<span>Beyond that, the rest of our application remains the same for
now, with the exception of the <code class="function">env_init()</code>
function, which we use to actually open our environment handle.
The flags </span>
we use to open the environment are slightly different for a
replicated application than they are for a non-replicated
application. Namely, replication requires the
<span>
<code class="literal">DB_INIT_REP</code> flag.
</span>
</p>
<p>
Also, because we are using the Replication Manager, we must prepare
our environment for threaded usage. For this reason, we also
need the <code class="literal">DB_THREAD</code> flag.
</p>
<pre class="programlisting">int
env_init(DB_ENV *dbenv, const char *home)
{
u_int32_t flags;
int ret;
(void)dbenv->set_cachesize(dbenv, 0, CACHESIZE, 0);
(void)dbenv->set_flags(dbenv, DB_TXN_NOSYNC, 1);
flags = DB_CREATE |
DB_INIT_LOCK |
DB_INIT_LOG |
DB_INIT_MPOOL |
<strong class="userinput"><code>DB_INIT_REP |</code></strong>
DB_INIT_TXN |
DB_RECOVER <strong class="userinput"><code>|
DB_THREAD;</code></strong>
if ((ret = dbenv->open(dbenv, home, flags, 0)) != 0)
dbenv->err(dbenv, ret, "can't open environment");
return (ret);
}</pre>
<p>
This completes our replication updates for the moment. We are not as
yet ready to actually run this program; there remains a few
critical pieces left to add to it. However, the work that we
performed in this section represents a solid foundation for the
remainder of our replication work.
</p>
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