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|
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<div class="libver">
<p>Library Version 11.2.5.3</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">Example Processing Loop</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="processingloop.html">Prev</a> </td>
<th width="60%" align="center">Chapter 4. Replica versus Master Processes</th>
<td width="20%" align="right"> <a accesskey="n" href="addfeatures.html">Next</a></td>
</tr>
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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="exampledoloop"></a>Example Processing Loop</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="exampledoloop.html#runningit">Running It</a>
</span>
</dt>
</dl>
</div>
<p>
In this section we take the example
processing loop that we presented in the
previous section and we flesh it out to
provide a more complete example. We do this
by updating the
<code class="function">doloop()</code>
function that our original transaction
application used
<span>(see <a class="xref" href="simpleprogramlisting.html#doloop_c" title="Function: doloop()">Function: doloop()</a>)</span>
to fully support our replicated application.
</p>
<p>
In the following example code, code that we
add to the original example is presented in
<strong class="userinput"><code>bold</code></strong>.
</p>
<p>
To begin, we include a new header file into
our application so that we can check for the
<code class="literal">ENOENT</code> return value later
in our processing loop. We also define our
<code class="literal">APP_DATA</code>
structure, and we define a
<code class="literal">sleeptime</code> value.
<span>
Finally, we
add a new forward declaration for our event
callback.
</span>
</p>
<pre class="programlisting">/*
* File: ex_rep_gsg_repmgr.c
*/
#include <stdlib.h>
#include <string.h>
<strong class="userinput"><code>#include <errno.h></code></strong>
#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"
<strong class="userinput"><code>#define SLEEPTIME 3</code></strong>
const char *progname = "ex_rep_gsg_repmgr";
<strong class="userinput"><code>typedef struct {
int is_master;
} APP_DATA;</code></strong>
int create_env(const char *, DB_ENV **);
int env_init(DB_ENV *, const char *);
int doloop (DB_ENV *);
static int print_stocks(DBC *);
<strong class="userinput"><code>void *event_callback(DB_ENV *, u_int32_t, void *);</code></strong> </pre>
<p>
In our <code class="function">main()</code> function, most of what we
have to add to it is some new variable declarations and
initializations:
</p>
<pre class="programlisting">int
main(int argc, char *argv[])
{
DB_ENV *dbenv;
DB_SITE *dbsite;
extern char *optarg;
const char *home;
char ch, *host, *portstr;
int ret, local_is_set, is_group_creator;
u_int32_t port;
<strong class="userinput"><code>/* Used to track whether this is a replica or a master */
APP_DATA my_app_data;
my_app_data.is_master = 0; /* Assume that we start as a replica */</code></strong>
dbenv = NULL;
ret = local_is_set = is_group_creator = 0;
home = NULL;</pre>
<p>
The rest of our <code class="function">main()</code> function
is unchanged, except that we make our
<code class="literal">APP_DATA</code> structure available through our
environment handle's <code class="methodname">app_private</code>
field:
</p>
<pre class="programlisting">
if ((ret = create_env(progname, &dbenv)) != 0)
goto err;
<strong class="userinput"><code>/* Make APP_DATA available through the environment handle */
dbenv->app_private = &my_app_data;</code></strong>
/* Default priority is 100 */
dbenv->rep_set_priority(dbenv, 100);
/* Permanent messages require at least one ack */
dbenv->repmgr_set_ack_policy(dbenv, DB_REPMGR_ACKS_ONE);
/* Give 500 microseconds to receive the ack */
dbenv->rep_set_timeout(dbenv, DB_REP_ACK_TIMEOUT, 500);
while ((ch = getopt(argc, argv, "h:l:L:p:r:")) != EOF)
switch (ch) {
case 'h':
home = optarg;
break;
/* 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 ((ret = dbenv->repmgr_site(dbenv, host, port, &dbsite
0)) != 0 ) {
fprintf(stderr,
"Could not set local address %s.\n", host);
goto err;
}
dbsite->set_config(dbsite, DB_LOCAL_SITE, 1);
if (is_group_creator)
dbsite->set_config(dbsite, DB_GROUP_CREATOR, 1);
if ((ret = dbsite->close(dbsite)) != 0) {
dbenv->(dbenv, ret, "DB_SITE->close");
goto err;
}
local_is_set = 1;
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_site(dbenv, host, port, &dbsite,
0)) != 0) {
fprintf(stderr,
"Could not add site %s.\n", host);
goto err;
}
dbenv->set_config(dbsite, DB_BOOTSTRAP_HELPER, 1);
if ((dbenv->close(dbsite)) != 0) {
dbenv->err(dbenv, ret, "DB_SITE->close");
goto err;
}
break;
case '?':
default:
usage();
}
/* Error check command line. */
if (home == NULL || !local_is_set)
usage();
if ((ret = env_init(dbenv, home)) != 0)
goto err;
if ((ret = dbenv->repmgr_start(dbenv, 3, DB_REP_ELECTION)) != 0)
goto err;
/* Sleep to give ourselves time to find a master. */
sleep(5);
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>
Having updated our <code class="function">main()</code>, we must also
update our <code class="function">create_env()</code> function to
register our <code class="function">event_callback</code> callback.
Notice that our <code class="function">env_init()</code> function, which is
responsible for actually opening our environment handle, is
unchanged:
</p>
<pre class="programlisting">int
create_env(const char *progname, DB_ENV **dbenvp)
{
DB_ENV *dbenv;
int ret;
if ((ret = db_env_create(&dbenv, 0)) != 0) {
fprintf(stderr, "can't create env handle: %s\n",
db_strerror(ret));
return (ret);
}
dbenv->set_errfile(dbenv, stderr);
dbenv->set_errpfx(dbenv, progname);
<strong class="userinput"><code>(void)dbenv->set_event_notify(dbenv, event_callback);</code></strong>
*dbenvp = dbenv;
return (0);
}
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 |
DB_INIT_REP |
DB_INIT_TXN |
DB_RECOVER |
DB_THREAD;
if ((ret = dbenv->open(dbenv, home, flags, 0)) != 0)
dbenv->err(dbenv, ret, "can't open environment");
return (ret);
}</pre>
<p>
That done, we need to implement our
<code class="function">event_callback()</code> callback. Note that what we use
here is no different from the callback that we described in
the previous section. However, for the sake of completeness we
provide the implementation here again.
</p>
<pre class="programlisting">
<strong class="userinput">
<code>/*
* A callback used to determine whether the local environment is a
* replica or a master. This is called by the Replication Manager
* when the local replication environment changes state.
*/
void *
event_callback(DB_ENV *dbenv, u_int32_t which, void *info)
{
APP_DATA *app = dbenv->app_private;
info = NULL; /* Currently unused. */
switch (which) {
case DB_EVENT_REP_MASTER:
app->is_master = 1;
break;
case DB_EVENT_REP_CLIENT:
app->is_master = 0;
break;
case DB_EVENT_REP_STARTUPDONE: /* fallthrough */
case DB_EVENT_REP_NEWMASTER:
/* Ignore. */
break;
default:
dbenv->errx(dbenv, "ignoring event %d", which);
}
}</code>
</strong>
</pre>
<p>
That done, we need to update our
<code class="function">doloop()</code>
<span>function. This is the place where we most
heavily modify our application.</span>
</p>
<p>
We begin by introducing <code class="literal">APP_DATA</code> to the
function:
</p>
<pre class="programlisting">/*
* Provides the main data processing function for our application.
* This function provides a command line prompt to which the user
* can provide a ticker string and a stock price. Once a value is
* entered to the application, the application writes the value to
* the database and then displays the entire database.
*/
#define BUFSIZE 1024
int
doloop(DB_ENV *dbenv)
{
DB *dbp;
<strong class="userinput"><code>APP_DATA *app_data;</code></strong>
DBT key, data;
char buf[BUFSIZE], *rbuf;
int ret;
u_int32_t flags;
dbp = NULL;
ret = 0;
memset(&key, 0, sizeof(key));
memset(&data, 0, sizeof(data));
<strong class="userinput"><code>app_data = dbenv->app_private;</code></strong></pre>
<p>
Next we begin to modify our main loop.
To start, upon entering the loop we create the database handle and configure it as
normal. But we also have to decide what flags we will use for
the open. Again, it depends on whether we are a replica or a
master.
</p>
<pre class="programlisting"> for (;;) {
if (dbp == NULL) {
if ((ret = db_create(&dbp, dbenv, 0)) != 0)
return (ret);
<strong class="userinput"><code>flags = DB_AUTO_COMMIT;
if (app_data->is_master)
flags |= DB_CREATE;</code></strong> </pre>
<p>
When we open the database, we modify our error handling to
account for the case where the database does not yet exist. This can
happen if our code is running as a replica and the Replication Manager has not
yet had a chance to create the databases for us. Recall that replicas never
write to their own databases directly, and so they cannot
create databases on their own.
</p>
<p>
If we detect that the database does not yet exist, we simply
close the database handle, sleep for a short period of time
and then continue processing. This gives the Replication Manager a chance to
create the database so that our replica can continue
operations.
</p>
<pre class="programlisting"> if ((ret = dbp->open(dbp,
NULL, DATABASE, NULL, DB_BTREE, flags, 0)) != 0) {
<strong class="userinput"><code>if (ret == ENOENT) {
printf(
"No stock database yet available.\n");
if ((ret = dbp->close(dbp, 0)) != 0) {
dbenv->err(dbenv, ret,
"DB->close");
goto err;
}
dbp = NULL;
sleep(SLEEPTIME);
continue;
}</code></strong>
dbenv->err(dbenv, ret, "DB->open");
goto err;
}
} </pre>
<p>
Next we modify our prompt, so that if the local process is running
as a replica, we can tell from the shell that the prompt is for a
read-only process.
</p>
<pre class="programlisting"> printf("QUOTESERVER<strong class="userinput"><code>%s</code></strong>> ",
<strong class="userinput"><code>app_data->is_master ? "" : " (read-only)");</code></strong>
fflush(stdout); </pre>
<p>
When we collect data from the prompt, there is a case that says
if no data is entered then show the entire stocks database.
This display is performed by our
<code class="function">print_stocks()</code>
<span>function</span>
(which has not
required a modification since we first introduced it in
<a class="xref" href="simpleprogramlisting.html#printstocks_c" title="Function: print_stocks()">
<span>Function: print_stocks()</span>
</a>).
</p>
<p>
When we call
<span><code class="function">print_stocks()</code>, </span>
we check for a dead replication handle. Dead
replication handles happen whenever a replication election
results in a previously committed transaction becoming
invalid. This is an error scenario caused by a new master having a
slightly older version of the data than the original
master and so all replicas must modify their database(s) to
reflect that of the new master. In this situation, some
number of previously committed transactions may have to be
unrolled. From the replica's perspective, the database
handles should all be closed and then opened again.
</p>
<pre class="programlisting"> if (fgets(buf, sizeof(buf), stdin) == NULL)
break;
if (strtok(&buf[0], " \t\n") == NULL) {
switch ((ret = print_stocks(dbp))) {
case 0:
continue;
<strong class="userinput"><code>case DB_REP_HANDLE_DEAD:
(void)dbp->close(dbp, DB_NOSYNC);
dbp = NULL;
dbenv->errx(dbenv, "Got a dead replication handle");
continue; </code></strong>
default:
dbp->err(dbp, ret, "Error traversing data");
goto err;
}
}
rbuf = strtok(NULL, " \t\n");
if (rbuf == NULL || rbuf[0] == '\0') {
if (strncmp(buf, "exit", 4) == 0 ||
strncmp(buf, "quit", 4) == 0)
break;
dbenv->errx(dbenv, "Format: TICKER VALUE");
continue;
}</pre>
<p>
That done, we need to add a little error checking to our
command prompt to make sure the user is not attempting to
modify the database at a replica. Remember, replicas must never
modify their local databases on their own. This guards against
that happening due to user input at the prompt.
</p>
<pre class="programlisting"> <strong class="userinput"><code>if (!app_data->is_master) {
dbenv->errx(dbenv, "Can't update at client");
continue;
}</code></strong>
key.data = buf;
key.size = (u_int32_t)strlen(buf);
data.data = rbuf;
data.size = (u_int32_t)strlen(rbuf);
if ((ret = dbp->put(dbp,
NULL, &key, &data, 0)) != 0) {
dbp->err(dbp, ret, "DB->put");
goto err;
}
}
err: if (dbp != NULL)
(void)dbp->close(dbp, DB_NOSYNC);
return (ret);
} </pre>
<p>
With that completed, we are all done updating our application
for replication.
The only remaining
<span>function, <code class="function">print_stocks()</code>,</span>
is unmodified from when we
originally introduced it. For details on that function, see
<a class="xref" href="simpleprogramlisting.html#printstocks_c" title="Function: print_stocks()">
<span>Function: print_stocks()</span>
</a>.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="runningit"></a>Running It</h3>
</div>
</div>
</div>
<p>
To run our replicated application, we need to make
sure each participating environment has its own unique
home directory. We can do this by running
each site on a separate networked machine, but that
is not strictly necessary; multiple instances of this
code can run on the same machine provided the
environment home restriction is observed.
</p>
<p>
To run a process, make sure the environment home exists and
then start the process using the <code class="literal">-h</code>
option to specify that directory. You must also use the
<code class="literal">-l</code> or <code class="literal">-L</code> option to
identify the local host and port that this process will use
to listen for replication messages (-L means that this is a
group creator), and the <code class="literal">-r</code> option to
identify the other processes in the replication group.
Finally, use the <code class="literal">-p</code> option to specify a
priority. The process that you designate to have the
highest priority will become the master.
</p>
<pre class="programlisting">> mkdir env1
> ./ex_rep_gsg_repmgr -h env1 -L localhost:8080 -p 10
No stock database yet available.
No stock database yet available. </pre>
<p>
Now, start another process. This time, change the environment
home to something else, use the <code class="literal">-l</code> flag to at
least change the port number the process is listening on, and
use the <code class="literal">-r</code> option to identify the host and
port of the other replication process:
</p>
<pre class="programlisting">> mkdir env2
> ./ex_rep_gsg_repmgr -h env2 -l localhost:8081 \
-r localhost:8080 -p 20</pre>
<p>
After a short pause, the second process should display the master
prompt:
</p>
<pre class="programlisting">
QUOTESERVER > </pre>
<p>
And the first process should
display the read-only prompt:
</p>
<pre class="programlisting">
QUOTESERVER (read-only)> </pre>
<p>
Now go to the master process and give it a couple of stocks and stock
prices:
</p>
<pre class="programlisting">QUOTESERVER> FAKECO 9.87
QUOTESERVER> NOINC .23
QUOTESERVER> </pre>
<p>
Then, go to the replica and hit <strong class="userinput"><code>return</code></strong> at the prompt to
see the new values:
</p>
<pre class="programlisting">QUOTESERVER (read-only)>
Symbol Price
====== =====
FAKECO 9.87
NOINC .23
QUOTESERVER (read-only)> </pre>
<p>
Doing the same at the master results in the same thing:
</p>
<pre class="programlisting">QUOTESERVER>
Symbol Price
====== =====
FAKECO 9.87
NOINC .23
QUOTESERVER> </pre>
<p>
You can change a stock by simply entering the stock value and
new price at the master's prompt:
</p>
<pre class="programlisting">QUOTESERVER> FAKECO 10.01
QUOTESERVER> </pre>
<p>
Then, go to either the master or the replica to see the updated
database. On the master:
</p>
<pre class="programlisting">QUOTESERVER>
Symbol Price
====== =====
FAKECO 10.01
NOINC .23
QUOTESERVER> </pre>
<p>
And on the replica:
</p>
<pre class="programlisting">QUOTESERVER (read-only)>
Symbol Price
====== =====
FAKECO 10.01
NOINC .23
QUOTESERVER (read-only)> </pre>
<p>
Finally, to quit the applications, simply type
<code class="literal">quit</code> at both prompts. On the replica:
</p>
<pre class="programlisting">QUOTESERVER (read-only)> quit
> </pre>
<p>
And on the master as well:
</p>
<pre class="programlisting">QUOTESERVER> quit
> </pre>
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