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<h2 class="title" style="clear: both"><a id="processingloop"></a>Processing Loop</h2>
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<p>
Typically the central part of any replication application
is some sort of a continuous loop that constantly
checks the state of the environment (whether it is a
replica or a master), opens and/or closes the
databases as is necessary, and performs other useful
work. A loop such as this one must of necessity
take special care to know whether it is operating
on a master or a replica environment because all of its
activities are dependent upon that state.
</p>
<p>
The flow of activities through the loop will
generally be as follows:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Check whether the environment has
changed state. If it has, you
might want to reopen your
database handles, especially if
you opened your replica's
database handles as read-only.
In this case, you might need to
reopen them as read-write.
However, if you always open your
database handles as read-write,
then it is not automatically necessary to
reopen the databases due to a
state change. Instead, you
could check for a
<span>
<tt class="literal">DB_REP_HANDLE_DEAD</tt>
return code
</span>
when you use your
database handle(s). If you see
this, then you need to reopen
your database handle(s).
</p>
</li>
<li>
<p>
If the databases are closed,
create new database handles,
configure the handle as is
appropriate, and then open the
databases. Note that handle
configuration will be different,
depending on whether the handle
is opened as a replica or a
master. At a minimum, the master
should be opened with database
creation privileges, whereas the
replica does not need to be. You
must also open the master such
that its databases are
read-write. You
<span class="emphasis"><em>can</em></span> open
replicas with read-only
databases, so long as you are
prepared to closed and the reopen
the handle in the event the
client becomes a master.
</p>
<p>
Also, note that if the local
environment
is a replica, then it is possible
that databases do not currently
exist. In this case, the database
open attempts will fail. Your
code will have to take this
corner case into account
(described below).
</p>
</li>
<li>
<p>
Once the databases are opened,
check to see if the local
environment is a
master. If it is, do whatever it is
a master should do for your
application.
</p>
<p>
Remember that the code for your
master should include some way
for you to tell the master
to exit gracefully.
</p>
</li>
<li>
<p>
If the local environment is not a
master, then do whatever it is
your replica environments should do.
Again, like the code for your
master environments, you should provide
a way for your replicas to exit
the processing loop gracefully.
</p>
</li>
</ol>
</div>
<p>
The following code fragment illustrates
these points (note that we fill out this
fragment with a working example
next in this chapter):
</p>
<pre class="programlisting">/* loop to manage replication activities */
Db *dbp;
int ret;
APP_DATA *app_data;
u_int32_t flags;
dbp = NULL;
ret = 0;
/*
* Remember that for this to work, an APP_DATA struct would have first
* had to been set to the environment handle's app_private data
* member. (dbenv is presumable declared and opened in another part of
* the code.)
*/
app_data = dbenv->get_app_private();
/*
* Infinite loop. We exit depending on how the master and replica code
* is written.
*/
for (;;) {
/* If dbp is not opened, we need to open it. */
if (dbp == NULL) {
/*
* Create the handle and then configure it. Before you open
* it, you have to decide what open flags to use:
*/
flags = DB_AUTO_COMMIT;
if (app_data->is_master)
flags |= DB_CREATE
/*
* Now you can open your database handle, passing to it the
* flags selected above.
*
* One thing to watch out for is a case where the databases
* you are trying to open do not yet exist. This can happen
* for replicas where the databases are being opened
* read-only. If this happens, ENOENT is returned by the
* open() call.
*/
try {
dbp->open(NULL, DATABASE, NULL, DB_BTREE,
app_data->is_master ? DB_CREATE | DB_AUTO_COMMIT :
DB_AUTO_COMMIT, 0);
} catch(DbException dbe) {
if (dbe.get_errno() == ENOENT) {
cout << "No stock db available yet - retrying." << endl;
try {
dbp->close(0);
} catch (DbException dbe2) {
cout << "Unexpected error closing after failed" <<
" open, message: " << dbe2.what() << endl;
dbp = NULL;
goto err;
}
dbp = NULL;
sleep(SLEEPTIME);
continue;
} else {
dbenv.err(ret, "DB->open");
throw dbe;
}
}
}
/*
* Now that the databases have been opened, continue with general
* processing, depending on whether we are a master or a replica.
*/
if (app_data->is_master) {
/*
* Do master stuff here. Don't forget to include a way to
* gracefully exit the loop. */
*/
} else {
/*
* Do replica stuff here. As is the case with the master
* code, be sure to include a way to gracefully exit the
* loop.
*/
}
} </pre>
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