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<h2 class="title" style="clear: both"><a id="rep_ex_chan"></a>Ex_rep_chan: a Replication Manager channel example</h2>
</div>
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<p>
Ex_rep_chan, found in the
<code class="filename">examples/c/ex_rep_chan</code> subdirectory
of the Berkeley DB distribution, is a simple but complete
demonstration of a replicated application that uses the
Replication Manager feature of channels to perform write
forwarding. The application is a mock stock ticker. Although
similar to the ex_rep_mgr example program, this example
differs in that it provides an example of using Replication
Manager message channels. Any site accepts a command to write
data to the database. If the site is a client, then, using the
channels feature, the application forwards the request to the
master site. If the site is a master then the request is
automatically handled locally. You can read and write stock
values at any site without needing to know what site is
currently the master.
</p>
<p>
The set of supported commands can be viewed with either the
<span class="bold"><strong>help</strong></span> or the <span class="bold"><strong>?</strong></span> command. Several commands work
with key/data pairs where the key is a stock symbol and the
data is its value.
</p>
<p>
The command to retrieve and print the current site's
database contents is <span class="bold"><strong>print</strong></span> or
simply an empty input line. To read the contents of the
master's database from any site use the <span class="bold"><strong>get key key ...</strong></span>
command. That command will
forward the read request to the master if necessary and return
the key/data pairs for all given keys.
</p>
<p>
There are two commands to put data into the database. Both
commands take one or more key/data pairs, all of which are
written into the database in a single transaction at the
master site. The <span class="bold"><strong>put</strong></span> command
sends the data to the master site, and simply waits for a
status response. The <span class="bold"><strong>put_sync</strong></span>
command sends the data to the master site, and uses a
transaction token returned by the master to wait for the
contents of that put to be available on the local site. This
serves as a demonstration of the <a class="link" href="rep_ryw.html" title="Read your writes consistency">read
your writes</a> consistency feature.
</p>
<p>
The Makefile created by the standard UNIX configuration
will build the ex_rep_chan example on most platforms. Enter
"make ex_rep_chan" to build it.
</p>
<p>
The synopsis for the program is as follows:
</p>
<p>
<code class="literal">ex_rep_chan <span class="bold"><strong>-h home</strong></span>
<span class="bold"><strong>-l host:port</strong></span> [<span class="bold"><strong>-MC</strong></span>] [<span class="bold"><strong>-r
host:port</strong></span>] [<span class="bold"><strong>-R
host:port</strong></span>] [<span class="bold"><strong>-p
priority</strong></span>] [<span class="bold"><strong>-v</strong></span>]</code>
</p>
<p>
The options are as follows:
</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">
<span class="bold"><strong>-h</strong></span>
</span>
</dt>
<dd>
Specify a home directory for the database
environment.
</dd>
<dt>
<span class="term">
<span class="bold"><strong>-l</strong></span>
</span>
</dt>
<dd>
Listen on local host "host" at port "port" for
incoming connections.
</dd>
<dt>
<span class="term">
<span class="bold"><strong>-M</strong></span>
</span>
</dt>
<dd>
Configure this process as a master.
</dd>
<dt>
<span class="term">
<span class="bold"><strong>-C</strong></span>
</span>
</dt>
<dd>
Configure this process as a client.
</dd>
<dt>
<span class="term">
<span class="bold"><strong>-r</strong></span>
</span>
</dt>
<dd>
Identifies the helper site used for joining the
group.
</dd>
<dt>
<span class="term">
<span class="bold"><strong>-R</strong></span>
</span>
</dt>
<dd>
Identifies a remote peer to be used for joining
the group. This peer is used for syncing purposes. See
<a class="xref" href="rep_mastersync.html#rep_c2c_sync" title="Client-to-client synchronization">Client-to-client synchronization</a> for more
information.
</dd>
<dt>
<span class="term">
<span class="bold"><strong>-p</strong></span>
</span>
</dt>
<dd>
Set the election priority. See <a class="xref" href="rep_elect.html" title="Elections">Elections</a>
for more information.
</dd>
<dt>
<span class="term">
<span class="bold"><strong>-v</strong></span>
</span>
</dt>
<dd>
Indicates that additional informational and
debugging output should be enabled.
</dd>
</dl>
</div>
<p>
A typical ex_rep_chan session begins with a command such as
the following, to start a master:
</p>
<pre class="programlisting">ex_rep_chan -M -h DIR1 -l localhost:30100</pre>
<p>
and several clients:
</p>
<pre class="programlisting">ex_rep_chan -C -h DIR2 -l localhost:30101 -r localhost:30100
ex_rep_chan -C -h DIR3 -l localhost:30102 -r localhost:30100
ex_rep_chan -C -h DIR4 -l localhost:30103 -r localhost:30100</pre>
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