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channel example</th>
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<div>
<h2 class="title" style="clear: both"><a id="rep_ex_chan"></a>Ex_rep_chan: a Replication Manager
channel example</h2>
</div>
</div>
</div>
<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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Distributed Transactions
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