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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<head>
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<title>Using Replication Manager message channels</title>
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<link rel="up" href="rep.html" title="Chapter 12. Berkeley DB Replication" />
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<div xmlns="" class="navheader">
<div class="libver">
<p>Library Version 12.1.6.1</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">Using Replication Manager message channels</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="rep_clock_skew.html">Prev</a> </td>
<th width="60%" align="center">Chapter 12. Berkeley DB Replication </th>
<td width="20%" align="right"> <a accesskey="n" href="rep_twosite.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="repmgr_channels"></a>Using Replication Manager message channels</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="repmgr_channels.html#dbchannel_class">DB_CHANNEL</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="repmgr_channels.html#dbchannel_send">Sending messages over a message channel</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="repmgr_channels.html#dbchannel_receive">Receiving messages</a>
</span>
</dt>
</dl>
</div>
<p>
The various sites comprising a replication group frequently
need to communicate with one another. Mostly, these messages
are handled for you internally by the Replication Manager.
However, your application may have a requirement to pass
messages beyond what the Replication Manager requires in order
to satisfy its own internal workings.
</p>
<p>
For this reason, you can access and use the Replication
Manager's internal message channels. You do this by using the
<code class="literal">DB_CHANNEL</code> class, and by implementing a
message handling function on each of your sites.
</p>
<p>
Note that an example of using Replication Manager message
channels is available in the distribution. See <a class="xref" href="rep_ex_chan.html" title="Ex_rep_chan: a Replication Manager channel example">Ex_rep_chan: a Replication Manager channel example</a> for
more information.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="dbchannel_class"></a>DB_CHANNEL</h3>
</div>
</div>
</div>
<p>
The <code class="literal">DB_CHANNEL</code> class provides a
series of methods which allow you to send messages to the
other sites in your replication group. You create a
<code class="literal">DB_CHANNEL</code> handle using the
<a href="../api_reference/C/repmgr_channel.html" class="olink">DB_ENV->repmgr_channel()</a> method. When you are done with the
handle, close it using the <a href="../api_reference/C/dbchannel_close.html" class="olink">DB_CHANNEL->close()</a> method. A
closed handle must never be accessed again. Note that all
channel handles should be closed before the associated
environment handle is closed. Also, allow all message
operations to complete on the channel before closing the
handle.
</p>
<p>
When you create a <code class="literal">DB_CHANNEL</code> handle,
you indicate what channel you want to use. Possibilities
are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
The numerical env ID of a remote site in the
replication group.
</p>
</li>
<li>
<p>
<code class="literal">DB_EID_MASTER</code>
</p>
<p>
Messages sent on this channel are sent only to
the master site. Note that messages are always
sent to the current master, even if the master has
changed since the channel was opened.
</p>
<p>
If the local site is the master, then sending
messages on this channel will result in the local
site receiving those messages echoed back to
itself.
</p>
</li>
</ul>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="dbchannel_send"></a>Sending messages over a message channel</h3>
</div>
</div>
</div>
<p>
You can send any message you want over a message
channel. The message can be as simple as a character
string and as complex as a large data structure. However,
before you can send the message, you must encapsulate it
within one or more <a href="../api_reference/C/dbt.html" class="olink">DBT</a>s. This means <a class="link" href="am_misc_struct.html" title="Storing C/C++ structures/objects">marshaling the message</a>
if it is contained within a complex data structure. </p>
<p>
The methods that you use to send messages all accept an
array of <a href="../api_reference/C/dbt.html" class="olink">DBT</a>s. This means that in most circumstances it
is perfectly acceptable to send multi-part messages.
</p>
<p>
Messages may be sent either asynchronously or
synchronously. To send a message asynchronously, use the
<a href="../api_reference/C/dbchannel_send_msg.html" class="olink">DB_CHANNEL->send_msg()</a> method. This method sends its message
and then immediately returns without waiting for any sort
of a response.
</p>
<p>
To send a message synchronously, use the
<a href="../api_reference/C/dbchannel_send_request.html" class="olink">DB_CHANNEL->send_request()</a> method. This method blocks until
it receives a response from the site to which it sent the
message (or until a timeout threshold is reached).
</p>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="dbchannel_respond"></a>Message Responses</h4>
</div>
</div>
</div>
<p>
Message responses are required if a message is sent
on a channel using the <a href="../api_reference/C/dbchannel_send_request.html" class="olink">DB_CHANNEL->send_request()</a>
method. That method accepts the address of a single
<a href="../api_reference/C/dbt.html" class="olink">DBT</a> which is used to receive the response from the
remote site.
</p>
<p>
Message responses are encapsulated in a single
<a href="../api_reference/C/dbt.html" class="olink">DBT</a>. The response can be anything from a complex
data structure, to a string, to a simple type, to no
information at all. In the latter case, receipt of the
<a href="../api_reference/C/dbt.html" class="olink">DBT</a> is sufficient to indicate that the request was
received at the remote site.
</p>
<p> Responses are sent back from the remote system
using its message handling function. Usually that
function calls <a href="../api_reference/C/dbchannel_send_msg.html" class="olink">DB_CHANNEL->send_msg()</a> to send a single
response. </p>
<p>
The response must be contained in a single <a href="../api_reference/C/dbt.html" class="olink">DBT</a>.
If a multi-part response is required by the
application, you can configure the response <a href="../api_reference/C/dbt.html" class="olink">DBT</a> that
you provide to <a href="../api_reference/C/dbchannel_send_request.html" class="olink">DB_CHANNEL->send_request()</a> for <a class="link" href="am_misc_bulk.html" title="Retrieving and updating records in bulk">bulk operations</a>.
</p>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="dbchannel_receive"></a>Receiving messages</h3>
</div>
</div>
</div>
<p>
Messages received at a remote site are handled using a
callback function. This function is configured for the
local environment using the <a href="../api_reference/C/repmgr_msg_dispatch.html" class="olink">DB_ENV->repmgr_msg_dispatch()</a>
method. For best results, the message dispatch function
should be configured for the local environment before
replication is started. In this way, you do not run the
risk of missing messages sent after replication has
started but before the message dispatch function is
configured for the environment.
</p>
<p>
The callback configured by <a href="../api_reference/C/repmgr_msg_dispatch.html" class="olink">DB_ENV->repmgr_msg_dispatch()</a>
accepts four parameters of note:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
A response channel. This is the channel the
function will use to respond to the message, if a
response is required. To respond to the message,
the function uses the <a href="../api_reference/C/dbchannel_send_msg.html" class="olink">DB_CHANNEL->send_msg()</a> method.
</p>
</li>
<li>
<p>
An array of <a href="../api_reference/C/dbt.html" class="olink">DBT</a>s. These hold the message that
this function must handle.
</p>
</li>
<li>
<p>
A numerical value that indicates how many
elements the previously described array holds.
</p>
</li>
<li>
<p>
A flag that indicates whether the message
requires a response. If the flag is set to
<code class="literal">DB_REPMGR_NEED_RESPONSE</code>,
then the function should send a single <a href="../api_reference/C/dbt.html" class="olink">DBT</a> in
response using the channel provided to this
function, and the <a href="../api_reference/C/dbchannel_send_msg.html" class="olink">DB_CHANNEL->send_msg()</a> method.
</p>
</li>
</ul>
</div>
<p>
For an example of using this callback, see the
<code class="literal">operation_dispatch()</code> function,
which is available with the <a class="link" href="rep_ex_chan.html" title="Ex_rep_chan: a Replication Manager channel example">
ex_rep_chan example</a> in your product
distribution.
</p>
</div>
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</td>
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</tr>
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<a accesskey="h" href="index.html">Home</a>
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