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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Ex_rep_base: a TCP/IP based communication infrastructure</title>
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="rep.html" title="Chapter 12. Berkeley DB Replication" />
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<link rel="prev" href="rep_ex.html" title="Ex_rep: a replication example" />
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<p>Library Version 11.2.5.2</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Ex_rep_base: a TCP/IP based communication infrastructure</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="rep_ex.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 12.
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Berkeley DB Replication
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="rep_ex_rq.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="rep_ex_comm"></a>Ex_rep_base: a TCP/IP based communication infrastructure</h2>
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</div>
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</div>
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</div>
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<p>
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Base API applications must implement a
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communication infrastructure. The communication infrastructure
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consists of three parts: a way to map environment IDs to particular
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sites, the functions to send and receive messages, and the application
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architecture that supports the particular communication infrastructure
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used (for example, individual threads per communicating site, a shared
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message handler for all sites, a hybrid solution). The communication
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infrastructure for ex_rep_base is implemented in the file
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<code class="filename">ex_rep/base/rep_net.c</code>, and each part of that infrastructure
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is described as follows.</p>
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<p>Ex_rep_base maintains a table of environment ID to TCP/IP port
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mappings. A pointer to this table is stored in a structure pointed to
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by the app_private field of the <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> object so it can be
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accessed by any function that has the database environment handle.
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The table is represented by a machtab_t structure which contains a
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reference to a linked list of member_t's, both of which are defined in
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<code class="filename">ex_rep/base/rep_net.c</code>. Each member_t contains the host and
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port identification, the environment ID, and a file descriptor.</p>
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<p>This design is particular to this application and communication
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infrastructure, but provides an indication of the sort of functionality
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that is needed to maintain the application-specific state for a
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TCP/IP-based infrastructure. The goal of the table and its interfaces
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is threefold: First, it must guarantee that given an environment ID,
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the send function can send a message to the appropriate place. Second,
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when given the special environment ID <a href="../api_reference/C/reptransport.html#transport_DB_EID_BROADCAST" class="olink">DB_EID_BROADCAST</a>, the send
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function can send messages to all the machines in the group. Third,
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upon receipt of an incoming message, the receive function can correctly
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identify the sender and pass the appropriate environment ID to the
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<a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a> method.</p>
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<p>Mapping a particular environment ID to a specific port is accomplished
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by looping through the linked list until the desired environment ID is
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found. Broadcast communication is implemented by looping through the
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linked list and sending to each member found. Since each port
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communicates with only a single other environment, receipt of a message
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on a particular port precisely identifies the sender.</p>
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<p>This is implemented in the quote_send, quote_send_broadcast and
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quote_send_one functions, which can be found in
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<code class="filename">ex_rep/base/rep_net.c</code>.</p>
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<p>The example provided is merely one way to satisfy these requirements,
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and there are alternative implementations as well. For instance,
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instead of associating separate socket connections with each remote
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environment, an application might instead label each message with a
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sender identifier; instead of looping through a table and sending a
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copy of a message to each member of the replication group, the
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application could send a single message using a broadcast protocol.</p>
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<p>The quote_send function is passed as the callback to
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<a href="../api_reference/C/reptransport.html" class="olink">DB_ENV->rep_set_transport()</a>; Berkeley DB automatically sends messages as needed
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for replication. The receive function is a mirror to the quote_send_one
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function. It is not a callback function (the application is responsible
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for collecting messages and calling <a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a> on them as is
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convenient). In the sample application, all messages transmitted are
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Berkeley DB messages that get handled by <a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a>, however, this
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is not always going to be the case. The application may want to pass
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its own messages across the same channels, distinguish between its own
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messages and those of Berkeley DB, and then pass only the Berkeley DB ones to
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<a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a>.</p>
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<p>The final component of the communication infrastructure is the process
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model used to communicate with all the sites in the replication group.
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Each site creates a thread of control that listens on its designated
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socket (as specified by the <span class="bold"><strong>-l</strong></span> command line argument) and
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then creates a new channel for each site that contacts it. In addition,
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each site explicitly connects to the sites specified in the
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<span class="bold"><strong>-r</strong></span> and <span class="bold"><strong>-R</strong></span>
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command line arguments. This is a fairly standard TCP/IP
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process architecture and is implemented by the connect_thread,
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connect_all and connect_site functions
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in <code class="filename">ex_rep/base/rep_msg.c</code> and supporting functions
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in <code class="filename">ex_rep/base/rep_net.c</code>.</p>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="rep_ex.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="rep.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="rep_ex_rq.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Ex_rep: a replication example </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Ex_rep_base: putting it all together</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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