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Yasuhiro KIMURA
156 lines
8.2 KiB
HTML
156 lines
8.2 KiB
HTML
<?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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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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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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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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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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<link rel="next" href="rep_ex_rq.html" title="Ex_rep_base: putting it all together" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 18.1.40</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. Berkeley DB Replication </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 communication
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infrastructure. The communication infrastructure consists of
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three parts: a way to map environment IDs to particular sites,
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the functions to send and receive messages, and the
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application architecture that supports the particular
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communication infrastructure used (for example, individual
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threads per communicating site, a shared message handler for
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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
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of that infrastructure is described as follows.
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</p>
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<p>
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Ex_rep_base maintains a table of environment ID to TCP/IP
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port mappings. A pointer to this table is stored in a
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structure pointed to by the app_private field of the <a href="../api_reference/C/env.html" class="olink">DB_ENV</a>
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object so it can be accessed by any function that has the
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database environment handle. The table is represented by a
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machtab_t structure which contains a reference to a linked
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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
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contains the host and port identification, the environment ID,
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and a file descriptor.
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</p>
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<p>
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This design is particular to this application and
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communication infrastructure, but provides an indication of
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the sort of functionality that is needed to maintain the
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application-specific state for a TCP/IP-based infrastructure.
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The goal of the table and its interfaces is threefold: First,
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it must guarantee that given an environment ID, the send
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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
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send function can send messages to all the machines in the
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group. Third, upon receipt of an incoming message, the receive
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function can correctly identify the sender and pass the
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appropriate environment ID to the <a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a> method.
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</p>
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<p>
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Mapping a particular environment ID to a specific port is
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accomplished by looping through the linked list until the
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desired environment ID is found. Broadcast communication is
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implemented by looping through the linked list and sending to
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each member found. Since each port communicates with only a
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single other environment, receipt of a message on a particular
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port precisely identifies the sender.
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</p>
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<p>
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This is implemented in the quote_send, quote_send_broadcast
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and quote_send_one functions, which can be found in
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<code class="filename">ex_rep/base/rep_net.c</code>.
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</p>
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<p>
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The example provided is merely one way to satisfy these
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requirements, and there are alternative implementations as
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well. For instance, instead of associating separate socket
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connections with each remote environment, an application might
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instead label each message with a sender identifier; instead
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of looping through a table and sending a copy of a message to
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each member of the replication group, the application could
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send a single message using a broadcast protocol.
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</p>
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<p>
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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
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needed for replication. The receive function is a mirror to
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the quote_send_one function. It is not a callback function
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(the application is responsible for collecting messages and
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calling <a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a> on them as is convenient). In the sample
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application, all messages transmitted are Berkeley DB messages
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that get handled by <a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a>, however, this is not always
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going to be the case. The application may want to pass its own
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messages across the same channels, distinguish between its own
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messages and those of Berkeley DB, and then pass only the
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Berkeley DB ones to <a href="../api_reference/C/repmessage.html" class="olink">DB_ENV->rep_process_message()</a>.
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</p>
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<p>
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The final component of the communication infrastructure is
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the process model used to communicate with all the sites in
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the replication group. Each site creates a thread of control
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that listens on its designated socket (as specified by the
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<span class="bold"><strong>-l</strong></span> command line argument)
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and then creates a new channel for each site that contacts it.
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In addition, each site explicitly connects to the sites
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specified in the <span class="bold"><strong>-r</strong></span> and
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<span class="bold"><strong>-R</strong></span> command line
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arguments. This is a fairly standard TCP/IP process
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architecture and is implemented by the connect_thread,
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connect_all and connect_site functions in
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<code class="filename">ex_rep/base/rep_msg.c</code> and supporting
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functions in
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<code class="filename">ex_rep/base/rep_net.c</code>.
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</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
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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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