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+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 55. Frontend/Backend Protocol</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="view-pg-views.html" title="54.35. pg_views" /><link rel="next" href="protocol-overview.html" title="55.1. Overview" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">Chapter 55. Frontend/Backend Protocol</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="view-pg-views.html" title="54.35. pg_views">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="internals.html" title="Part VII. Internals">Up</a></td><th width="60%" align="center">Part VII. Internals</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="protocol-overview.html" title="55.1. Overview">Next</a></td></tr></table><hr /></div><div class="chapter" id="PROTOCOL"><div class="titlepage"><div><div><h2 class="title">Chapter 55. Frontend/Backend Protocol</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="sect1"><a href="protocol-overview.html">55.1. Overview</a></span></dt><dd><dl><dt><span class="sect2"><a href="protocol-overview.html#PROTOCOL-MESSAGE-CONCEPTS">55.1.1. Messaging Overview</a></span></dt><dt><span class="sect2"><a href="protocol-overview.html#PROTOCOL-QUERY-CONCEPTS">55.1.2. Extended Query Overview</a></span></dt><dt><span class="sect2"><a href="protocol-overview.html#PROTOCOL-FORMAT-CODES">55.1.3. Formats and Format Codes</a></span></dt></dl></dd><dt><span class="sect1"><a href="protocol-flow.html">55.2. Message Flow</a></span></dt><dd><dl><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-FLOW-START-UP">55.2.1. Start-up</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-FLOW-SIMPLE-QUERY">55.2.2. Simple Query</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-FLOW-EXT-QUERY">55.2.3. Extended Query</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-FLOW-PIPELINING">55.2.4. Pipelining</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-FLOW-FUNCTION-CALL">55.2.5. Function Call</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-COPY">55.2.6. COPY Operations</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-ASYNC">55.2.7. Asynchronous Operations</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-FLOW-CANCELING-REQUESTS">55.2.8. Canceling Requests in Progress</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-FLOW-TERMINATION">55.2.9. Termination</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-FLOW-SSL">55.2.10. <acronym class="acronym">SSL</acronym> Session Encryption</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#PROTOCOL-FLOW-GSSAPI">55.2.11. <acronym class="acronym">GSSAPI</acronym> Session Encryption</a></span></dt></dl></dd><dt><span class="sect1"><a href="sasl-authentication.html">55.3. SASL Authentication</a></span></dt><dd><dl><dt><span class="sect2"><a href="sasl-authentication.html#SASL-SCRAM-SHA-256">55.3.1. SCRAM-SHA-256 Authentication</a></span></dt></dl></dd><dt><span class="sect1"><a href="protocol-replication.html">55.4. Streaming Replication Protocol</a></span></dt><dt><span class="sect1"><a href="protocol-logical-replication.html">55.5. Logical Streaming Replication Protocol</a></span></dt><dd><dl><dt><span class="sect2"><a href="protocol-logical-replication.html#PROTOCOL-LOGICAL-REPLICATION-PARAMS">55.5.1. Logical Streaming Replication Parameters</a></span></dt><dt><span class="sect2"><a href="protocol-logical-replication.html#PROTOCOL-LOGICAL-MESSAGES">55.5.2. Logical Replication Protocol Messages</a></span></dt><dt><span class="sect2"><a href="protocol-logical-replication.html#PROTOCOL-LOGICAL-MESSAGES-FLOW">55.5.3. Logical Replication Protocol Message Flow</a></span></dt></dl></dd><dt><span class="sect1"><a href="protocol-message-types.html">55.6. Message Data Types</a></span></dt><dt><span class="sect1"><a href="protocol-message-formats.html">55.7. Message Formats</a></span></dt><dt><span class="sect1"><a href="protocol-error-fields.html">55.8. Error and Notice Message Fields</a></span></dt><dt><span class="sect1"><a href="protocol-logicalrep-message-formats.html">55.9. Logical Replication Message Formats</a></span></dt><dt><span class="sect1"><a href="protocol-changes.html">55.10. Summary of Changes since Protocol 2.0</a></span></dt></dl></div><a id="id-1.10.6.2" class="indexterm"></a><p>
+  <span class="productname">PostgreSQL</span> uses a message-based protocol
+  for communication between frontends and backends (clients and servers).
+  The protocol is supported over <acronym class="acronym">TCP/IP</acronym> and also over
+  Unix-domain sockets.  Port number 5432 has been registered with IANA as
+  the customary TCP port number for servers supporting this protocol, but
+  in practice any non-privileged port number can be used.
+ </p><p>
+  This document describes version 3.0 of the protocol, implemented in
+  <span class="productname">PostgreSQL</span> 7.4 and later.  For descriptions
+  of the earlier protocol versions, see previous releases of the
+  <span class="productname">PostgreSQL</span> documentation.  A single server
+  can support multiple protocol versions.  The initial startup-request
+  message tells the server which protocol version the client is attempting to
+  use.  If the major version requested by the client is not supported by
+  the server, the connection will be rejected (for example, this would occur
+  if the client requested protocol version 4.0, which does not exist as of
+  this writing).  If the minor version requested by the client is not
+  supported by the server (e.g., the client requests version 3.1, but the
+  server supports only 3.0), the server may either reject the connection or
+  may respond with a NegotiateProtocolVersion message containing the highest
+  minor protocol version which it supports.  The client may then choose either
+  to continue with the connection using the specified protocol version or
+  to abort the connection.
+ </p><p>
+   In order to serve multiple clients efficiently, the server launches
+   a new <span class="quote">“<span class="quote">backend</span>”</span> process for each client.
+   In the current implementation, a new child
+   process is created immediately after an incoming connection is detected.
+   This is transparent to the protocol, however.  For purposes of the
+   protocol, the terms <span class="quote">“<span class="quote">backend</span>”</span> and <span class="quote">“<span class="quote">server</span>”</span> are
+   interchangeable; likewise <span class="quote">“<span class="quote">frontend</span>”</span> and <span class="quote">“<span class="quote">client</span>”</span>
+   are interchangeable.
+  </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="view-pg-views.html" title="54.35. pg_views">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="internals.html" title="Part VII. Internals">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="protocol-overview.html" title="55.1. Overview">Next</a></td></tr><tr><td width="40%" align="left" valign="top">54.35. <code class="structname">pg_views</code> </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 55.1. Overview</td></tr></table></div></body></html>
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