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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 15.4 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#id-1.10.6.7.3">55.2.1. Start-up</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#id-1.10.6.7.4">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#id-1.10.6.7.7">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#id-1.10.6.7.10">55.2.8. Canceling Requests in Progress</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#id-1.10.6.7.11">55.2.9. Termination</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#id-1.10.6.7.12">55.2.10. <acronym class="acronym">SSL</acronym> Session Encryption</a></span></dt><dt><span class="sect2"><a href="protocol-flow.html#id-1.10.6.7.13">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 15.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 55.1. Overview</td></tr></table></div></body></html>
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