<!-- doc/src/sgml/protocol.sgml -->

<chapter id="protocol">
 <title>Frontend/Backend Protocol</title>

 <indexterm zone="protocol">
  <primary>protocol</primary>
  <secondary>frontend-backend</secondary>
 </indexterm>

 <para>
  <productname>PostgreSQL</productname> uses a message-based protocol
  for communication between frontends and backends (clients and servers).
  The protocol is supported over <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.
 </para>

 <para>
  This document describes version 3.2 of the protocol, introduced in
  <productname>PostgreSQL</productname> version 18. The server and the libpq
  client library are backwards compatible with protocol version 3.0,
  implemented in <productname>PostgreSQL</productname> 7.4 and later.
 </para>

  <para>
   In order to serve multiple clients efficiently, the server launches
   a new <quote>backend</quote> 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 <quote>backend</quote> and <quote>server</quote> are
   interchangeable; likewise <quote>frontend</quote> and <quote>client</quote>
   are interchangeable.
  </para>

 <sect1 id="protocol-overview">
  <title>Overview</title>

  <para>
   The protocol has separate phases for startup and normal operation.
   In the startup phase, the frontend opens a connection to the server
   and authenticates itself to the satisfaction of the server.  (This might
   involve a single message, or multiple messages depending on the
   authentication method being used.)  If all goes well, the server then sends
   status information to the frontend, and finally enters normal operation.
   Except for the initial startup-request message, this part of the
   protocol is driven by the server.
  </para>

  <para>
   During normal operation, the frontend sends queries and
   other commands to the backend, and the backend sends back query results
   and other responses.  There are a few cases (such as <command>NOTIFY</command>)
   wherein the
   backend will send unsolicited messages, but for the most part this portion
   of a session is driven by frontend requests.
  </para>

  <para>
   Termination of the session is normally by frontend choice, but can be
   forced by the backend in certain cases.  In any case, when the backend
   closes the connection, it will roll back any open (incomplete) transaction
   before exiting.
  </para>

  <para>
   Within normal operation, SQL commands can be executed through either of
   two sub-protocols.  In the <quote>simple query</quote> protocol, the frontend
   just sends a textual query string, which is parsed and immediately
   executed by the backend.  In the <quote>extended query</quote> protocol,
   processing of queries is separated into multiple steps: parsing,
   binding of parameter values, and execution.  This offers flexibility
   and performance benefits, at the cost of extra complexity.
  </para>

  <para>
   Normal operation has additional sub-protocols for special operations
   such as <command>COPY</command>.
  </para>

 <sect2 id="protocol-message-concepts">
  <title>Messaging Overview</title>

  <para>
   All communication is through a stream of messages.  The first byte of a
   message identifies the message type, and the next four bytes specify the
   length of the rest of the message (this length count includes itself, but
   not the message-type byte).  The remaining contents of the message are
   determined by the message type.  For historical reasons, the very first
   message sent by the client (the startup message) has no initial
   message-type byte.
  </para>

  <para>
   To avoid losing synchronization