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>Chapter 51. Frontend/Backend Protocol</H1
><DIV
CLASS="TOC"
><DL
><DT
><B
>Table of Contents</B
></DT
><DT
>51.1. <A
HREF="protocol-overview.html"
>Overview</A
></DT
><DD
><DL
><DT
>51.1.1. <A
HREF="protocol-overview.html#PROTOCOL-MESSAGE-CONCEPTS"
>Messaging Overview</A
></DT
><DT
>51.1.2. <A
HREF="protocol-overview.html#PROTOCOL-QUERY-CONCEPTS"
>Extended Query Overview</A
></DT
><DT
>51.1.3. <A
HREF="protocol-overview.html#PROTOCOL-FORMAT-CODES"
>Formats and Format Codes</A
></DT
></DL
></DD
><DT
>51.2. <A
HREF="protocol-flow.html"
>Message Flow</A
></DT
><DD
><DL
><DT
>51.2.1. <A
HREF="protocol-flow.html#AEN113806"
>Start-up</A
></DT
><DT
>51.2.2. <A
HREF="protocol-flow.html#AEN113886"
>Simple Query</A
></DT
><DT
>51.2.3. <A
HREF="protocol-flow.html#PROTOCOL-FLOW-EXT-QUERY"
>Extended Query</A
></DT
><DT
>51.2.4. <A
HREF="protocol-flow.html#AEN113997"
>Function Call</A
></DT
><DT
>51.2.5. <A
HREF="protocol-flow.html#PROTOCOL-COPY"
>COPY Operations</A
></DT
><DT
>51.2.6. <A
HREF="protocol-flow.html#PROTOCOL-ASYNC"
>Asynchronous Operations</A
></DT
><DT
>51.2.7. <A
HREF="protocol-flow.html#AEN114083"
>Canceling Requests in Progress</A
></DT
><DT
>51.2.8. <A
HREF="protocol-flow.html#AEN114091"
>Termination</A
></DT
><DT
>51.2.9. <A
HREF="protocol-flow.html#AEN114100"
><ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> Session Encryption</A
></DT
></DL
></DD
><DT
>51.3. <A
HREF="protocol-replication.html"
>Streaming Replication Protocol</A
></DT
><DT
>51.4. <A
HREF="protocol-message-types.html"
>Message Data Types</A
></DT
><DT
>51.5. <A
HREF="protocol-message-formats.html"
>Message Formats</A
></DT
><DT
>51.6. <A
HREF="protocol-error-fields.html"
>Error and Notice Message Fields</A
></DT
><DT
>51.7. <A
HREF="protocol-changes.html"
>Summary of Changes since Protocol 2.0</A
></DT
></DL
></DIV
><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"
>"backend"</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"
>"backend"</SPAN
> and <SPAN
CLASS="QUOTE"
>"server"</SPAN
> are
   interchangeable; likewise <SPAN
CLASS="QUOTE"
>"frontend"</SPAN
> and <SPAN
CLASS="QUOTE"
>"client"</SPAN
>
   are interchangeable.
  </P
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