path: root/src/backend/libpq/README

Date: Wed, 18 Feb 1998 19:44:52 +0000
From: Phil Thompson <phil@river-bank.demon.co.uk>
To: "Thomas G. Lockhart" <lockhart@alumni.caltech.edu>
Subject: [DOCS] Re: [HACKERS] Frontend/Backend Protocol

1. Introduction

This document describes V1.0 of the protocol used between PostgreSQL frontends
and backends.  It is a message based protocol running over TCP.  V6.3 of
PostgreSQL introduced version numbers into the protocol.  This was done in such
a way as to still allow connections from earlier versions of frontends, but
this document does not cover the protocol used by those earlier versions.

This document does not cover how different frontend interfaces may use the
protocol to implement certain features, eg. the way in which libpq passes
certain environment variables after the connection is established.


2. Overview

The three major components are the frontend (running on the client) and the
postmaster and backend (running on the server).  The postmaster and backend
have different roles but may be implemented by the same executable.

A frontend sends a startup packet to the postmaster.  This includes the names
of the user and the database the user wants to connect to.  The postmaster then
uses this, and the information in the pg_hba.conf(5) file to determine what
further authentication information it requires the frontend to send (if any)
and responds to the frontend accordingly.

The frontend then sends any required authentication information.  Once the
postmaster validates this it responds to the frontend that it is authenticated
and hands over to a backend.

Subsequent communications are query and result packets exchanged between the
frontend and the backend.  The postmaster takes no further part in the
communication.

When the frontend wishes to disconnect it sends an appropriate packet and
closes the connection without waiting for a response for the backend.

Packets are sent as a data stream.  The first byte determines what should be
expected in the rest of the packet.  The exception is packets send from a
frontend to the postmaster, which comprise a packet length then the packet
itself.  The difference is historical.


3. Protocol

This section describes the message flow.  There are four different types of
flows depending on the state of the connection.

3.1 Authentication

The frontend sends a StartupPacket.  The postmaster uses this and the contents
of the pg_hba.conf(5) file to determine what authentication method the frontend
must use.  The postmaster then responds with one of the following messages.

	ErrorResponse
		The postmaster then immediately closes the connection.

	AuthenticationOk
		The postmaster then hands over to the backend.  The postmaster
		takes no further part in the communication.

	AuthenticationKerberosV4
		The frontend must then take part in a Kerberos V4
		authentication dialog (not described here) with the postmaster.
		If this is succesful, the postmaster responds with an
		AuthenticationOk, otherwise it responds with an ErrorResponse.

	AuthenticationKerberosV5
		The frontend must then take part in a Kerberos V5
		authentication dialog (not described here) with the postmaster.
		If this is succesful, the postmaster responds with an
		AuthenticationOk, otherwise it responds with an ErrorResponse.

	AuthenticationUnencryptedPassword
		The frontend must then send an UnencryptedPasswordPacket.
		If this is the correct password, the postmaster responds with
		an AuthenticationOk, otherwise it responds with an
		ErrorResponse.

	AuthenticationEncryptedPassword
		The frontend must then send an EncryptedPasswordPacket.
		If this is the correct password, the postmaster responds with
		an AuthenticationOk, otherwise it responds with an
		ErrorResponse.

If the frontend does not support the authentication method requested by the
postmaster, then it should immediately close the connection.

3.2 Query

The frontend sends a Query message to the backend.  The response sent by the
backend depends on the contents of the query.  The possible responses are as
follows.

	CompletedResponse
		The query completed normally.

	CopyInResponse
		The backend is ready to copy data from the frontend to a
		relation.  The frontend should then send a CopyDataRows
		message.  The backend will then respond with a
		CompletedResponse message with a tag of "COPY".

	CopyOutResponse
		The backend is ready to copy data from a relation to the
		frontend.  It then sends a CopyDataRows message, and then a
		CompletedResponse message with a tag of "COPY".

	CursorResponse
		The query was either an insert(l), delete(l), update(l),
		fetch(l) or a select(l) command.  If the transaction has been
		aborted then the backend sends a CompletedResponse message with
		a tag of "*ABORT STATE*".  Otherwise the following responses
		are sent.

		For an insert(l) command, the backend then sends a
		CompletedResponse message with a tag of "INSERT <oid> <rows>"
		where <rows> is the number of rows inserted, and <oid> is the
		object ID of the inserted row if <rows> is 1, otherwise <oid>
		is 0.

		For a delete(l) command, the backend then sends a
		CompletedResponse message with a tag of "DELETE <rows>" where
		<rows> is the number of rows deleted.

		For an update(l) command, the backend then sends a
		CompletedResponse message with a tag of "UPDATE <rows>" where
		<rows> is the number of rows deleted.

		For a fetch(l) or select(l) command, the backend sends a
		RowDescription message.  This is then followed by an AsciiRow
		or BinaryRow message (depending on if a binary cursor was
		specified) for each row being returned to the frontend.
		Finally, the backend sends a CompletedResponse message with a
		tag of "SELECT".

	EmptyQueryResponse
		The query was empty.

	ErrorResponse
		An error has occured.

	NoticeResponse
		A warning message has been issued in relation to the query.
		Notices are in addition to other responses, ie. the backend
		will send another response message immediately afterwards.

	NotificationResponse
		A notify(l) command has been executed for a relation for
		which a previous listen(l) command was executed.  Notifications
		are in addition to other responses, ie. the backend will send
		another response message immediately afterwards.

A frontend must be prepared to accept ErrorResponse and NoticeResponse
messages whenever it is expecting any other type of message.

3.3 Function Call

The frontend sends a FunctionCall message to the backend.  The response sent by
the backend depends on the result of the function call.  The possible responses
are as follows.

	ErrorResponse
		An error has occured.

	FunctionResultResponse
		The function call was executed and returned a result.

	FunctionVoidResponse
		The function call was executed and returned no result.

	NoticeResponse
		A warning message has been issued in relation to the function
		call.  Notices are in addition to other responses, ie. the
		backend will send another response message immediately
		afterwards.

A frontend must be prepared to accept ErrorResponse and NoticeResponse
messages whenever it is expecting any other type of message.

3.4 Termination

The frontend sends a Terminate message and immediately closes the connection.
On receipt of the message, the backend immediately closes the connection and
terminates.


4. Message Data Types

This section describes the base data types used in messages.

	Int<n>(i)
		An <n> bit integer in network byte order.  If i is specified it
		is the literal value.  Eg. Int16, Int32(42).

	LimString<n>(s)
		A character array of exactly <n> bytes interpreted as a '\0'
		terminated string.  The '\0' is omitted if there is
		insufficient room.  If s is specified it is the literal value.
		Eg. LimString32, LimString64("user").

	String(s)
		A conventional C '\0' terminated string with no length
		limitation.  A frontend should always read the full string
		even though it may have to discard characters if it's buffers
		aren't big enough.  If s is specified it is the literal value.
		Eg. String, String("user").

	Byte<n>(c)
		Exactly <n> bytes.  If c is specified it is the literal
		value.  Eg. Byte, Byte1('\n').


5. Messages Formats

This section describes the detailed format of each message.  Each can be sent
by either a frontend (F), a postmaster/backend (B), or both (F & B).

AsciiRow (B)
	Byte1('D')
		Identifies the message, in the context in which it is sent (see
		CopyInResponse), as an ASCII row.
	Byte<n>
		A bit map with one bit for each field in the row.  The 1st
		field corresponds to bit 7 of the 1st byte, the 2nd field
		corresponds to bit 6 of the 1st byte, the 8th field corresponds
		to bit 0 of the 1st byte, the 9th field corresponds to bit 8 of
		the 2nd byte, and so on.  The bit is set if the value of the
		corresponding field is not NULL.

	Then, for each field, there is the following.
		Int32
			Specifies the size of the value of the field, including
			this size.
		Byte<n>
			Specifies the value of the field itself in ASCII
			characters.  <n> is the above size minus 4.

AuthenticationOk (B)
	Byte1('R')
		Identifies the message as an authentication request.
	Int32(0)
		Specifies that the authentication was succesful.

AuthenticationKerberosV4 (B)
	Byte1('R')
		Identifies the message as an authentication request.
	Int32(1)
		Specifies that Kerberos V4 authentication is required.

AuthenticationKerberosV5 (B)
	Byte1('R')
		Identifies the message as an authentication request.
	Int32(2)
		Specifies that Kerberos V5 authentication is required.

AuthenticationUnencryptedPassword (B)
	Byte1('R')
		Identifies the message as an authentication request.
	Int32(3)
		Specifies that an unencrypted password is required.

AuthenticationEncryptedPassword (B)
	Byte1('R')
		Identifies the message as an authentication request.
	Int32(4)
		Specifies that an encrypted password is required.
	Byte2
		The salt to use when encrypting the password.

BinaryRow (B)
	Byte1('B')
		Identifies the message, in the context in which it is sent (see
		CopyOutResponse), as a binary row.
	Byte<n>
		A bit map with one bit for each field in the row.  The 1st
		field corresponds to bit 7 of the 1st byte, the 2nd field
		corresponds to bit 6 of the 1st byte, the 8th field corresponds
		to bit 0 of the 1st byte, the 9th field corresponds to bit 8 of
		the 2nd byte, and so on.  The bit is set if the value of the
		corresponding field is not NULL.

	Then, for each field, there is the following.
		Int32
			Specifies the size of the value of the field, excluding
			this size.
		Byte<n>
			Specifies the value of the field itself in binary
			format.  <n> is the above size.

CompletedResponse (B)
	Byte1('C')
		Identifies the message as a completed response.
	String
		The command tag.  This is usually (but not always) a single
		word that identifies which SQL command was completed.

CopyDataRows (B & F)
	This is a stream of rows where each row is terminated by a Char1('\n').
	This is then followed by the sequence Char1('\\'), Char1('.'),
	Char1('\n').

CopyInResponse (B)
	Byte1('D')
		Identifies the message, in the context in which it is sent (see
		AsciiRow), as a copy in started response.

CopyOutResponse (B)
	Byte1('B')
		Identifies the message, in the context in which it is sent (see
		BinaryRow), as a copy out started response.

CursorResponse (B)
	Byte1('P')
		Identifies the message as a cursor response.
	String
		The name of the cursor.  This will be "blank" if the cursor is
		implicit.

EmptyQueryResponse (B)
	Byte1('I')
		Identifies the message as an empty query response.
	String("")
		Unused.

EncryptedPasswordPacket (F)
	Int32
		The size of the packet in bytes.
	String
		The encrypted (using crypt()) password.

ErrorResponse (B)
	Byte1('E')
		Identifies the message as an error.
	String
		The error message itself.

FunctionCall (F)
	Byte1('F')
		Identifies the message as a function call.
	String("")
		Unused.
	Int32
		Specifies the object ID of the function to call.
	Int32
		Specifies the number of arguments being supplied to the
		function.

	Then, for each argument, there is the following.
		Int32
			Specifies the size of the value of the argument,
			excluding this size.
		Byte<n>
			Specifies the value of the field itself in binary
			format.  <n> is the above size.

FunctionResultResponse (B)
	Byte1('V')
		Identifies the message as a function call result.
	Byte1('G')
		Specifies that an actual result was returned.
	Int32
		Specifies the size of the value of the result, excluding this
		size.
	Byte<n>
		Specifies the value of the result itself in binary format.
		<n> is the above size.
	Byte1('0')
		Unused.  (Strictly speaking, FunctionResultResponse and
		FunctionVoidResponse are the same thing but with some optional
		parts to the message.)

FunctionVoidResponse (B)
	Byte1('V')
		Identifies the message as a function call result.
	Byte1('0')
		Specifies that no actual result was returned.

NoticeResponse (B)
	Byte1('N')
		Identifies the message as a notice.
	String
		The notice message itself.

NotificationResponse (B)
	Byte1('A')
		Identifies the message as a notification response.
	Int32
		The process ID of the backend process.
	String
		The name of the relation that the notify has been raised on.

Query (F)
	Byte1('Q')
		Identifies the message as query.
	String
		The query itself.

RowDescription (B)
	Byte1('T')
		Identifies the message as a row description.
	Int16
		Specifies the number of fields in a row (and may be zero).

	Then, for each field, there is the following.
		String
			Specifies the field name.
		Int32
			Specifies the object ID of the field type.
		Int16
			Specifies the type size.

StartupPacket (F)
	Int32(296)
		The size of the packet in bytes.
	Int32
		The protocol version number.  The most significant 16 bits are
		the major version number.  The least 16 significant bits are
		the minor version number.
	LimString64
		The database name, defaults to the user name if omitted.
	LimString32
		The user name.
	LimString64
		Any additional command line arguments to be passed to the
		backend by the postmaster.
	LimString64
		Unused.
	LimString64
		The optional tty the backend should use for debugging messages.

Terminate (F)
	Byte1('X')
		Identifies the message as a termination.

UnencryptedPasswordPacket (F)
	Int32
		The size of the packet in bytes.
	String
		The unencrypted password.

--------------1B9BA35856C95E22453E911A--