# Frontend/Backend Protocol

Written by Phil Thompson (phil@river-bank.demon.co.uk). Updates for protocol 2.0 by Tom Lane (tgl@sss.pgh.pa.us).

PostgreSQL uses a message-based protocol for communication between frontends and backends. The protocol is implemented over TCP/IP and also on Unix domain sockets. PostgreSQL 6.3 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 describes version 2.0 of the protocol, implemented in PostgreSQL 6.4 and later.

Higher level features built on this protocol (for example, how libpq passes certain environment variables after the connection is established) are covered elsewhere.

## Overview

A frontend opens a connection to the server and sends a start-up packet. This includes the names of the user and of the database the user wants to connect to. The server then uses this, and the information in the `pg_hba.conf` 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 server validates this it responds to the frontend that it is authenticated and sends a message indicating successful start-up (normal case) or failure (for example, an invalid database name).

In order to serve multiple clients efficiently, the server launches a new backend process for each client. This is transparent to the protocol, however. In the current implementation, a new child process is created immediately after an incoming connection is detected.

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

Packets are sent as a data stream. The first byte determines what should be expected in the rest of the packet. The exceptions are packets sent as part of the start-up and authentication exchange, which comprise a packet length followed by the packet itself. The difference is historical.

## Protocol

This section describes the message flow. There are four different types of flows depending on the state of the connection: start-up, query, function call, and termination. There are also special provisions for notification responses and command cancellation, which can occur at any time after the start-up phase.

### Start-up

Initially, the frontend sends a StartupPacket. The server uses this info and the contents of the `pg_hba.conf` file to determine what authentication method the frontend must use. The server then responds with one of the following messages:

ErrorResponse

- The server then immediately closes the connection.

AuthenticationOk

- The authentication exchange is completed.

AuthenticationKerberosV4

- The frontend must then take part in a Kerberos V4 authentication dialog (not described here, part of the Kerberos specification) with the server. If this is successful, the server 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, part of the Kerberos specification) with the server. If this is successful, the server responds with an AuthenticationOk, otherwise it responds with an ErrorResponse.

AuthenticationCleartextPassword

- The frontend must then send a PasswordPacket containing the password in clear-text form. If this is the correct password, the server responds with an AuthenticationOk, otherwise it responds with an ErrorResponse.

AuthenticationCryptPassword

- The frontend must then send a PasswordPacket containing the password encrypted via crypt(3), using the 2-character salt specified in the AuthenticationCryptPassword packet. If this is the correct password, the server responds with an AuthenticationOk, otherwise it responds with an ErrorResponse.

AuthenticationMD5Password

- The frontend must then send a PasswordPacket containing the password encrypted via MD5, using the 4-character salt specified in the AuthenticationMD5Password packet. If this is the correct password, the server responds with an AuthenticationOk, otherwise it responds with an ErrorResponse.

AuthenticationSCMCredential

- This method is only possible for local Unix-domain connections on platforms that support SCM credential messages. The frontend must issue an SCM credential message and then send a single data byte. (The contents of the data byte are uninteresting; it's only used to ensure that the server waits long enough to receive the credential message.) If the credential is acceptable, the server responds with an AuthenticationOk, otherwise it responds with an ErrorResponse.

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

After having received AuthenticationOk, the frontend should wait for further messages from the server. The possible messages from the backend in this phase are:

BackendKeyData

- This message provides secret-key data that the frontend must save if it wants to be able to issue cancel requests later. The frontend should not respond to this message, but should continue listening for a ReadyForQuery message.

ReadyForQuery

- Start-up is completed. The frontend may now issue query or function call messages.

ErrorResponse

- Start-up failed. The connection is closed after sending this message.

NoticeResponse

- A warning message has been issued. The frontend should display the message but continue listening for ReadyForQuery or ErrorResponse.

The ReadyForQuery message is the same one that the backend will issue after each query cycle. Depending on the coding needs of the frontend, it is reasonable to consider ReadyForQuery as starting a query cycle (and then BackendKeyData indicates successful conclusion of the start-up phase), or to consider ReadyForQuery as ending the start-up phase and each subsequent query cycle.

### Query

A Query cycle is initiated by the frontend sending a Query message to the backend. The backend then sends one or more response messages depending on the contents of the query command string, and finally a ReadyForQuery response message. ReadyForQuery informs the frontend that it may safely send a new query or function call.

The possible response messages from the backend are:

CompletedResponse

- An SQL command completed normally.

CopyInResponse

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

CursorResponse

- Beginning of the response to a `SELECT`, `FETCH`, `INSERT`, `UPDATE`, or `DELETE` query. In the `FETCH` case the name of the cursor being fetched from is included in the message. Otherwise the message always mentions the blank cursor.

RowDescription

- Indicates that rows are about to be returned in response to a `SELECT` or `FETCH` query. The message contents describe the layout of the rows. This will be followed by an AsciiRow or BinaryRow message (depending on whether a binary cursor was specified) for each row being returned to the frontend.

EmptyQueryResponse

- An empty query string was recognized.

ErrorResponse

- An error has occurred.

ReadyForQuery

- Processing of the query string is complete. A separate message is sent to indicate this because the query string may contain multiple SQL commands. (CompletedResponse marks the end of processing one SQL command, not the whole string.) ReadyForQuery will always be sent, whether processing terminates successfully or with an error.

NoticeResponse

- A warning message has been issued in relation to the query. Notices are in addition to other responses, i.e., the backend will continue processing the command.

The response to a `SELECT` or `FETCH` query normally consists of CursorResponse, RowDescription, zero or more AsciiRow or BinaryRow messages, and finally CompletedResponse. `INSERT`, `UPDATE`, and `DELETE` queries produce CursorResponse followed by CompletedResponse. `COPY` to or from the frontend invokes special protocol as mentioned above. All other query types normally produce only a CompletedResponse message.

Since a query string could contain several queries (separated by semicolons), there might be several such response sequences before the backend finishes processing the query string. ReadyForQuery is issued when the entire string has been processed and the backend is ready to accept a new query string.

If a completely empty (no contents other than whitespace) query string is received, the response is EmptyQueryResponse followed by ReadyForQuery. (The need to specially distinguish this case is historical.)

In the event of an error, ErrorResponse is issued followed by ReadyForQuery. All further processing of the query string is aborted by ErrorResponse (even if more queries remained in it). Note that this may occur partway through the sequence of messages generated by an individual query.

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

Actually, it is possible for NoticeResponse to arrive even when the frontend is not expecting any kind of message, that is, the backend is nominally idle. (In particular, the backend can be commanded to terminate by its parent process. In that case it will send a NoticeResponse before closing the connection.) It is recommended that the frontend check for such asynchronous notices just before issuing any new command.

Also, if the frontend issues any `LISTEN` commands then it must be prepared to accept NotificationResponse messages at any time; see below.

Recommended practice is to code frontends in a state-machine style that will accept any message type at any time that it could make sense, rather than wiring in assumptions about the exact sequence of messages.

### Function Call

A Function Call cycle is initiated by the frontend sending a FunctionCall message to the backend. The backend then sends one or more response messages depending on the results of the function call, and finally a ReadyForQuery response message. ReadyForQuery informs the frontend that it may safely send a new query or function call.

The possible response messages from the backend are:

ErrorResponse

- An error has occurred.

FunctionResultResponse

- The function call was executed and returned a result.

FunctionVoidResponse

- The function call was executed and returned no result.

ReadyForQuery

- Processing of the function call is complete. ReadyForQuery will always be sent, whether processing terminates successfully or with an error.

NoticeResponse

- A warning message has been issued in relation to the function call. Notices are in addition to other responses, i.e., the backend will continue processing the command.

A frontend must be prepared to accept ErrorResponse and NoticeResponse messages whenever it is expecting any other type of message. Also, if it issues any `LISTEN` commands then it must be prepared to accept NotificationResponse messages at any time; see below.

### Notification Responses

If a frontend issues a `LISTEN` command, then the backend will send a NotificationResponse message (not to be confused with NoticeResponse!) whenever a `NOTIFY` command is executed for the same notification name.

Notification responses are permitted at any point in the protocol (after start-up), except within another backend message. Thus, the frontend must be prepared to recognize a NotificationResponse message whenever it is expecting any message. Indeed, it should be able to handle NotificationResponse messages even when it is not engaged in a query.

NotificationResponse

- A `NOTIFY` command has been executed for a name for which a previous `LISTEN` command was executed. Notifications may be sent at any time.

It may be worth pointing out that the names used in listen and notify commands need not have anything to do with names of relations (tables) in the SQL database. Notification names are simply arbitrarily chosen condition names.

### Cancelling Requests in Progress

During the processing of a query, the frontend may request cancellation of the query. The cancel request is not sent directly on the open connection to the backend for reasons of implementation efficiency: we don't want to have the backend constantly checking for new input from the frontend during query processing. Cancel requests should be relatively infrequent, so we make them slightly cumbersome in order to avoid a penalty in the normal case.

To issue a cancel request, the frontend opens a new connection to the server and sends a CancelRequest message, rather than the StartupPacket message that would ordinarily be sent across a new connection. The server will process this request and then close the connection. For security reasons, no direct reply is made to the cancel request message.

A CancelRequest message will be ignored unless it contains the same key data (PID and secret key) passed to the frontend during connection start-up. If the request matches the PID and secret key for a currently executing backend, the processing of the current query is aborted. (In the existing implementation, this is done by sending a special signal to the backend process that is processing the query.)

The cancellation signal may or may not have any effect --- for example, if it arrives after the backend has finished processing the query, then it will have no effect. If the cancellation is effective, it results in the current command being terminated early with an error message.

The upshot of all this is that for reasons of both security and efficiency, the frontend has no direct way to tell whether a cancel request has succeeded. It must continue to wait for the backend to respond to the query. Issuing a cancel simply improves the odds that the current query will finish soon, and improves the odds that it will fail with an error message instead of succeeding.

Since the cancel request is sent across a new connection to the server and not across the regular frontend/backend communication link, it is possible for the cancel request to be issued by any process, not just the frontend whose query is to be canceled. This may have some benefits of flexibility in building multiple-process applications. It also introduces a security risk, in that unauthorized persons might try to cancel queries. The security risk is addressed by requiring a dynamically generated secret key to be supplied in cancel requests.

### Termination

The normal, graceful termination procedure is that the frontend sends a Terminate message and immediately closes the connection. On receipt of the message, the backend immediately closes the connection and terminates.

An ungraceful termination may occur due to software failure (i.e., core dump) at either end. If either frontend or backend sees an unexpected closure of the connection, it should clean up and terminate. The frontend has the option of launching a new backend by recontacting the server if it doesn't want to terminate itself.

For either normal or abnormal termination, any open transaction is rolled back, not committed. One should note however that if a frontend disconnects while a query is being processed, the backend will probably finish the query before noticing the disconnection. If the query is outside any transaction block (`BEGIN` ... `COMMIT` sequence) then its results may be committed before the disconnection is recognized.

### SSL Session Encryption

Recent releases of PostgreSQL allow frontend/backend communication to be encrypted using SSL. This provides communication security in environments where attackers might be able to capture the session traffic.

To initiate an SSL-encrypted connection, the frontend initially sends an SSLRequest message rather than a StartupPacket. The server then responds with a single byte containing `Y` or `N`, indicating that it is willing or unwilling to perform SSL, respectively. The frontend may close the connection at this point if it is dissatisfied with the response. To continue after `Y`, perform an SSL startup handshake (not described here, part of the SSL specification) with the server. If this is successful, continue with sending the usual StartupPacket. In this case the StartupPacket and all subsequent data will be SSL-encrypted. To continue after `N`, send the usual StartupPacket and proceed without encryption.

The frontend should also be prepared to handle an ErrorMessage response to SSLRequest from the server. This would only occur if the server predates the addition of SSL support to PostgreSQL. In this case the connection must be closed, but the frontend may choose to open a fresh connection and proceed without requesting SSL.

An initial SSLRequest may also be used in a connection that is being opened to send a CancelRequest message.

While the protocol itself does not provide a way for the server to force SSL encryption, the administrator may configure the server to reject unencrypted sessions as a byproduct of authentication checking.

## 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 null-terminated string. The zero-byte 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 null-terminated string with no length limitation. If `s` is specified it is the literal value. Eg. String, String("user").

  There is no predefined limit on the length of a string that can be returned by the backend. Good coding strategy for a frontend is to use an expandable buffer so that anything that fits in memory can be accepted. If that's not feasible, read the full string and discard trailing characters that don't fit into your fixed-size buffer.

Byte`n`(`c`)

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

## Message Formats

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

AsciiRow (B)

- Byte1('D')

- Identifies the message as an ASCII data row. (A prior RowDescription message defines the number of fields in the row and their data types.)

Byte`n`

- A bit map with one bit for each field in the row. The 1st field corresponds to bit 7 (MSB) of the 1st byte, the 2nd field corresponds to bit 6 of the 1st byte, the 8th field corresponds to bit 0 (LSB) of the 1st byte, the 9th field corresponds to bit 7 of the 2nd byte, and so on. Each bit is set if the value of the corresponding field is not NULL. If the number of fields is not a multiple of 8, the remainder of the last byte in the bit map is wasted.

  Then, for each field with a non-NULL value, 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. There is no trailing zero-byte in the field data; the front end must add one if it wants one.

AuthenticationOk (B)

- Byte1('R')

- Identifies the message as an authentication request.

Int32(0)

- Specifies that the authentication was successful.

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.

AuthenticationCleartextPassword (B)

- Byte1('R')

- Identifies the message as an authentication request.

Int32(3)

- Specifies that a cleartext password is required.

AuthenticationCryptPassword (B)

- Byte1('R')

- Identifies the message as an authentication request.

Int32(4)

- Specifies that a crypt()-encrypted password is required.

Byte2

- The salt to use when encrypting the password.

AuthenticationMD5Password (B)

- Byte1('R')

- Identifies the message as an authentication request.

Int32(5)

- Specifies that an MD5-encrypted password is required.

Byte4

- The salt to use when encrypting the password.

AuthenticationSCMCredential (B)

- Byte1('R')

- Identifies the message as an authentication request.

Int32(6)

- Specifies that an SCM credentials message is required.

BackendKeyData (B)

- Byte1('K')

- Identifies the message as cancellation key data. The frontend must save these values if it wishes to be able to issue CancelRequest messages later.

Int32

- The process ID of this backend.

Int32

- The secret key of this backend.

BinaryRow (B)

- Byte1('B')

- Identifies the message as a binary data row. (A prior RowDescription message defines the number of fields in the row and their data types.)

Byte`n`

- A bit map with one bit for each field in the row. The 1st field corresponds to bit 7 (MSB) of the 1st byte, the 2nd field corresponds to bit 6 of the 1st byte, the 8th field corresponds to bit 0 (LSB) of the 1st byte, the 9th field corresponds to bit 7 of the 2nd byte, and so on. Each bit is set if the value of the corresponding field is not NULL. If the number of fields is not a multiple of 8, the remainder of the last byte in the bit map is wasted.

  Then, for each field with a non-NULL value, 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.

CancelRequest (F)

- Int32(16)

- The size of the packet in bytes.

Int32(80877102)

- The cancel request code. The value is chosen to contain `1234` in the most significant 16 bits, and `5678` in the least 16 significant bits. (To avoid confusion, this code must not be the same as any protocol version number.)

Int32

- The process ID of the target backend.

Int32

- The secret key for the target backend.

CompletedResponse (B)

- Byte1('C')

- Identifies the message as a completed response.

String

- The command tag. This is usually a single word that identifies which SQL command was completed.

  For an `INSERT` command, the tag is `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` command, the tag is `DELETE `rows`` where `rows` is the number of rows deleted.

  For an `UPDATE` command, the tag is `UPDATE `rows`` where `rows` is the number of rows updated.

CopyDataRows (B & F)

- This is a stream of rows where each row is terminated by a Byte1('\n'). This is then followed by the sequence Byte1('\\'), Byte1('.'), Byte1('\n').

CopyInResponse (B)

- Byte1('G')

- Identifies the message as a Start Copy In response. The frontend must now send a CopyDataRows message.

CopyOutResponse (B)

- Byte1('H')

- Identifies the message as a Start Copy Out response. This message will be followed by a CopyDataRows message.

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 a response to an empty query string.

String("")

- Unused.

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 a nonempty 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 an empty 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 notifying backend process.

String

- The name of the condition that the notify has been raised on.

PasswordPacket (F)

- Int32

- The size of the packet in bytes.

String

- The password (encrypted, if requested).

Query (F)

- Byte1('Q')

- Identifies the message as a query.

String

- The query string itself.

ReadyForQuery (B)

- Byte1('Z')

- Identifies the message type. ReadyForQuery is sent whenever the backend is ready for a new query cycle.

RowDescription (B)

- Byte1('T')

- Identifies the message as a row description.

Int16

- Specifies the number of fields in a row (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.

Int32

- Specifies the type modifier.

SSLRequest (F)

- Int32(8)

- The size of the packet in bytes.

Int32(80877103)

- The SSL request code. The value is chosen to contain `1234` in the most significant 16 bits, and `5679` in the least 16 significant bits. (To avoid confusion, this code must not be the same as any protocol version number.)

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 empty.

LimString32

- The user name.

LimString64

- Any additional command line arguments to be passed to the backend child process by the server.

LimString64

- Unused.

LimString64

- The optional tty the backend should use for debugging messages. (Currently, this field is unsupported and ignored.)

Terminate (F)

- Byte1('X')

- Identifies the message as a termination.