Phil Thompson 1998-08-08 Written by Phil Thompson. Updates for protocol 2.0 by Tom Lane. Postgres uses a message-based protocol for communication between frontends and backends. The protocol is implemented over TCP/IP and also on Unix sockets. Postgres v6.3 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 Postgres v6.4 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. 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 the connection to a backend. The backend then sends a message indicating successful startup (normal case) or failure (for example, an invalid database name). Subsequent communications are query and result packets exchanged between the frontend and the backend. The postmaster takes no further part in ordinary query/result communication. (However, the postmaster is involved when the frontend wishes to cancel a query currently being executed by its backend. Further details about that appear below.) 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 sent from a frontend to the postmaster, which comprise a packet length then the packet itself. The difference is historical. This section describes the message flow. There are four different types of flows depending on the state of the connection: startup, query, function call, and termination. There are also special provisions for notification responses and command cancellation, which can occur at any time after the startup phase. Startup is divided into an authentication phase and a backend startup phase. Initially, the frontend sends a StartupPacket. The postmaster uses this info 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 successful, 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 successful, 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. After sending AuthenticationOk, the postmaster attempts to launch a backend process. Since this might fail, or the backend might encounter a failure during startup, the frontend must wait for the backend to acknowledge successful startup. The frontend should send no messages at this point. The possible messages from the backend during this phase are: BackendKeyData This message is issued after successful backend startup. It 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 Backend startup is successful. The frontend may now issue query or function call messages. ErrorResponse Backend startup 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 startup phase), or to consider ReadyForQuery as ending the startup phase and each subsequent query cycle. 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 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 whether 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 An empty query string was recognized. (The need to specially distinguish this case is historical.) 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, ie. 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. 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 postmaster. 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(l) commands then it must be prepared to accept NotificationResponse messages at any time; see below. 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, ie. 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(l) commands then it must be prepared to accept NotificationResponse messages at any time; see below. If a frontend issues a listen(l) command, then the backend will send a NotificationResponse message (not to be confused with NoticeResponse!) whenever a notify(l) command is executed for the same notification name. Notification responses are permitted at any point in the protocol (after startup), 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(l) command has been executed for a name for which a previous listen(l) 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. During the processing of a query, the frontend may request cancellation of the query by sending an appropriate request to the postmaster. The cancel request is not sent directly 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 postmaster and sends a CancelRequest message, rather than the StartupPacket message that would ordinarily be sent across a new connection. The postmaster 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 startup. If the request matches the PID and secret key for a currently executing backend, the postmaster signals the backend to abort processing of the current 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 to the postmaster 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. 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 postmaster, if it doesn't want to terminate itself. This section describes the base data types used in messages. Intn(i) An n bit integer in network byte order. If i is specified it is the literal value. Eg. Int16, Int32(42). LimStringn(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 its buffers aren't big enough. Is 8193 bytes the largest allowed size? If s is specified it is the literal value. Eg. String, String("user"). Byten(c) Exactly n bytes. If c is specified it is the literal value. Eg. Byte, Byte1('\n'). 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 as an ASCII data row. (A prior RowDescription message defines the number of fields in the row and their data types.) Byten 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 with a non-NULL value, there is the following: Int32 Specifies the size of the value of the field, including this size. Byten 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 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. 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. 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.) Byten 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 with a non-NULL value, there is the following: Int32 Specifies the size of the value of the field, excluding this size. Byten 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 (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 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. 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. Byten 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. Byten 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 backend process. String The name of the relation that the notify has been raised on. 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 (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. Int32 Specifies the type modifier. 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.