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 is divided into an authentication phase and a backend start-up phase.
Initially, the frontend sends a StartupPacket. The postmaster uses this info and the contents of the pg_hba.conf file to determine what authentication method the frontend must use. The postmaster then responds with one of the following messages:
The postmaster then immediately closes the connection.
The postmaster then hands over to the backend. The postmaster takes no further part in the communication.
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.
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.
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.
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 start-up, the frontend must wait for the backend to acknowledge successful start-up. The frontend should send no messages at this point. The possible messages from the backend during this phase are:
This message is issued after successful backend start-up. 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.
Backend start-up is successful. The frontend may now issue query or function call messages.
Backend start-up failed. The connection is closed after sending this message.
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.
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:
An SQL command completed normally.
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".
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".
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".
An empty query string was recognized. (The need to specially distinguish this case is historical.)
An error has occurred.
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.
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.
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:
An error has occurred.
The function call was executed and returned a result.
The function call was executed and returned no result.
Processing of the function call is complete. ReadyForQuery will always be sent, whether processing terminates successfully or with an error.
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(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 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.
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 start-up. 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.