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.
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:
The server then immediately closes the connection.
The authentication exchange is completed.
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.
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.
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.
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.
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.
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:
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.
Start-up is completed. The frontend may now issue query or function call messages.
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 table. 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 table to the frontend. It then sends a CopyDataRows message, and then a CompletedResponse message with a tag of COPY.
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.
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.
An empty query string was recognized.
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.
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.
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 commands then it must be prepared to accept NotificationResponse messages at any time; see below.
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.
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.
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.
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.
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.
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