Asynchronous Notification

PostgreSQL 8.4.21 Documentation
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# 30.7. Asynchronous Notification

PostgreSQL offers asynchronous notification via the `LISTEN` and `NOTIFY` commands. A client session registers its interest in a particular notification condition with the `LISTEN` command (and can stop listening with the `UNLISTEN` command). All sessions listening on a particular condition will be notified asynchronously when a `NOTIFY` command with that condition name is executed by any session. No additional information is passed from the notifier to the listener. Thus, typically, any actual data that needs to be communicated is transferred through a database table. Commonly, the condition name is the same as the associated table, but it is not necessary for there to be any associated table.

libpq applications submit `LISTEN` and `UNLISTEN` commands as ordinary SQL commands. The arrival of `NOTIFY` messages can subsequently be detected by calling `PQnotifies`.

The function `PQnotifies` returns the next notification from a list of unhandled notification messages received from the server. It returns a null pointer if there are no pending notifications. Once a notification is returned from `PQnotifies`, it is considered handled and will be removed from the list of notifications.

       PGnotify *PQnotifies(PGconn *conn);

       typedef struct pgNotify {
           char *relname;              /* notification condition name */
           int  be_pid;                /* process ID of notifying server process */
           char *extra;                /* notification parameter */
       } PGnotify;

After processing a `PGnotify` object returned by `PQnotifies`, be sure to free it with `PQfreemem`. It is sufficient to free the `PGnotify` pointer; the `relname` and `extra` fields do not represent separate allocations. (At present, the `extra` field is unused and will always point to an empty string.)

Example 30-2 gives a sample program that illustrates the use of asynchronous notification.

`PQnotifies` does not actually read data from the server; it just returns messages previously absorbed by another libpq function. In prior releases of libpq, the only way to ensure timely receipt of `NOTIFY` messages was to constantly submit commands, even empty ones, and then check `PQnotifies` after each `PQexec`. While this still works, it is deprecated as a waste of processing power.

A better way to check for `NOTIFY` messages when you have no useful commands to execute is to call `PQconsumeInput`, then check `PQnotifies`. You can use `select()` to wait for data to arrive from the server, thereby using no CPU power unless there is something to do. (See `PQsocket` to obtain the file descriptor number to use with `select()`.) Note that this will work OK whether you submit commands with `PQsendQuery`/`PQgetResult` or simply use `PQexec`. You should, however, remember to check `PQnotifies` after each `PQgetResult` or `PQexec`, to see if any notifications came in during the processing of the command.

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