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.