PostgreSQL supports asynchronous notification via the LISTEN and NOTIFY commands. A backend registers its interest in a particular notification condition with the LISTEN command (and can stop listening with the UNLISTEN command). All backends listening on a particular condition will be notified asynchronously when a NOTIFY of that condition name is executed by any backend. 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 relation. Commonly the condition name is the same as the associated relation, but it is not necessary for there to be any associated relation.
libpq applications submit LISTEN and UNLISTEN commands
as ordinary SQL command. Subsequently, arrival of NOTIFY messages can be detected by calling
PQnotifies
.
PQnotifies
Returns the next
notification from a list of unhandled notification messages
received from the backend. Returns NULL 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[NAMEDATALEN]; /* name of relation * containing data */ int be_pid; /* process id of backend */ } PGnotify;
After processing a PGnotify
object returned by PQnotifies
,
be sure to free it with free()
to avoid a memory leak.
Note: In PostgreSQL 6.4 and later, the be_pid is that of the notifying backend, whereas in earlier versions it was always the PID of your own backend.
The second sample program gives an example of the use of asynchronous notification.
PQnotifies()
does not actually
read backend data; 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 queries, 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 queries to make is to call PQconsumeInput()
, then check PQnotifies()
. You can use select()
to wait for backend data to arrive,
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 queries 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 query.