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; /* 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.
