36.8. Asynchronous Notification
Postgres Pro offers asynchronous notification via the LISTEN
and NOTIFY
commands. A client session registers its interest in a particular notification channel with the LISTEN
command (and can stop listening with the UNLISTEN
command). All sessions listening on a particular channel will be notified asynchronously when a NOTIFY
command with that channel name is executed by any session. A “payload” string can be passed to communicate additional data to the listeners.
libpq applications submit LISTEN
, UNLISTEN
, and NOTIFY
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 channel name */ int be_pid; /* process ID of notifying server process */ char *extra; /* notification payload string */ } 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. (The names of these fields are historical; in particular, channel names need not have anything to do with relation names.)
Example 36.2 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 ancient 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.