PostgreSQL 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 33.2 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.