PostgreSQL 8.3.23 Documentation | ||||
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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.