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PostgreSQL 8.1.4 Documentation | ||||
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Prev | Fast Backward | Chapter 28. libpq - C Library | Fast Forward | Next |
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 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.)
Note: In PostgreSQL 6.4 and later, the be_pid is that of the notifying server process, whereas in earlier versions it was always the PID of your own server process.
Example 28-2 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.