command sends a notification event together with an optional
string to each client application that has previously executed
for the specified channel name in the current database. Notifications are visible to all users.
provides a simple interprocess communication mechanism for a collection of processes accessing the same
database. A payload string can be sent along with the notification, and higher-level mechanisms for passing structured data can be built by using tables in the database to pass additional data from notifier to listener(s).
The information passed to the client for a notification event includes the notification channel name, the notifying sessions server process
PID, and the payload string, which is an empty string if it has not been specified.
It is up to the database designer to define the channel names that will be used in a given database and what each one means. Commonly, the channel name is the same as the name of some table in the database, and the notify event essentially means,
I changed this table, take a look at it to see whats new. But no such association is enforced by the
commands. For example, a database designer could use several different channel names to signal different sorts of changes to a single table. Alternatively, the payload string could be used to differentiate various cases.
is used to signal the occurrence of changes to a particular table, a useful programming technique is to put the
in a rule that is triggered by table updates. In this way, notification happens automatically when the table is changed, and the application programmer cannot accidentally forget to do it.
interacts with SQL transactions in some important ways. Firstly, if a
is executed inside a transaction, the notify events are not delivered until and unless the transaction is committed. This is appropriate, since if the transaction is aborted, all the commands within it have had no effect, including
NOTIFY. But it can be disconcerting if one is expecting the notification events to be delivered immediately. Secondly, if a listening session receives a notification signal while it is within a transaction, the notification event will not be delivered to its connected client until just after the transaction is completed (either committed or aborted). Again, the reasoning is that if a notification were delivered within a transaction that was later aborted, one would want the notification to be undone somehow but the server cannot
a notification once it has sent it to the client. So notification events are only delivered between transactions. The upshot of this is that applications using
for real-time signaling should try to keep their transactions short.
If the same channel name is signaled multiple times from the same transaction with identical payload strings, the database server can decide to deliver a single notification only. On the other hand, notifications with distinct payload strings will always be delivered as distinct notifications. Similarly, notifications from different transactions will never get folded into one notification. Except for dropping later instances of duplicate notifications,
guarantees that notifications from the same transaction get delivered in the order they were sent. It is also guaranteed that messages from different transactions are delivered in the order in which the transactions committed.
It is common for a client that executes
to be listening on the same notification channel itself. In that case it will get back a notification event, just like all the other listening sessions. Depending on the application logic, this could result in useless work, for example, reading a database table to find the same updates that that session just wrote out. It is possible to avoid such extra work by noticing whether the notifying sessions server process
(supplied in the notification event message) is the same as ones own sessions
libpq). When they are the same, the notification event is ones own work bouncing back, and can be ignored.