Provided by: postgres-xc-client_1.1-2ubuntu2_amd64 
NAME
NOTIFY - generate a notification
SYNOPSIS
NOTIFY channel [ , payload ]
DESCRIPTION
Note
The following description applies only to Postgres-XC
NOTIFY statement works only locally to the Coordinator or the Database. That is, it can send an event
only locally to the node where the command is issued.
The NOTIFY command sends a notification event together with an optional “payload” string to each client
application that has previously executed LISTEN channel for the specified channel name in the current
database.
NOTIFY provides a simple interprocess communication mechanism for a collection of processes accessing the
same PostgreSQL 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 session's 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 what's new”. But
no such association is enforced by the NOTIFY and LISTEN 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.
When NOTIFY is used to signal the occurrence of changes to a particular table, a useful programming
technique is to put the NOTIFY 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.
NOTIFY interacts with SQL transactions in some important ways. Firstly, if a NOTIFY 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
somehowbut the server cannot “take back” 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 NOTIFY 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, NOTIFY 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 NOTIFY 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 session's server process PID (supplied in the notification event message)
is the same as one's own session's PID (available from libpq). When they are the same, the notification
event is one's own work bouncing back, and can be ignored.
PARAMETERS
channel
Name of the notification channel to be signaled (any identifier).
payload
The “payload” string to be communicated along with the notification. This must be specified as a
simple string literal. In the default configuration it must be shorter than 8000 bytes. (If binary
data or large amounts of information need to be communicated, it's best to put it in a database table
and send the key of the record.)
NOTES
There is a queue that holds notifications that have been sent but not yet processed by all listening
sessions. If this queue becomes full, transactions calling NOTIFY will fail at commit. The queue is quite
large (8GB in a standard installation) and should be sufficiently sized for almost every use case.
However, no cleanup can take place if a session executes LISTEN and then enters a transaction for a very
long time. Once the queue is half full you will see warnings in the log file pointing you to the session
that is preventing cleanup. In this case you should make sure that this session ends its current
transaction so that cleanup can proceed.
A transaction that has executed NOTIFY cannot be prepared for two-phase commit.
pg_notify
To send a notification you can also use the function pg_notify(text, text). The function takes the
channel name as the first argument and the payload as the second. The function is much easier to use than
the NOTIFY command if you need to work with non-constant channel names and payloads.
EXAMPLES
Configure and execute a listen/notify sequence from psql:
LISTEN virtual;
NOTIFY virtual;
Asynchronous notification "virtual" received from server process with PID 8448.
NOTIFY virtual, 'This is the payload';
Asynchronous notification "virtual" with payload "This is the payload" received from server process with PID 8448.
LISTEN foo;
SELECT pg_notify('fo' || 'o', 'pay' || 'load');
Asynchronous notification "foo" with payload "payload" received from server process with PID 14728.
COMPATIBILITY
There is no NOTIFY statement in the SQL standard.
SEE ALSO
LISTEN(7), UNLISTEN(7)