NOTIFY - generate a notification
NOTIFY channel [ , payload ]
The NOTIFY statement is not yet supported in Postgres-XL.
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. Notifications are visible to all users.
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 somehow — but 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.
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.)
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.
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.
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.
There is no
NOTIFY statement in the SQL standard.
LISTEN(7),
UNLISTEN(7)