Description
The NOTIFY command sends a notify event to each
frontend application that has previously executed
LISTEN notifyname
for the specified notify condition in the current database.
The information passed to the frontend for a notify event includes the notify
condition name and the notifying backend process's PID. It is up to the
database designer to define the condition names that will be used in a given
database and what each one means.
Commonly, the notify condition 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 condition names
to signal different sorts of changes to a single table.
NOTIFY provides a simple form of signal or
IPC (interprocess communication) mechanism for a collection of processes
accessing the same PostgreSQL database.
Higher-level mechanisms can be built by using tables in the database to
pass additional data (beyond a mere condition name) from notifier to
listener(s).
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 can't 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 we would like all the commands within it to have had no
effect, including NOTIFY. But it can be disconcerting if one
is expecting the notify events to be delivered immediately. Secondly, if
a listening backend receives a notify signal while it is within a transaction,
the notify event will not be delivered to its connected frontend until just
after the transaction is completed (either committed or aborted). Again, the
reasoning is that if a notify were delivered within a transaction that was
later aborted, one would want the notification to be undone somehow---but
the backend cannot "take back" a notify once it has sent it to the frontend.
So notify 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.
NOTIFY behaves like Unix signals in one important
respect: if the same condition name is signaled multiple times in quick
succession, recipients may get only one notify event for several executions
of NOTIFY. So it is a bad idea to depend on the number
of notifies received. Instead, use NOTIFY to wake up
applications that need to pay attention to something, and use a database
object (such as a sequence) to keep track of what happened or how many times
it happened.
It is common for a frontend that sends NOTIFY to be
listening on the same notify name itself. In that case it will get back a
notify event, just like all the other listening frontends. Depending on the
application logic, this could result in useless work---for example,
re-reading a database table to find the same updates that that frontend just
wrote out. In PostgreSQL 6.4 and later, it is
possible to avoid such extra work by noticing whether the notifying backend
process's PID (supplied in the notify event message) is the same as one's own
backend's PID (available from libpq). When they are the same, the notify
event is one's own work bouncing back, and can be ignored. (Despite what was
said in the preceding paragraph, this is a safe technique.
PostgreSQL keeps self-notifies separate from notifies
arriving from other backends, so you cannot miss an outside notify by ignoring
your own notifies.)
Notes
name
can be any string valid as a name;
it need not correspond to the name of any actual table. If
name
is enclosed in double-quotes, it need not even be a syntactically
valid name, but can be any string up to 63 characters long.
In some previous releases of
PostgreSQL,
name
had to be enclosed in double-quotes when it did not correspond to any existing
table name, even if syntactically valid as a name. That is no longer required.
In PostgreSQL releases prior to 6.4, the backend
PID delivered in a notify message was always the PID of the frontend's own
backend. So it was not possible to distinguish one's own notifies from other
clients' notifies in those earlier releases.
