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pg_listen — sets or changes a callback for asynchronous NOTIFY messages
pg_listen dbHandlenotifyNamecallbackCommand
Inputs
dbHandle
Specifies a valid database handle.
notifyName
Specifies the notify condition name to start or stop listening to.
callbackCommand
If present and not empty, provides the command string to execute
when a matching notification arrives.
Outputs
None
Description
pg_listen creates, changes, or cancels a request
to listen for asynchronous NOTIFY messages from the
Postgres backend. With a callbackCommand
parameter, the request is established, or the command string of an already
existing request is replaced. With no callbackCommand parameter, a prior
request is canceled.
After a pg_listen request is established,
the specified command string is executed whenever a NOTIFY message bearing
the given name arrives from the backend. This occurs when any
Postgres client application issues a NOTIFY command
referencing that name. (Note that the name can be, but does not have to be,
that of an existing relation in the database.)
The command string is executed from the Tcl idle loop. That is the normal
idle state of an application written with Tk. In non-Tk Tcl shells, you can
execute update or vwait to cause
the idle loop to be entered.
You should not invoke the SQL statements LISTEN or UNLISTEN directly when
using pg_listen. Pgtcl takes care of issuing those
statements for you. But if you want to send a NOTIFY message yourself,
invoke the SQL NOTIFY statement using pg_exec.
