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NAME="libpqpp-notify"
>1.6. Asynchronous Notification</A
></H1
>
<P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> supports asynchronous
notification via the <TT
CLASS="COMMAND"
>LISTEN</TT
> and
<TT
CLASS="COMMAND"
>NOTIFY</TT
> commands. A backend registers its
interest in a particular notification condition with the
<TT
CLASS="COMMAND"
>LISTEN</TT
> command. All backends that are
listening on a particular condition will be notified
asynchronously when a <TT
CLASS="COMMAND"
>NOTIFY</TT
> of that name is
executed by another backend. No additional information is passed
from the notifier to the listener. Thus, typically, any actual
data that needs to be communicated is transferred through a
relation.
</P
>
<P
> <SPAN
CLASS="APPLICATION"
>libpq++</SPAN
> applications are notified whenever a
connected backend has
received an asynchronous notification. However, the communication from
the backend to the frontend is not asynchronous.
The <SPAN
CLASS="APPLICATION"
>libpq++</SPAN
> application
must poll the backend to see if there is any pending notification
information. After the execution of a command, a frontend may call
<TT
CLASS="FUNCTION"
>PgDatabase::Notifies</TT
>
to see if any notification data is currently available from the backend.
<TT
CLASS="FUNCTION"
>PgDatabase::Notifies</TT
>
returns the notification from a list of unhandled notifications from the
backend. The function returns <TT
CLASS="SYMBOL"
>NULL</TT
> if there are no pending notifications
from the backend.
<TT
CLASS="FUNCTION"
>PgDatabase::Notifies</TT
>
behaves like the popping of a stack. Once a notification is returned
from <TT
CLASS="FUNCTION"
>PgDatabase::Notifies</TT
>,
it is considered handled and will be removed from the list of
notifications.
<P
></P
></P><UL
><LI
> <P
> <TT
CLASS="FUNCTION"
>PgDatabase::Notifies</TT
>
retrieves pending notifications from the server.
</P><PRE
CLASS="SYNOPSIS"
>PGnotify* PgDatabase::Notifies()</PRE
><P>
</P
>
</LI
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><P>
</P
>
<P
> The second sample program gives an example of the use of asynchronous
notification.
</P
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