Re: Can the frontend get notifications from the postgres server?

On Tue, Jun 17, 2003 at 08:39:00 AM weigelt(at)metux(dot)de [mailto:weigelt(at)metux(dot)de] wrote:

<snip?
<snip>
|>> > But the problem still is: how to receive the NOTIFY from an php script ?
|>> > which function of the c-client lib can be used for this ?
|>>
|>> If you're talking about a PHP script running under apache as a module or
|>> CGI, then you probably can't (or if you can, you shouldn't due to time out
|>> issues and such.)
|>No. The script is running as an standalone process (works as application server).
|>The Problem is, how to get the NOTIFY message, which is received from the
|>RDBMS somewhere in the client lib, to the php script.
|>When using psql, the NOTIFY is displayed after sending an query. It seems
|>that it is sent within the server's reply to the query. But i dont know
|>how the clientlib hands it to the application.
|>
|>cu
|>--

I believe Postgres supports asynchronous notification via the LISTEN and NOTIFY commands. If the backend registers its interest in a particular relation with the LISTEN command, then all backends listening on a particular relation will be notified asynchronously when a NOTIFY of that relation 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 the relation.

libpq applications are notified whenever a connected backend has received an asynchronous notification. However, the communication from the backend to the frontend is not asynchronous. Notification comes piggy-backed on other query results. Thus, an application must submit queries, even empty ones, in order to receive notice of backend notification. In effect, the libpq application must poll the backend to see if there is any pending notification information. After the execution of a query, a frontend may call PQNotifies to see if any notification data is available from the backend.

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