- uses a service already in my environment,
- message is temporary, and
- is automatically sent to all listening.
Decided on the PostgreSQL NOITFY - LISTEN pair to send simple asynchronous event notifications. All that's required is a database connection (already there) to a shared database (already done). The PHP code is pretty straightforward. Here's a snippit...
#!/usr/bin/php
<?php
// Open some connections to our database
$dbh1 = pg_connect("dbname=postgres");
$dbh2 = pg_connect("dbname=postgres");
$dbh3 = pg_connect("dbname=postgres");
// Prime the connection
Database::asyncListen($dbh2, 'listen_test');
Database::asyncListen($dbh3, 'listen_test');
// Send notification
Database::asyncSend($dbh1, 'listen_test');
// Listen for notification on current connections
Database::asyncListen($dbh2, 'listen_test');
Database::asyncListen($dbh3, 'listen_test');
// Test connection signing on after the event
$dbh4 = pg_connect("dbname=postgres");
Database::asyncListen($dbh4, 'listen_test');
class Database {
public function asyncSend($conn, $event) {
pg_query($conn, "NOTIFY {$event};");
echo "event created\n";
}
public function asyncListen($conn, $event) {
pg_query($conn, "LISTEN {$event};");
if ( pg_get_notify($conn) !== false ) {
echo "event '{$event}' registered\n";
} else {
echo "nothing\n";
}
}
}
?>
|
Here's the output:
nothing nothing event created event 'listen_test' registered event 'listen_test' registered nothing |
What's this good for? As we race towards distributed computing there has to be a means to communicate effectively. Such communications within PostgreSQL is handy in that LISTEN is incredibly light thus a body to the message / event is not necessary. Multiple LISTEN's can be executed looking for many events for many things. The name is specific to the message. Rather than multiple constant SELECT's on a table looking for a record, using NOTIFY can tell the code when to check the table.
Just one of those little discoveries I felt the need to scream from the rooftops.
