Question

I have an “article” table on a Postgresql 9.1 database and a trigger that notifies a channel on each insert.

I’d like to create a node.js script that catches those inserts and pushes notifications to connected clients using Socket.io. So far I’m using the node-postgres module to LISTEN to the channel but it seems the LISTEN query times out after about 10-15 seconds and stops catching the inserts. I could query a new listen when the timeout happens, but I’m not sure how to properly implement the continuation.

Here’s my postgresql notification procedure:

CREATE FUNCTION article_insert_notify() RETURNS trigger AS $$
BEGIN
  NOTIFY "article_watcher";
  RETURN NULL;
END;
$$ LANGUAGE plpgsql;

The trigger:

CREATE TRIGGER article_insert_trigger
AFTER INSERT ON article
FOR EACH ROW EXECUTE PROCEDURE article_insert_notify();

And the node.js code:

var pg = require ('pg'),
    pgConnection = "postgres://user:pass@localhost/db"

pg.connect(pgConnection, function(err, client) {
    client.query('LISTEN "article_watcher"');
    client.on('notification', function(data) {
        console.log(data.payload);
    });
});

How can I ensure a fulltime LISTEN or how could I catch those timeouts to reissue a listen query ? Or maybe a module other than node-postgres offers more appropriate tools to do so ?

Answer 1

I got answer to my issue on the node-postgres repo. To quote Brianc:

pg.connect is use to create pooled connections. Using a connection
pool connection for listen events really isn’t supported or a good
idea though.
[…]
To ‘listen’ a connection by definition must stay open permanently. For
a connection to stay open permanently it can never be returned to the
connection pool.

The correct way to listen in this case is to use a standalone client:

var pg = require ('pg'),
    pgConnectionString = "postgres://user:pass@localhost/db";

var client = new pg.Client(pgConnectionString);
client.connect();
client.query('LISTEN "article_watcher"');
client.on('notification', function(data) {
    console.log(data.payload);
});