pgee

PostgreSQL asynchronous notification event emitter. pgee allows PostgreSQL's LISTEN and NOTIFY features to be used as a Node.js EventEmitter.

An example use of pgee is shown below. pgee exports a single constructor that extends EventEmitter. The constructor accepts anything that can be passed to pg.connect(), or an already connected pg.Client instance. The pgee.prototype.listen() and pgee.prototype.unlisten() methods are used to LISTEN and UNLISTEN channels in the database. Notifications received from the database are emitted as events. Similarly, the pgee.prototype.emit() method triggers a NOTIFY on the database channel.

const CONNECT_STRING = `postgres://${process.env.USER}@localhost/postgres`;
const PgEe = require('pgee');

// The constructor accepts a pg.Client, connection string, or any valid
// pg connection configuration.
const pgee = new PgEe(CONNECT_STRING);

pgee.connect((err) => {
  if (err) {
    return console.error(err);
  }

  // Listen on "foo" channel. When a notification is received, emit a "foo"
  // event, triggering the listener function.
  pgee.listen({
    channel: 'foo',
    listener: (data) => {
      console.log(`Received 'foo' event: ${JSON.stringify(data)}`);
    }
  }, (err, channel) => {
    if (err) {
      return console.error(err);
    }

    pgee.emit('foo', {bar: 'baz'});
  });
});

API

pgee(options) Constructor

• Arguments
  • options (varies) - Options to pass to pg.connect() or a connected instance of pg.Client (see the pg module for details).
• Returns
  • object - A newly constructed pgee instance.

Constructs a new pgee instance, which extends EventEmitter. The constructor does not attempt to connect to a database.

pgee.prototype.connect([callback])

• Arguments
  • callback (function) - An optional callback function that is invoked after connecting to the database. The callback only receives a possible error argument.
• Returns
  • Nothing

Connects to a PostgreSQL database using the credentials provided to the constructor. The connection is taken from the pg module's connection pool. If an instance of pg.Client was passed to the constructor, it is assumed to already be connected, so no further action is taken. If callback is a function, it is called upon completion. If a function was not provided, a 'connect' event is emitted.

pgee.prototype.close()

• Arguments
  • None
• Returns
  • Nothing

Closes the pgee instance. If the connection was taken from the pg connection pool during connect(), it will be returned to the pool. If an external pg.Client was provided to the constructor, no action will be taken on the client. All attached event listeners are removed from the instance.

pgee.prototype.listen(channel [, callback])

• Arguments
  • channel (string or object) - If this is a string, it represents the name of the channel to subscribe to. If is is an option, it supports the following properties.
    • channel (string) - The channel name to subscribe to.
    • listener (function) - An optional event listener that triggers on channel events. The data associated with the database notification is passed as the only argument.
  • callback (function) - An optional callback function. If an error occurs, it is passed as the first argument of the callback. On success, the second argument of the callback is the channel name as a string.
• Returns
  • Nothing

Issues a LISTEN "channel" to the database. If a listener is provided, it is attached using the on() method. Additional listeners can be attached later using on(), addListener(), once(), or listen(). After the LISTEN command is issued, the callback function is invoked (if one is present). If no callback was provided, a 'listen' event is emitted, containing the name of the channel that was added.

pgee.prototype.unlisten(channel [, callback])

• Arguments
  • channel (string or object) - If this is a string, it represents the name of the channel to unsubscribe from. If is is an option, it supports the following properties.
    • channel (string) - The channel name to subscribe to.
    • removeListeners (boolean) - An optional value that, if true, removes all channel listeners. Defaults to false.
  • callback (function) - An optional callback function. If an error occurs, it is passed as the first argument of the callback. On success, the second argument of the callback is the channel name as a string.
• Returns
  • Nothing

Issues an UNLISTEN "channel" to the database. If removeListeners is true, any existing channel listeners are removed using removeAllListeners(channel). Listeners can also be removed later using removeAllListeners(), unlisten(), etc. After the UNLISTEN command is issued, the callback function is invoked (if one is present). If no callback was provided, an 'unlisten' event is emitted, containing the name of the channel that was removed.

pgee.prototype.emit(event, message)

• Arguments
  • event (string) - Name of the event to emit. If the event name is 'error', an error is emitted immediately. Any other value triggers a database notification, where the channel name is equal to event.
  • message (varies) - Data attached to the emitted event.
• Returns
  • Nothing

Issues a pg_notify() to the database, where the channel name is event, and message represents the notification payload. If event is 'error', then no request is made to the database, and an error is immediately emitted. Note that the names 'connect', 'listen', and 'unlisten' have the potential to conflict with other events used in pgee.