PG PubSub

A Publish/Subscribe implementation on top of PostgreSQL NOTIFY/LISTEN

Installation

npm install pg-pubsub --save

Requirements

• Node.js ^14.17.0 || >=16.0.0
• Postgres >= 9.4

Usage

const PGPubsub = require('pg-pubsub');
const pubsubInstance = new PGPubsub(uri[, options]);

Options

{
  [log]: Function // default: silent when NODE_ENV=production, otherwise defaults to console.log(...)
}

Methods

• addChannel(channelName[, eventListener]) – starts listening on a channel and optionally adds an event listener for that event. As PGPubsub inherits from EventEmitter one can also add it oneself. Returns a Promise that resolves when the listening has started.
• removeChannel(channelName[, eventListener]) – either removes all event listeners and stops listeneing on the channel or removes the specified event listener and stops listening on the channel if that was the last listener attached.
• publish(channelName, data) – publishes the specified data JSON-encoded to the specified channel. It may be better to do this by sending the NOTIFY channelName, '{"hello":"world"}' query yourself using your ordinary Postgres pool, rather than relying on the single connection of this module. Returns a Promise that will become rejected or resolved depending on the success of the Postgres call.
• close(): Promise – closes down the database connection and removes all listeners. Useful for graceful shutdowns.
• All EventEmitter methods are inherited from EventEmitter

Examples

Simple

const pubsubInstance = new PGPubsub('postgres://username@localhost/database');

await pubsubInstance.addChannel('channelName', function (channelPayload) {
  // Process the payload – if it was JSON that JSON has been parsed into an object for you
});

await pubsubInstance.publish('channelName', { hello: "world" });

The above sends NOTIFY channelName, '{"hello":"world"}' to PostgreSQL, which will trigger the above listener with the parsed JSON in channelPayload.

Advanced

const pubsubInstance = new PGPubsub('postgres://username@localhost/database');

await pubsubInstance.addChannel('channelName');

// pubsubInstance is a full EventEmitter object that sends events on channel names
pubsubInstance.once('channelName', channelPayload => {
  // Process the payload
});

Description

Creating a PGPubsub instance will not do much up front. It will prepare itself to start a Postgres connection once the first channel is added and then it will keep a connection open until its shut down, reconnecting it if it gets lost, so that it can constantly listen for new notifications.

Lint / Test

• setup a postgres database to run the integration tests
  • the easist way to do this is via docker, docker run -it -p 5432:5432 -e POSTGRES_DB=pgpubsub_test postgres
• npm test

For an all-in-one command, try:

# fire up a new DB container, run tests against it, and clean it up!
docker rm -f pgpubsub_test || true && \
docker run -itd -p 5432:5432 -e POSTGRES_DB=pgpubsub_test --name pgpubsub_test postgres && \
npm test && \
docker rm -f pgpubsub_test