Graphql PG Subscription

A graphql subscriptions implementation using postgres and apollo's graphql-subscriptions.

This package implements the PubSubEngine Interface from the graphql-subscriptions package and also the new AsyncIterator interface. It allows you to connect your subscriptions manger to a postgres based Pub Sub mechanism to support multiple subscription manager instances.

Features

• Properly Maintained
• Small In Size
• Larger Payload Problem Solved (I use pg table for the payload. So now larger payload can pass) - (PG Default Notify has size limit, that's why you can't provide larger payload.);
• Fully TypeScript Support
• Worked with any framework (ExpressJS, NestJS)

Installation

npm i graphql-pg-subscriptions

Usage

First of all, follow the instructions in graphql-subscriptions to add subscriptions to your app.

Afterwards replace PubSub with PostgresPubSub:

// Before
import { PubSub } from "graphql-subscriptions";

export const pubsub = new PubSub();

//After
import { PostgresPubSub } from "graphql-postgres-subscriptions";
import { Client } from "pg";

const client = new Client({
    user: 'dbuser',
    host: 'database.server.com',
    database: 'mydb',
    password: 'secretpassword',
    port: 3211,
});

client.connect();

const pubsub = new PostgresPubSub({ client, maxListeners: 15 });

//You can increase max event listeners if you need, default is 15

commonMessageHandler

The second argument to new PostgresPubSub() is the commonMessageHandler. The common message handler gets called with the received message from PostgreSQL. You can transform the message before it is passed to the individual filter/resolver methods of the subscribers. This way it is for example possible to inject one instance of a DataLoader which can be used in all filter/resolver methods.

const getDataLoader = () => new DataLoader(...)
const commonMessageHandler = ({attributes: {id}, data}) => ({id, dataLoader: getDataLoader()})
const pubsub = new PostgresPubSub({ client, commonMessageHandler });

export const resolvers = {
  Subscription: {
    somethingChanged: {
      resolve: ({ id, dataLoader }) => dataLoader.load(id)
    }
  }
};

Error handling

PostgresPubSub instances emit a special event called "error". This event's payload is an instance of Javascript's Error. You can get the error's text using error.message.

const ps = new PostgresPubSub({ client });

ps.subscribe("error", err => {
  console.log(err.message); // -> "payload string too long"
}).then(() => ps.publish("a", "a".repeat(9000)));

For example you can log all error messages (including stack traces and friends) using something like this:

ps.subscribe("error", console.error);

Stay in touch

• Author - Siam Ahnaf
• Website - https://www.siamahnaf.com/
• Twitter - https://twitter.com/siamahnaf198
• Github - https://github.com/siamahnaf