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Building a realtime chat app with GraphQL Subscriptions

03 February, 2022 | 4 min read

Learn how subscriptions in Hasura GraphQL Engine can be used to build a full-fledged real-time group chat app on Postgres with React and Apollo.

Pro Tip: There's no backend code involved. Everything is declarative.

TLDR

  • Hasura allows us to build a realtime GraphQL API without writing any backend code.

  • To save bandwith we use a subscription to listen for message events, then run a query to fetch only the new messages.

  • Every two seconds our frontend runs a user_online mutation to populate an online users list.

  • A user_typing mutation is run whenever we type few characters and a subscription is used to fetch the last typing user.

Introduction

Hasura allows us to instantly create a realtime GraphQL API from our data. In this tutorial we walk through creating a group chat application, without needing to write any backend code, using React and Apollo. The focus is on data models that we store in Postgres rather than full chat functionality.

Data Modelling

Users

When a user signs up we insert their chosen username and generate their ID. We also track when they last typed and were seen.

user (
  id SERIAL PRIMARY KEY
  username TEXT UNIQUE
  last_seen timestamp with time zone
  last_typed timestamp with time zone
)

Messages

For our tutorial we will just be inserting messages, not editing or deleting, but if we wanted to in the future Hasura will autogenerate the mutations for us. We could also extend this by adding features such as multiple different chatrooms.

message (
  id SERIAL NOT NULL PRIMARY KEY
  "text" TEXT NOT NULL
  username INT FOREIGN KEY REFERENCES user(username) NOT NULL
  "timestamp" timestamp with time zone DEFAULT now() NOT NULL
)

Online users

To query the users online we create a Postgres view that fetches all users with last_seen less than 10 seconds ago.

CREATE OR REPLACE VIEW public."user_online" AS
SELECT "user".id,
  "user".username,
  "user".last_typed,
  "user".last_seen
FROM "user"
WHERE ("user".last_seen > (now() - '00:00:10'::interval));

Typing Indicator

To query the last person typing we create a similar view with last_typed within the past 2 seconds.

CREATE OR REPLACE VIEW public."user_typing" AS
SELECT "user".id,
  "user".username,
  "user".last_typed,
  "user".last_seen
FROM "user"
WHERE ("user".last_typed > (now() - '00:00:02'::interval));

Frontend

Creating a user

At user signup we insert a single row into the users table

mutation ($username: String!) {
  insert_user_one(object: { username: $username }) {
    id
    username
  }
}

We take the returned id and username and store it in our app's state management.

User online events

Every two seconds we run a mutation to update our user's last_seen value. For example, with React using UseEffect or componentDidMount we could call the mutation inside a setInterval. Be mindful when using setInterval inside a React hook.

mutation ($userId: Int!) {
  update_user_by_pk(pk_columns: { id: $userId }, _set: { last_seen: "now()" }) {
    id
  }
}

Subscribing to new messages

We can use Graphql subscriptions in two ways:

  1. Subscribe to a query and render all the data that comes back.

  2. Use the subscription as an event notification then run our own custom fetching logic.

Option two is best for our chatroom use case because we can do some work to just fetch new messages. Otherwise every new message we would also come with all previous ones.

  1. On load we fetch all existing messages by setting $last_received_id to -1 and $last_received_ts to a timestamp well into the past such as '2018-08-21T19:58:46.987552+00:00'

    query ($last_received_id: Int, $last_received_ts: timestamptz) {
      message(
        order_by: { timestamp: asc }
        where: {
          _and: {
            id: { _neq: $last_received_id }
            timestamp: { _gte: $last_received_ts }
          }
        }
      ) {
        id
        text
        username
        timestamp
      }
    }
    
  2. Now we subscribe to new message events

    subscription {
      message(order_by: { id: desc }, limit: 1) {
        id
        username
        text
        timestamp
      }
    }
    
  3. Once we detect a new message we call our message query with our last known id and timestamp from our client state.

With a bit of magic from realtime subscriptions we saved our users a ton of data by only fetching new messages.

Sending messages

A user sends a message by inserting a row into the messages table. In the future if we setup JWT based authentication in the future we can automatically set the username from the JWT custom claims.

mutation insert_message($message: message_insert_input!) {
  insert_message_one(object: $message) {
    id
    timestamp
    text
    username
  }
}

UI Typing Indicator

  1. Similar to the user online events, when a user types a few characters we run a mutation on their last_typing timestamp.

    mutation ($userId: Int!) {
      update_user_by_pk(
        pk_columns: { id: $userId }
        _set: { last_typed: "now()" }
      ) {
        id
      }
    }
    
  2. Then we subscribe to our Postgres view of the last user typing, making sure to exclude ourselves.

    subscription ($selfId: Int) {
      user_typing(
        where: { id: { _neq: $selfId } }
        limit: 1
        order_by: { last_typed: desc }
      ) {
        last_typed
        username
      }
    }
    

Online Users list

We can easily get a list of all users online by subscribing to the user_online view we created.

subscription {
  user_online(order_by: { username: asc }) {
    id
    username
  }
}

Try it out on Hasura Cloud to create your own backend.

Live Demo

Head to https://realtime-chat.demo.hasura.io/ and try out the Live demo of the chat app.

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Rishichandra Wawhal

Rishichandra Wawhal

I like to write code and lift heavy things

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