GraphQL PubSub

PubSub sits at the heart of subscriptions in GraphQL. This guide walks through the concept step by step, with examples, a cheatsheet, and common mistakes to avoid.

PubSub Overview

PubSub is a building block you will reach for often in GraphQL. It keeps related logic together and makes your intent obvious to reviewers and future maintainers.

When you learn pubsub properly, you avoid the guesswork that leads to bugs and rework. The example below shows the shape you will use in most real GraphQL projects.

const resolvers = {
  Subscription: {
    postAdded: {
      subscribe: () => pubsub.asyncIterator(['POST_ADDED']),
    },
  },
};

// elsewhere, when a post is created:
pubsub.publish('POST_ADDED', { postAdded: newPost });

Subscriptions push real-time updates to clients over a persistent connection.

PubSub Example

const typeDefs = gql`
  type Query { hello: String! }
`;
const resolvers = { Query: { hello: () => 'world' } };
const server = new ApolloServer({ typeDefs, resolvers });

Start from a minimal PubSub example and grow it only as needed.
Keep configuration explicit so PubSub behaves the same in every environment.
Name things clearly so teammates understand your PubSub at a glance.
Add tests around PubSub early to lock in expected behaviour.

GraphQL Cheatsheet

Quick GraphQL reference related to pubsub.

Concept	Example	Purpose
Schema	`type Query { user(id: ID!): User }`	Define the API shape
Resolver	`Query: { user: (_, { id }) => ... }`	Provide field data
Query	`query { user(id: 1) { name } }`	Read exactly what you need
Mutation	`mutation { createUser(input) { id } }`	Change data
Subscription	`subscription { postAdded { id } }`	Real-time updates
Context	`context: ({ req }) => ({ user })`	Auth and shared state
DataLoader	`loader.load(id)`	Batch to avoid N+1

How PubSub Works in GraphQL

PubSub fits into GraphQL's model of a single typed schema that clients query for exactly the data they need. The server resolves each requested field through resolver functions.

Subscriptions push real-time updates to clients over a persistent connection.

The schema is the contract between client and server.
Resolvers fetch data field by field, including nested types.
Clients request only the fields they use, avoiding over-fetching.
Context carries auth and shared services into every resolver.

Practical Guidance for PubSub

In production, pubsub should be efficient and secure. Batch data access with DataLoader, guard resolvers with authorization, and limit query depth and complexity.

Concern	Recommendation
N+1 queries	Batch with DataLoader
Security	Auth in context, depth/complexity limits
Errors	Typed GraphQLError with extension codes
Performance	Cache and paginate large lists

Common Mistakes

Skipping error handling and edge cases when wiring up pubsub.
Leaving pubsub untested, so regressions slip into production.
Over-engineering pubsub before you actually need the extra flexibility.
Ignoring documentation, which makes pubsub hard for the next developer to change.

Key Takeaways

PubSub is a core part of working effectively with GraphQL.
Start small and keep pubsub focused on a single responsibility.
Apply consistent patterns so pubsub scales across your project.
Test and document pubsub to keep it maintainable over time.

Pro Tip

Pair pubsub with automated tests from day one. It is far cheaper to catch GraphQL regressions in CI than in production.