Register PubSub as a provider using an injection token. In the mutation, publish the event after the write. In the subscription resolver method, return pubSub.asyncIterator() with the event name. The @Subscription() return type arrow function and the event payload key must match the asyncIterator event name.

PubSub subscription with publish and subscribe

PubSub subscription rules:

1
Register PubSub with an injection token — inject it into resolvers via @Inject(PUB_SUB).
2
The object key in publish() must match the subscription field name: { postCreated: post }.
3
asyncIterator() argument must match the event name used in publish().
4
In-memory PubSub works only in single-process deployments — use Redis PubSub for multiple instances.
5
Publish after the database write completes — never publish on a failed or rolled-back write.

Difficulty: 5/10

Topics: GraphQL Subscriptions, NestJS PubSub, Resolver design

Scenario Questions

0-2 years experience

1
We need a simple real‑time chat feature where a client subscribes to new messages. Walk me through how you'd set up a GraphQL subscription using NestJS's PubSub in a single module.
2
If you forget to call pubSub.publish after creating a message, what will the subscriber see and why?
3
How would you write a unit test to verify that your subscription resolver receives the published events?

2-5 years experience

1
Our product team wants to add a subscription for order status updates, but the status changes are emitted from a separate microservice via a message queue. How would you integrate that with NestJS PubSub and what trade‑offs would you consider?
2
During a load test, you notice that some subscription events are being dropped. What debugging steps would you take in the NestJS GraphQL layer?
3
Explain why you might choose NestJS's built‑in PubSub versus an external Redis PubSub adapter in a mid‑scale deployment.

5-8 years experience

1
Design a scalable subscription system for a live dashboard that streams metrics from thousands of devices. How would you structure the PubSub, resolver, and any back‑pressure handling in NestJS?
2
What are the implications of using the default in‑memory PubSub in a horizontally scaled NestJS cluster, and how would you modify the architecture to support multiple instances?
3
How would you secure subscription connections to ensure only authorized users receive events, and what changes are needed in the GraphQL subscription handshake?

8+ years experience

1
Our organization is moving from a monolith to a micro‑frontend architecture, and we need to migrate existing GraphQL subscriptions to a shared event bus. What architectural changes would you propose for the NestJS PubSub layer to avoid coupling and support versioned schemas?
2
Consider long‑term maintenance of subscription contracts across several teams. How would you establish a strategy for deprecating fields, handling breaking changes, and ensuring backward compatibility in the NestJS GraphQL subscription implementation?
3
If we wanted to replace the PubSub implementation with a cloud‑native service like AWS AppSync, what migration path would you outline, and what pitfalls should we watch for in the existing NestJS codebase?

Follow-up Questions

❓What would you monitor in production to ensure the subscription pipeline stays healthy?
❓Can you compare the performance characteristics of in‑memory PubSub versus Redis?
❓How does the GraphQL subscription handshake differ from a regular query in NestJS?