How do you filter subscription events so each client only receives events relevant to them in NestJS?

Q: How do you filter subscription events so each client only receives events relevant to them in NestJS?

// Filter by subscription argument — only receive messages for the subscribed channel @Subscription(() => Message, { filter: (payload, variables) => payload.messageAdded.channelId === variables.channelId, resolve: (payload) => payload.messageAdded, // reshape payload before delivery }) messageAdded( @Args('channelId', { type: () => Int }) channelId: number, ) { return this.pubSub.asyncIterator('messageAdded'); } // Auth filter — only deliver to the post's author @Subscription(() => Post, { filter(payload, _variables, context) { return payload.postUpdated.authorId === context.req.user.id; }, }) @UseGuards(GqlAuthGuard) postUpdated(@CurrentUser() user: User) { return this.pubSub.asyncIterator('postUpdated'); } filter(payload, variables, context) — payload is the published event, variables are the subscription args. Return true to deliver the event; return false or undefined to skip it for this subscriber. The context third argument provides the authenticated user — use it for per-user security filtering. resolve option reshapes the payload before delivery — useful to unwrap a nested field. Always filter server-side — never rely on clients to discard events they should not see.

Difficulty: 5/10

WebSocket gateway, GraphQL subscription filter, RxJS operators

Use the filter option on @Subscription(). The filter function receives the event payload as the first argument and the subscription variables as the second. Return true to deliver the event to the subscriber, false to skip it. The context (third argument) provides the authenticated user for per-user authorization filters.

Subscription filter function with variables and context

Subscription filter best practices:

1
filter(payload, variables, context) — payload is the published event, variables are the subscription args.
2
Return true to deliver the event; return false or undefined to skip it for this subscriber.

How do you filter subscription events so each client only receives events relevant to them in NestJS?

Difficulty: 5/10

WebSocket gateway, GraphQL subscription filter, RxJS operators

Use the filter option on @Subscription(). The filter function receives the event payload as the first argument and the subscription variables as the second. Return true to deliver the event to the subscriber, false to skip it. The context (third argument) provides the authenticated user for per-user authorization filters.

Subscription filter function with variables and context

Subscription filter best practices:

1
filter(payload, variables, context) — payload is the published event, variables are the subscription args.
2
Return true to deliver the event; return false or undefined to skip it for this subscriber.

How do you filter subscription events so each client only receives events relevant to them in NestJS?

How do you filter subscription events so each client only receives events relevant to them in NestJS?

Scenario Questions

Follow-up Questions