What is the difference between request-response and event-based (fire-and-forget) message patterns in NestJS microservices?

Q: What is the difference between request-response and event-based (fire-and-forget) message patterns in NestJS microservices?

// Microservice — request-response handler @MessagePattern({ cmd: 'get_user' }) getUser(@Payload() data: { id: number }): Promise { return this.usersService.findOne(data.id); // Return value is serialized and sent back to caller } // Microservice — fire-and-forget event handler @EventPattern('user_created') handleUserCreated(@Payload() data: UserCreatedEvent): void { this.notificationService.sendWelcomeEmail(data.email); // Return value is ignored — no reply sent } // Client — request-response (returns Observable, awaited with firstValueFrom) const user = await firstValueFrom( this.client.send ({ cmd: 'get_user' }, { id: 1 }), ); // Client — fire-and-forget (subscribe to trigger the send) this.client.emit('user_created', { email: 'a@b.com' }).subscribe(); @MessagePattern + send() — use when the caller needs data back (query operations, synchronous lookups). @EventPattern + emit() — use when notifying other services that something happened (domain events). send() returns an Observable — always convert with firstValueFrom() or lastValueFrom() for async/await code. emit() also returns an Observable — call .subscribe() to trigger the actual send. Fire-and-forget decouples services temporally — the emitter does not wait for any consumer to be ready.

Difficulty: 5/10

request-response, event-based, NestJS microservices

Request-response uses @MessagePattern() on the handler and client.send() on the caller — the sender waits for a reply returned as an Observable. Event-based uses @EventPattern() on the handler and client.emit() on the caller — the sender fires and forgets with no reply expected or returned.

@MessagePattern vs @EventPattern and their client counterparts

When to use each pattern:

1
@MessagePattern + send() — use when the caller needs data back (query operations, synchronous lookups).
2
@EventPattern + emit() — use when notifying other services that something happened (domain events).

What is the difference between request-response and event-based (fire-and-forget) message patterns in NestJS microservices?

Difficulty: 5/10

request-response, event-based, NestJS microservices

Request-response uses @MessagePattern() on the handler and client.send() on the caller — the sender waits for a reply returned as an Observable. Event-based uses @EventPattern() on the handler and client.emit() on the caller — the sender fires and forgets with no reply expected or returned.

@MessagePattern vs @EventPattern and their client counterparts

When to use each pattern:

1
@MessagePattern + send() — use when the caller needs data back (query operations, synchronous lookups).
2
@EventPattern + emit() — use when notifying other services that something happened (domain events).

What is the difference between request-response and event-based (fire-and-forget) message patterns in NestJS microservices?

Request-response uses @MessagePattern() on the handler and client.send() on the caller — the sender waits for a reply returned as an Observable. Event-based uses @EventPattern() on the handler and client.emit() on the caller — the sender fires and forgets with no reply expected or returned.

What is the difference between request-response and event-based (fire-and-forget) message patterns in NestJS microservices?

Request-response uses @MessagePattern() on the handler and client.send() on the caller — the sender waits for a reply returned as an Observable. Event-based uses @EventPattern() on the handler and client.emit() on the caller — the sender fires and forgets with no reply expected or returned.

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