Questions
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1What is the difference between request-response and event-based (fire-and-forget) message patterns in NestJS microservices?
2What is the @Payload() decorator and how do you use @Ctx() to access transport-specific context in NestJS microservices?
3How do you connect a NestJS microservice to RabbitMQ?
4How do you implement manual message acknowledgement with RabbitMQ in NestJS?
5How do you implement a RabbitMQ topic exchange for routing messages to multiple queues based on routing keys in NestJS?
6How does gRPC differ from REST and when would you choose it for inter-service communication in NestJS?
7How do you implement an API Gateway pattern in NestJS that aggregates multiple microservices?
8How do you implement distributed tracing across NestJS microservices?
9What transport layers does NestJS support for microservices and how do you bootstrap a microservice?
10How do you connect a NestJS microservice to Apache Kafka?
11How do you handle Kafka message patterns and access message headers and partition info via KafkaContext in NestJS?
12How do you set up gRPC in a NestJS microservice?
13How do you implement a circuit breaker pattern when calling a downstream microservice in NestJS?
14How do you inject and use a ClientProxy to communicate with another microservice in NestJS?
15How do you handle errors in microservice request-response patterns and propagate them to the caller in NestJS?
16How do you implement Kafka consumer groups and what guarantees does partition assignment give you in NestJS?
17How do you implement the outbox pattern with Kafka in NestJS to guarantee exactly-once message delivery?
18How do you implement gRPC streaming including server-side, client-side, and bidirectional streaming in NestJS?
19What is the difference between @EventPattern() and @MessagePattern() in NestJS microservices?
20How do you implement the Saga pattern for distributed transactions across NestJS microservices?
21How do you implement idempotent event handlers to safely handle duplicate message delivery in NestJS?
22How do you implement event sourcing with NestJS microservices?
23How do you implement retry logic with exponential backoff for failed microservice calls in NestJS?
24How do you test a NestJS microservice controller in isolation without a real message broker?
25How do you implement service discovery and load balancing across NestJS microservice instances?
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How do you implement idempotent event handlers to safely handle duplicate message delivery in NestJS?

Store processed event IDs in a deduplification table. Use a database transaction to atomically attempt inserting the event ID and execute the handler — if the insert is skipped due to a duplicate key, the handler is not called. This ensures each event is processed exactly once even with at-least-once delivery semantics.

Idempotency via deduplication table
Idempotency implementation rules:
  1. 1

    Every event must carry a unique eventId — generate a UUID on the producer side before publishing.

  2. 2

    The deduplication insert and the handler run in the same database transaction — atomic together.

  3. 3

    orIgnore() (or ON CONFLICT DO NOTHING in SQL) makes the insert a no-op for duplicate event IDs.

  4. 4

    Clean up old processed events periodically — retain them only long enough to cover maximum redelivery windows.

  5. 5

    Idempotency is essential for Kafka (at-least-once), RabbitMQ with requeuing, and any retry mechanism.