Synchronous architectures are simple and intuitive: Service A calls Service B, waits for a response, and uses the result. This pattern breaks down in several important scenarios — when Service B is temporarily unavailable, when the call volume spikes beyond what B can handle synchronously, or when you want Service A to continue working regardless of whether B succeeds.
Event-driven architecture introduces a message queue between producers and consumers, decoupling their operation in time and in failure modes.
When to Reach for a Queue
- Background processing: Any operation that does not need to complete before the HTTP response (sending emails, generating reports, processing uploads).
- Load levelling: When bursts of traffic would overwhelm a downstream service, queues absorb the burst and drain at a sustainable rate.
- Decoupling: When you want to add consumers to an event stream without changing the producer.
- Retry semantics: When failures should be retried with backoff rather than propagated immediately.
- Fan-out: When a single event should trigger multiple independent consumers.
Choosing Between RabbitMQ, SQS, and Kafka
- AWS SQS: Managed, zero operational overhead. The right choice for most applications already on AWS.
- RabbitMQ: Rich routing features (topic exchanges, routing keys). Good for complex message routing with modest throughput.
- Apache Kafka: Designed for very high throughput and durable event log semantics. Right for event sourcing, stream processing, and millions of events per second.
Idempotency: The Non-Negotiable Requirement
In any queue-based system, messages can be delivered more than once. Your consumers must handle duplicate delivery correctly — processing the same message twice should produce the same result as processing it once. Idempotency keys, conditional updates, and deduplication windows are the standard mechanisms.
Building a system that needs queue-based architecture?
Asquarify designs and implements event-driven systems that handle load spikes and maintain resilience. Talk to us about your architecture requirements.
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