What is a Message Queue?

A Message Queue is a form of asynchronous service-to-service communication. Messages are stored in a queue until they are processed and deleted by a consumer.

How It Works

Producer                  Queue                  Consumer
    │                       │                       │
    │─── Send Message ─────►│                       │
    │                       │                       │
    │◄── ACK ───────────────│                       │
    │                       │                       │
    │                       │◄── Poll/Pull ─────────│
    │                       │                       │
    │                       │─── Deliver Message ──►│
    │                       │                       │
    │                       │◄── ACK (processed) ───│
    │                       │                       │
    │                       │    (Delete message)   │

Key Concepts

Concept	Description
Producer	Sends messages to queue
Consumer	Receives and processes messages
Queue	Stores messages until consumed
Message	Unit of data being transferred
ACK	Acknowledgment of receipt/processing

Delivery Guarantees

Guarantee	Description	Use Case
At-most-once	May lose messages	Metrics, logs
At-least-once	May duplicate messages	Most applications
Exactly-once	No loss, no duplicates	Financial transactions

Benefits

Benefit	How
Decoupling	Producers don't need to know consumers
Resilience	Messages persist if consumer down
Scalability	Add more consumers for throughput
Load leveling	Absorb traffic spikes
Async processing	Non-blocking operations

Message Queue Patterns

Point-to-Point

One message → one consumer:

Producer ──► Queue ──► Consumer
                  └──► Consumer (competing)

Work Queue

Distribute tasks among workers:

Producer ──► Queue ──► Worker 1
                  ├──► Worker 2
                  └──► Worker 3

Popular Message Queues

System	Strengths
RabbitMQ	Flexible routing, multiple protocols
Amazon SQS	Managed, serverless
Redis (Lists)	Simple, in-memory
ActiveMQ	Enterprise features

Use Cases

Email sending
Image/video processing
Order processing
Log aggregation
Background jobs

Interview Tips

Explain producer-consumer pattern
Discuss delivery guarantees and trade-offs
Mention dead letter queues for failed messages
Compare with pub/sub pattern
Give examples: order processing, email sending

Message Queues

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