Basics

• What's a message queue?
• What does a message queue get you?
• Why RabbitMQ specifically?

Message Queues

• Some sort of broker that you send message payloads to over the wire (usually via TCP)
• Figures out who should get the message, and gets it to them.

Why A Message Queue

• Performance
• Simplifies complex communication topologies
• Implicitly handles retries and buffering
• Promotes decoupled software design

Why RabbitMQ?

• Pros:
  • It's pretty much the fastest
  • It's pretty much the most reliable
  • It scales really well
  • It's massively flexible
• Cons:
  • Client libs not as clean and simple as e.g. Redis.
  • Corollary: AMPQ protocol is...complex.

Charting The Seas - Topology Building Blocks

Basic Topology Blocks

• Queues (obviously)
• Exchanges
• Bindings
• Routing Keys (optional)

Fundamental Concept

• Publishers just publish
• Subscribers just listen
• Neither one is aware of, dependent on, or directly interacts with the other

Queues

• The buckets wot messages go in
• Always the Final Destination
• Can be persisted to disk
• Can be replicated across instances for HA
• Messages sit here until subscribers pull them off

Exchanges

• The only things publishers can publish to
• 3 basic types (will come back to this)

Bindings

• The thing that links, or binds exchanges to queues
• Once an exchange is bound to a queue, the message has a complete pathway to travel along.

Routing Keys

• Specialized message metadata
• Used by the binding + exchange to make routing decisions on a message-by-message basis

Charting The Seas - Basic Topology Patterns

Exchange Types

• Remember I said there were 3 basic exchange types
  • Direct Exchanges
  • Fanout Exchanges
  • Topic Exchanges

Direct Exchange

Each message is routed to one queue based on binding, where routing key matches exactly.

Think "unicast"

Fanout Exchange

Routing Key is ignored. A copy of the message is sent to each queue the exchange is bound to.

Think "multicast"

Topic Exchange

Routing key must be dot-delimited. Binding defines regex-style matching.

Imagine keys like "sad.orange.rabbit" and "lazy.purple.lizard", and a queue that wants all "color" keys, and another queue that only wants "reptile" keys.