[README](../README.md) · next: [Ordering](02-ordering.md)

# 1. Three storage models, and everything that follows from them

Almost every difference between these brokers follows from one sentence about where a message
lives.

- **Kafka** is an append-only log, partitioned. The broker keeps every record for the retention
  period and remembers nothing about individual consumers except an offset. Reading does not
  remove anything.
- **RabbitMQ** is a router with queues. The broker owns each message until a consumer
  acknowledges it, and then deletes it. It can route, expire, and dead-letter on its own.
- **Pulsar** is a log too, but the cursor and the storage are separate services: brokers are
  stateless and BookKeeper holds the data. Acknowledgement moves a cursor, and a message is
  deleted once every subscription has passed it — unless a retention policy says otherwise.

Read those three sentences again before reading any comparison table, including the one in
[chapter 7](07-the-decision-table.md). They predict most of it.

| | Kafka | RabbitMQ | Pulsar |
|---|---|---|---|
| unit of parallelism | partition | queue | subscription |
| set when | topic is created | queue is declared | consumer subscribes |
| consuming | moves an offset | deletes the message | moves a cursor |
| a second reader | new consumer group | another queue, bound in advance | new subscription |

The last row is the one people underestimate. In Kafka and Pulsar you can add a reader that sees
history you have already processed. In RabbitMQ you cannot add one after the fact at all — the
messages are gone — so the decision to have a second consumer has to be made *before* the
messages arrive.

## What was measured, and how

Three questions, asked identically of all three brokers by driving the client libraries directly
rather than through three different Spring abstractions:

| Question | Chapter |
|---|---|
| What ordering survives when you add a second consumer? | [02](02-ordering.md) |
| Can you read the same message twice? | [03](03-replay.md) |
| How far do consumers scale, and what stops them? | [04](04-consumer-scaling.md) |
| What does each one cost to run? | [05](05-operational-footprint.md) |

Versions are the ones a Spring Boot 4.1.1 application gets: kafka-clients 4.2.1, amqp-client
5.30.0, pulsar-client 4.2.4, all read from `spring-boot-dependencies-4.1.1.pom`. The brokers are
Kafka 4.2.1, RabbitMQ 3.10.25 and Pulsar 4.2.4, each started from its own distribution by a script
in `scripts/`.

next: [Ordering](02-ordering.md)