Brokers

Brokers are concrete messaging substrates that mediate delivery between producers and consumers.

Examples include queues, topics, streams, pub/sub systems, event buses, and log-based messaging systems.

Brokers can realize interaction patterns such as:

Queue delivery.
Pub/sub.
Stream subscription.
Broadcast or fanout.
Retained or replayable delivery.
Dead-letter or retry paths.

Broker guarantees must be interpreted through the model's delivery semantics. Ordering, durability, deduplication, acknowledgment, and replay are always scoped to the broker's own boundaries and keys.