Messaging and Events

The messaging layer routes commands, queries, and events between application
and domain components. Three building blocks compose to form the full pipeline:

• Message Bus — dispatches any message to the right handler (commands, queries, events).
  MessageBusCommandSender, MessageBusEventPublisher, and MessageBusQueryFetcher are thin port-satisfying wrappers
  around the bus that expose narrower, role-specific interfaces.
• Event Store — an append-only log that records domain events chronologically.
  Enables rebuilding aggregate state from history (event sourcing).
• Event Bus — publish/subscribe delivery of domain events to registered handlers.
  Sits behind EventPublisherPort so subscribers can be composed and replaced.

Message Bus

• In-Memory Message Bus — Synchronous dispatcher routing commands, queries, and events to registered handlers.
• Command Sender — Thin adapter implementing CommandSenderPort; fire-and-forget.
• Event Publisher — Thin adapter implementing EventPublisherPort; publishes domain events.
• Query Fetcher — Thin adapter implementing QueryFetcherPort; dispatches queries, returns typed results.

Event Store

Append-only event log storing domain events chronologically. Supports append with optimistic concurrency (expected version check) and get_events for aggregate rebuilding.

Event Bus

Publish/subscribe mechanism delivering domain events to registered handlers. Synchronous delivery to all subscribers.

When to use

Use the in-memory message bus for tests and development. Register handlers at startup, dispatch messages at runtime. Use MessageBusCommandSender, MessageBusEventPublisher, and MessageBusQueryFetcher as thin wrappers that satisfy the corresponding port protocols.