command-bus | Java implementation of the Command-Bus pattern

I see there's some confusion between the various Buses and the Event Store. Before attempting to implement an Event Bus, you need to answer one important question that lies at the foundation of any Event Sourcing implementation:

• How to preserve the Event Store as the Single Source of Truth?

That is, your Event Store contains the complete state of the domain. This also means that the consumers of the Event Bus (whatever it ends up being - a message queue, a streaming platform, Redis, etc.) should only get the events that are persisted. Therefore, the goals become:

• Only deliver events on the Bus that are persisted to the Store (so if you get an error writing to the Store, or maybe a Concurrency Exception, do not deliver via bus!)
• Deliver all events to all interested consumers, without losing any events

These two goals intuitively translate to "I want atomic commit between the Event Store and the Event Bus". This is simplest to achieve when they're the same thing!

So, instead of thinking about how to connect an "Event Bus" to command handlers and send events back and forth, think about how to retrieve already persisted events from the Event Store and subscribe to that. This also removes any dependency between command handlers and event subscribers - they live on different sides of the Event Store (writer vs. reader), and could be in different processes, on different machines.