advanced~2h
Event Processors: Subscribing vs. Tracking
Every @EventHandler in a projector has to actually be invoked by something as events flow past — Axon calls that something an event processor, and it comes in two fundamentally different modes with very different operational consequences.
Learning objectives
- Explain what an event processor does and how it differs from the event bus itself
- Contrast subscribing event processors (synchronous, publisher-coupled) with tracking event processors (asynchronous, replayable)
- Identify why only tracking processors support running multiple instances for the same processing group
- Explain the mechanical link between tracking tokens and the ability to replay events later
- Choose the correct processor type for a given projector based on consistency and scaling requirements
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Event Processors: Subscribing vs. Tracking