ADR-018 — Unified in-process event bus (KairoEventBus) (v0.10)

Status

Accepted — implemented in v0.10.0 (API + default implementation + optional publisher bridging).

Context

Kairo emits important lifecycle signals across multiple domains:

• durable execution events (ExecutionEvent / ExecutionEventEmitter)
• evolution governance lifecycle (EvolutionEventType)
• security observability (SecurityEvent / SecurityEventSink)

Downstream observability (OTel, metrics, audit sinks) needs a single subscription surface without forcing every emitter to learn transport details.

Decision

Add KairoEventBus + KairoEvent envelope:

• KairoEvent is a domain-tagged envelope (execution, evolution, security, team) with optional strongly-typed payload.
• DefaultKairoEventBus uses a multicast Reactor sink for fan-out.

Bridge points (optional, caller-driven in v0.10):

• ExecutionEventEmitter(store, executionId, bus?) publishes after append preparation.
• EvolutionPipelineOrchestrator(..., bus?) publishes lifecycle milestones.
• BusBridgingSecurityEventSink composes an existing sink + bus publish.

Consequences

• Pros: one subscription API for cross-domain observability; avoids overloading ExecutionEventType with unrelated domains.
• Cons: Spring auto-wiring is not yet mandatory; callers must opt-in until a follow-up auto-configuration lands.

Non-goals (v0.10)

• Replacing durable execution persistence with the bus (the bus is not a store).
• Merging enums across domains (each domain keeps its own *EventType).