ADR 0002: Last Event Data Access via Visitor

• Status: Implemented
• Date: 2026-03-19
• Scope: StaticEventSystem

Context

StaticEventSystem stores last event payloads as erased byte buffers.

A direct "return payload" API creates tradeoffs:

• Returning references can expose lifetime and synchronization hazards.
• Returning deep-copied raw buffers might create condition, when source data stored might become invalid.
• Returning typed values from the core queue would couple core code to payload type headers.

At the same time, the generated API should remain typed and convenient for app code.

Decision

Expose callback-based last-event access in queue/system core:

• getLastEventData(eventId, visitor) is the single low-level access path.
• Core keeps erased payload storage and does not expose typed payload dependencies.
• Generated system/LastEventGetters.* performs typed conversion and returns typed results via bool + out parameter.

Rationale

• Keeps critical section small and scoped to callback execution.
• Avoids returning raw deep-copy buffers to callers.
• Preserves layering: core remains type-erased, generated system layer handles payload typing.
• Reduces API surface duplication by standardizing on one low-level access pattern.

Consequences

• Callers of core/system low-level API must use a visitor lambda.
• Typed convenience remains available through generated last-event getters.
• Header bloat is reduced by using forward declarations in generated getter headers.

Notes

This decision complements ADR 0001 (discard-on-stop policy) and follows the same safety-first shutdown and lifetime model.