Sacrifice GetAtTime time-travel across compaction

Date: 2026-06-26 Status: Accepted Decision: router-hosts-4w2 Deciders: sean

Context

GetAtTime (event-replay point-in-time, projection.go:111) is declared in the HostProjection interface and unit-tested, but has no production caller — no gRPC RPC, no CLI, no operator. Compaction irrevocably deletes the pre-compaction event log, which permanently breaks point-in-time replay for compacted aggregates.

Decision

Accept that compaction destroys pre-compaction event history. GetAtTime for a compacted aggregate returns only the single seed event's state. No effort is spent preserving time-travel.

Rationale

• GetAtTime has no production consumer; preserving it would add cost for zero runtime benefit.
• If a future caller is ever added, the per-aggregate snapshot table (named in the spec's Future section) is the correct solution — not retaining the full log indefinitely.

Alternatives Considered

• Accept GetAtTime breakage; no caller to protect (chosen): compaction stays simple (delete-all + insert-seed) with no extra storage. Cost: GetAtTime returns incomplete results for compacted aggregates if a caller is ever added.
• Retain pre-compaction events / no delete (rejected): keeps time-travel fully intact but negates the entire purpose of compaction (the log stays large).
• Pre-compaction snapshot preserving time-travel up to the compaction point (rejected): partial time-travel, but requires the per-aggregate snapshot table this design explicitly defers.

Consequences

• Positive: no additional storage or retention bookkeeping.
• Negative: GetAtTime semantics are silently broken for compacted aggregates; a future caller must account for this at the call site.
• Neutral: the interface method remains declared; the behavior change is not reflected in its signature, so it must be documented if a caller is added.