Tianyi Cui
1.8 KiB
RFC: Event-sourced sessions with derived message history
Status: implemented
Problem
The MVP requires strict event-based tracing with fully replayable sessions (严格的基于事件的trace、logging系统，session完全可回放).
Decision
A Session is an append-only log of typed SessionEvents — the single source of truth. The LLM message history is derived from the log (deriveMessages()); raw stream chunks are logged for token-level replay fidelity while the assembled assistant/message event is authoritative for derivation. Replay/fork = seed a new session with an existing log.
Appends are synchronous (the hot path never blocks on I/O); session/event is a sync notification; persistence plugins buffer write-behind and drain at the awaited session/flush checkpoint fired at every turn end.
Ordering contract: the loop appends to the session before emitting the corresponding Cordis event, and the agent/step-result waterfall runs before the assistant/message append so the log records the message tool dispatch actually used. Regression tests pin that ordering.
Alternatives considered
A mutable message array with events fired as notifications — simpler, but state and log can diverge; with event-sourcing the log IS the state, so divergence is structurally impossible.
Consequences
- Replay, trace, and telemetry are structurally guaranteed, not bolted on.
- Persistence stays a plugin concern; the in-memory store ships in dsh-session.
- The event vocabulary is merge-extensible (plugins add e.g. compaction events); session persistence froze its shape once the log became durable.
- Derivation cost grows with log length — compaction (future plugin) is the intended mitigation, not log mutation.