27 lines
1.8 KiB
Markdown
27 lines
1.8 KiB
Markdown
# RFC: Event-sourced sessions with derived message history
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Status: implemented
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## Problem
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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).
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## Decision
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A `Session` is an append-only log of typed `SessionEvent`s — 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.
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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.
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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.
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## Alternatives considered
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**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.
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## Consequences
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- Replay, trace, and telemetry are structurally guaranteed, not bolted on.
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- Persistence stays a plugin concern; the in-memory store ships in dsh-session.
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- The event vocabulary is merge-extensible (plugins add e.g. compaction events); [session persistence](2026-06-14-session-persistence.md) froze its shape once the log became durable.
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- Derivation cost grows with log length — compaction (future plugin) is the intended mitigation, not log mutation.
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