docs/rfc/implemented/architecture/2026-06-11-event-sourced-sessions.md

# 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 `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.

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](2026-06-14-session-persistence.md) froze its shape once the log became durable.
- Derivation cost grows with log length — compaction (future plugin) is the intended mitigation, not log mutation.
docs: unify ADR/RFC trees into one lifecycle-organized RFC tree 2026-06-18 02:18:24 +08:00			`# RFC: Event-sourced sessions with derived message history`
Backfill architecture decision records 2026-06-11 15:24:14 +08:00
docs(rfc): define and enforce a uniform RFC format; adopt it across the corpus 2026-07-05 22:58:25 +08:00			`Status: implemented`
docs: unify ADR/RFC trees into one lifecycle-organized RFC tree 2026-06-18 02:18:24 +08:00
docs(rfc): define and enforce a uniform RFC format; adopt it across the corpus 2026-07-05 22:58:25 +08:00			`## Problem`
Backfill architecture decision records 2026-06-11 15:24:14 +08:00
docs(rfc): define and enforce a uniform RFC format; adopt it across the corpus 2026-07-05 22:58:25 +08:00			`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).`
Backfill architecture decision records 2026-06-11 15:24:14 +08:00
			`## Decision`

docs: unwrap hard-wrapped Markdown to one line per paragraph 2026-06-13 18:39:20 +08:00			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.
Backfill architecture decision records 2026-06-11 15:24:14 +08:00
docs: unwrap hard-wrapped Markdown to one line per paragraph 2026-06-13 18:39:20 +08:00			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.
Backfill architecture decision records 2026-06-11 15:24:14 +08:00
docs: rebalance prose cleanup and add trimming skill 2026-07-13 23:27:00 +08:00			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.
Backfill architecture decision records 2026-06-11 15:24:14 +08:00
docs(rfc): define and enforce a uniform RFC format; adopt it across the corpus 2026-07-05 22:58:25 +08:00			`## 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.`

Backfill architecture decision records 2026-06-11 15:24:14 +08:00			`## Consequences`

			`- Replay, trace, and telemetry are structurally guaranteed, not bolted on.`
			`- Persistence stays a plugin concern; the in-memory store ships in dsh-session.`
docs(rfc): define and enforce a uniform RFC format; adopt it across the corpus 2026-07-05 22:58:25 +08:00			`- 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.`
docs: unwrap hard-wrapped Markdown to one line per paragraph 2026-06-13 18:39:20 +08:00			`- Derivation cost grows with log length — compaction (future plugin) is the intended mitigation, not log mutation.`