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Codex review of the PR2 diff caught doc/comment sites doc-sync does not gate (core-data-structures prose) and a latent test-helper race: - docs/core-data-structures/persistence.md + bash.md, sqlite README, and two source comments (coordinator.ts, jsonl.spec.ts) still listed the removed has/delete/get/list methods — updated to the surviving four-method persistence surface and the get/list-free bash seam. - doneFor(ctx, id) attached its onTaskDone listener lazily, after the task could already have closed (e.g. `true`), so it could miss the completion and hang. Replaced with trackCompletions(ctx): one eagerly-installed listener (mounted in setup() before any task starts) records every completion, and doneFor resolves immediately for an already-finished task or on completion otherwise. Race-free, and there is no get-by-id seam left to poll instead.
64 lines
4.9 KiB
Markdown
64 lines
4.9 KiB
Markdown
# Session Persistence
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The **durability seam** for the event log. [session.md](session.md) describes the in-memory `Session` — the append-only `SessionEvent` log that is the source of truth. This page describes how that log is made durable: the abstract `SessionPersistence` service, its backends, the flush checkpoint, crash recovery, and the metadata header that travels alongside the log.
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The seam is a textbook [capability seam](../rfc/implemented/architecture/2026-06-13-capability-seams.md): one abstract service ([dsh-session-persistence](../../packages/session-persistence/session-persistence), `ctx.sessionPersistence`) defining create/append/load/list over the existing `SessionEvent` — **no parallel persisted type** — and two interchangeable backends that pass the same `runPersistenceContract` suite. See the [session-persistence RFC](../rfc/implemented/architecture/2026-06-14-session-persistence.md).
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## The flush checkpoint
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`session/event` is a *synchronous* notification; persistence plugins buffer it (write-behind) and drain at the awaited `session/flush` checkpoint the loop fires at every turn end. Flush is `ctx.parallel` (awaited): a turn's events are durably committed before the next turn starts, and the turn boundary is the commit boundary. A rejecting flush is reported via `agent/error` and the logger — never as a session event (it would land past the commit boundary), so the backend keeps its buffered events for the next flush.
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## Crash recovery preserves an interrupted turn
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A backend that reloads a log crashed mid-turn finds an open `turn/start` with no `turn/end`. It does **not** truncate — a single turn can be huge in a long-horizon task (many steps, large tool output), and those events were durably appended before the crash. Instead it closes the orphaned turn with a synthetic `turn/end { reason: { kind: 'interrupted' } }`, keeping the log balanced and the turn-enclosure invariant intact. `interrupted` is the one `TurnEndReason` no loop emits (see [session.md](session.md#why-a-turn-ended-turnendreasonmap)).
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## `SessionHeader` — metadata beside the log
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Per-session metadata travels **separately** from the event log: format version, cwd, and lineage are storage concerns, not conversation events, so they stay out of `SessionEventMap` and never reach `deriveMessages()`. The header is attached to a `Session` via `session.header`.
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Source: [`packages/core/session/src/types.ts`](../../packages/core/session/src/types.ts)
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```ts type-equiv
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interface SessionHeader {
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/** On-disk format version; a persistence backend rejects unknown versions. */
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version: number
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/** The session's id (mirrors the {@link Session}'s id). */
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id: SessionId
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/** Unix epoch milliseconds when the session was created. */
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createdAt: number
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/** Absolute working directory the session was created in (if any). */
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cwd?: string
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/** The session this one was forked from (seed lineage), if any. */
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parentSession?: SessionId
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}
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```
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## `CreateSessionOptions` — seeding and metadata
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Creating a `Session` through the store takes a `seed` (replay/fork an existing event log) and `meta` (the storage-level fields the store folds into a `SessionHeader`). The store fills in `version`/`id` and defaults `createdAt`; the caller supplies the validated absolute `cwd`, the `parentSession` lineage, and — only when reconstructing a persisted session — the original `createdAt` to preserve it.
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```ts type-equiv
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interface CreateSessionOptions {
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/** Events to seed the new session with (replay/fork). */
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seed?: SessionEvent[]
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/**
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* Creation metadata. The store fills in `version`/`id` and defaults
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* `createdAt` to now; the caller supplies the storage-level fields (validated
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* absolute `cwd`, `parentSession` lineage, and — when reconstructing a
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* persisted session — the original `createdAt` to preserve it).
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*/
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meta?: { cwd?: string; parentSession?: SessionId; createdAt?: number }
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}
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```
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Replay/fork is therefore `ctx.sessions.create(id, { seed: seedEvents })`; resuming a *persisted* session into a live agent is `ctx.agents.resume({ resumeSessionId })`.
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## The backends
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Both implement the same abstract `SessionPersistence` (create/append/load/list over `SessionEvent`) and pass `runPersistenceContract`, proving the seam is genuinely backend-agnostic:
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- **[dsh-session-persistence-jsonl](../../packages/session-persistence/session-persistence-jsonl)** — an append-only JSONL log per session with crash-safe atomic writes, the interrupted-turn crash recovery above, and a read/replay path.
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- **[dsh-session-persistence-sqlite](../../packages/session-persistence/session-persistence-sqlite)** — `node:sqlite`, one row per `SessionEvent`. The row shape `(session_id, seq, type, time, data)` maps 1:1 onto the event, so there is no parallel persisted schema to keep in sync.
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Multiple backends sharing one on-disk session coordinate writes through the [shared persistence write-coordinator](../rfc/implemented/architecture/2026-06-18-shared-persistence-write-coordinator.md).
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