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131 lines
8.4 KiB
Markdown
131 lines
8.4 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. The event vocabulary the log carries is enumerated, member by member, in the generated [persistence log event catalog](../persistence-catalog.md).
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The seam is a textbook [capability seam](../../.agents/notes/implemented/architecture/2026-06-13-capability-seams.md): one abstract service ([dsh-session-persistence](../../packages/session-persistence/session-persistence), `ctx.sessionPersistence`) defining locate/create/append, crash-repairing load, non-mutating inspect, and lightweight list/snapshot observation over the existing `SessionEvent` — **no parallel persisted type** — and two interchangeable backends that pass the same `runPersistenceContract` suite. See the [session-persistence Agent Note](../../.agents/notes/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) until `session/flush`. The loop awaits an ordinary turn's checkpoint before claiming the next queue item; synchronous idle `inject()` schedules its checkpoint without blocking `send()`, and disposal still drains it. A successful flush durably commits the closed turn as one unit; a rejecting flush is reported through `agent/error` and the logger — never as a session event past the closed turn — while 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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`SessionPersistence.inspect(id)` is the observer counterpart to recovery: it returns a detached valid stored prefix without truncating a torn record, adding interruption closers, or publishing write state. Same-id serialization keeps it coherent with backend writes. Derived read models use `inspect`, never `load`, so observing a checkpointed open turn cannot mutate the log if live ownership begins concurrently.
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## `SessionLocation` — optional per-session artifact target
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`SessionPersistence.locate(meta)` synchronously resolves a backend-owned independent artifact without reading, creating, or flushing it. JSONL returns its absolute target path; SQLite returns `undefined` because sessions share one database. A returned path can therefore name a file that does not yet exist or lacks the current unflushed turn; it is a location hint, not authorization or a freshness guarantee.
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```ts type-equiv
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/**
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* A backend-resolved, per-session local artifact location. The path is an
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* absolute target path and can name an artifact that has not materialized yet.
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* Consumers must treat it as a location hint, never as an authorization token.
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*/
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interface SessionLocation {
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/** Backend-specific artifact kind, for example `jsonl`. */
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readonly kind: string
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/** Absolute path to this session's backend-owned artifact. */
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readonly path: string
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}
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```
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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, lineage, and the seed boundary 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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/**
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* Immutable validated storage metadata, kept outside the conversation event log.
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*/
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interface SessionHeader {
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/**
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* On-disk format version, stamped from {@link SESSION_FORMAT_VERSION} when the
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* session is created. A persistence backend rejects any other version on load
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* (no migration — see the constant).
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*/
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readonly version: number
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/** The session's id (mirrors the {@link Session}'s id). */
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readonly id: SessionId
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/** Unix epoch milliseconds when the session was created. */
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readonly createdAt: number
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/** Absolute working directory the session was created in (if any). */
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readonly cwd?: string
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/** The session this one was forked from (seed lineage), if any. */
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readonly parentSession?: SessionId
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/**
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* How many leading events were inherited through a seed. Persisting this
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* boundary lets resume and replay distinguish parent history from child work.
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*/
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readonly seedLength?: number
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/**
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* Delegation depth: absent (zero) for a top-level session, parent depth + 1
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* for a subagent child. Persisted so a recursion budget survives restart and
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* resume — a runtime-only depth would reset a resumed child to top-level.
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*/
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readonly delegationDepth?: number
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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, the `seedLength` seed boundary, the `delegationDepth`, and — only when reconstructing a persisted session — the original `createdAt` to preserve it.
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```ts type-equiv
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/**
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* Options for creating a {@link Session} via the store. `seed` replays/forks
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* an existing event log; `meta` carries the caller-supplied storage fields the
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* store folds into a {@link SessionHeader}.
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*/
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interface CreateSessionOptions {
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/** Events to seed the new session with (replay/fork). */
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readonly seed?: readonly SessionEvent[]
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/**
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* Storage metadata read once before publication. `seedLength` is explicit
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* because a resumed seed contains the full stored log, not only its inherited prefix.
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*/
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readonly meta?: {
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readonly cwd?: string
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readonly parentSession?: SessionId
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readonly createdAt?: number
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readonly seedLength?: number
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readonly delegationDepth?: number
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}
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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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## Lightweight source revisions
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Consumers of derived state compare a cheap opaque revision before loading a full event log. The persistence backend owns its representation and changes it transactionally with append or mutating load repair; callers compare it only for equality.
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```ts type-equiv
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/**
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* Backend-owned token that identifies both one storage source and one revision
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* of a persisted session log.
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*/
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type SessionPersistenceRevision = Branded<'SessionPersistenceRevision'>
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```
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```ts type-equiv
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/** Lightweight immutable source identity returned without loading a full log. */
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interface SessionPersistenceSnapshot {
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/** Detached metadata for one materialized session. */
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header: SessionHeader
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/** Opaque source-qualified token that changes whenever this stored log changes. */
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revision: SessionPersistenceRevision
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}
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```
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## The backends
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Both implement the same abstract `SessionPersistence` (locate/create/append/load/inspect/list/listSnapshots 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 logical JSONL log per session, stored as checksummed concatenated Zstandard frames by default or raw lines by configuration, with crash-safe atomic writes, interrupted-turn recovery, 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, source_event_seqs, surface_op)` maps 1:1 onto the event, including optional surface metadata, so there is no parallel persisted schema to keep in sync.
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