deepseek-harness/docs/core-data-structures/persistence.md

Session Persistence

The durability seam for the event log. 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 seam is a textbook capability seam: one abstract service (dsh-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.

The flush checkpoint

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.

Crash recovery preserves an interrupted turn

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).

SessionHeader — metadata beside the log

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.

Source: packages/core/session/src/types.ts

interface SessionHeader {
  /**
   * On-disk format version, stamped from {@link SESSION_FORMAT_VERSION} when the
   * session is created. A persistence backend rejects any other version on load
   * (no migration — see the constant).
   */
  version: number
  /** The session's id (mirrors the {@link Session}'s id). */
  id: SessionId
  /** Unix epoch milliseconds when the session was created. */
  createdAt: number
  /** Absolute working directory the session was created in (if any). */
  cwd?: string
  /** The session this one was forked from (seed lineage), if any. */
  parentSession?: SessionId
}

CreateSessionOptions — seeding and metadata

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.

interface CreateSessionOptions {
  /** Events to seed the new session with (replay/fork). */
  seed?: SessionEvent[]
  /**
   * Creation metadata. The store fills in `version`/`id` and defaults
   * `createdAt` to now; the caller supplies the storage-level fields (validated
   * absolute `cwd`, `parentSession` lineage, and — when reconstructing a
   * persisted session — the original `createdAt` to preserve it).
   */
  meta?: { cwd?: string; parentSession?: SessionId; createdAt?: number }
}

Replay/fork is therefore ctx.sessions.create(id, { seed: seedEvents }); resuming a persisted session into a live agent is ctx.agents.resume({ resumeSessionId }).

The backends

Both implement the same abstract SessionPersistence (create/append/load/list over SessionEvent) and pass runPersistenceContract, proving the seam is genuinely backend-agnostic:

• dsh-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.
• dsh-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.

Multiple backends sharing one on-disk session coordinate writes through the shared persistence write-coordinator.