# Session Projections

English | [中文](session-projection.zh.md)

The session-projection seam — a [capability seam](../capability-seams.md) through which domain host plugins serve whole current values of log-derived per-session state to client carriers: the Service Definition and registry ([dsh-session-projection](../../packages/session/session-projection), `ctx.sessionProjections`), domain contributors (each registering one pure unit), and carriers ([dsh-host-apiproxy](../../packages/host/apiproxy)'s history tail page and `session/projection` push frame). It is one optional capability, not part of the agent-loop spine. The framework drives, the domain computes: the registry subscribes to `session/event` once and folds every committed event through every unit; domains hold no subscriptions and clients never fold domain events — they receive finished values. Design authority: the [session-projection RFC](../../.agents/notes/proposed/architecture/2026-07-27-session-projection-and-command-log.md); drive/cache/feed contracts: the [package README](../../packages/session/session-projection/README.md).

Source: [`packages/session/session-projection/src/index.ts`](../../packages/session/session-projection/src/index.ts)

## The unit

`SessionProjectionMap` is the merge-extensible type table for the whole chain (host unit, wire block, client hook); values are wire-JSON whole values, and rendering belongs to the slot system, never this layer. A domain contributes one `ProjectionDefinition` per key:

```ts type-equiv
/**
 * One domain's state-driven computation unit: three pure synchronous
 * functions plus declarations — never an opaque getter. The framework drives
 * `apply` on every committed session event; the domain holds no
 * subscriptions and owns only the mathematics. All three functions MUST be
 * synchronous (an async unit would tear the carriers' consistency cut) and
 * `state` MUST be plain JSON (the persisted-cache precondition).
 */
interface ProjectionDefinition<K extends keyof SessionProjectionMap, S> {
  /** The projection key this unit owns (its `SessionProjectionMap` entry). */
  key: K
  /** Validates the wire payload (`view` output) before it leaves the host. */
  schema: ZodType<SessionProjectionMap[K]>
  /**
   * State for the empty log.
   * @returns the initial state.
   */
  init(): S
  /**
   * Pure transition: previous state + one committed event → next state. A
   * unit uninterested in an event MUST return the same state reference — an
   * unchanged reference (`Object.is`) produces zero downstream work.
   * @param state - the state covering all prior events.
   * @param event - the next committed session event.
   * @returns the next state (same reference when the event is not the unit's).
   */
  apply(state: S, event: SessionEvent): S
  /**
   * State → wire payload (the read-side projection).
   * @param state - the current state.
   * @returns the whole current value for this unit's key.
   */
  view(state: S): SessionProjectionMap[K]
  /**
   * Persisted-cache invalidation version: bump whenever the serialized state fields or the
   * fold semantics change, so persisted `(sessionId, key, ver, seq, val)`
   * rows from an older unit are discarded instead of being forward-applied
   * into garbage. Non-negative integer.
   */
  stateVersion: number
}
```

The whole-value event rule is load-bearing: a state-carrying log event carries the complete post-change state, never a bare delta — it keeps every transition trivially cheap and every served value self-describing (last-wins for consumers).

## The snapshot and the change feed

```ts type-equiv
/**
 * One consistent read cut over every registered unit for one session.
 * `asOfSeq` is the shared watermark — the seq of the last event every value
 * reflects (`-1` for an empty log, mirroring `session/subscribed.lastSeq`).
 */
interface ProjectionSnapshot {
  /** Seq of the last event the values reflect; -1 for an empty log. */
  asOfSeq: number
  /** Whole current value per registered key. */
  values: Partial<SessionProjectionMap>
}
```

```ts type-equiv
/**
 * Change-feed listener: one unit's value changed for one session. `value` is
 * the schema-validated `view` output; `seq` is the unit's watermark at
 * emission (the seq of the event that caused the change).
 */
type ProjectionChangeListener = (
  session: Session,
  key: Extract<keyof SessionProjectionMap, string>,
  value: unknown,
  seq: number,
) => void
```

`snapshot(session)` is fully synchronous: a carrier reads it in the same tick as its page slice, so `asOfSeq` covers both reads at one sequence number. Every value passes its unit's schema before return; an accidentally async `view` returns a Promise, which schema validation rejects. The change feed fires once per unit whose state *reference* changed for each committed event; `apply` must return the same reference when its state did not change.

## The registry: `ctx.sessionProjections`

`SessionProjectionRegistry` ([signatures](#ctxsessionprojections--sessionprojectionregistry)) owns the drive: one `session/event` subscription, eager `apply` over every registered unit, and per-session per-unit watermark cells. Cells build lazily — a unit registered after events flowed, or a session older than the registry, folds `init` over the in-memory log on first touch (event or read). Registration is an effect whose disposer rides the calling fiber: an unloaded domain plugin's key (with its cached cells) disappears from subsequent drives and snapshots, and clients read that as capability absence; duplicate keys throw. Domain plugins register under `ctx.inject(['sessionProjections'], …)` so headless assemblies without the registry stay unaffected.

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## Cordis surface

Generated from source by `scripts/gen-cordis-catalog.ts` (verified fresh by `pnpm run verify-cordis-catalog` in doc-sync; regenerate with `pnpm run gen-cordis-catalog`) — this section is byte-identical in both language sides of the page. Signature blocks use a `ts cordis-catalog` fence and keep the original source JSDoc; dispatch modes are defined in the [primer](../cordis-primer.md#dispatch-modes), and the framework-inherited `ctx` surface lives in [cordis-api/inherited.md](../cordis-api/inherited.md).

<a id="ctxsessionprojectioncache--sessionprojectioncache"></a>

### `ctx.sessionProjectionCache` — `SessionProjectionCache`

The persisted projection cache service. Opens the `session_projcache` domain at init, checkpoints live sessions on a throttled write-behind (count/interval triggers from Config) plus two mandatory points — `turn/end` and session disposal (the live-to-cold moment) — and serves the cold-read ladder: cached row, persistence `readFrom` tail, registry `restore`, durable write-back. Every durable write is fail-soft: failures log a warning and the cache self-heals on the next write or cold read.

```ts cordis-catalog
/**
 * The zero-I/O listing read: whole values viewed straight from the stored
 * rows (version-matching keys only), each cut carried with its watermark
 * so a client value store can seed under its higher-seq-wins rule — as
 * stale as the last durable checkpoint but never wrong, and never from an
 * unrelated log (the caller's header is the identity witness). Fresher
 * paths (the history tail baseline, {@link coldSnapshot}) supersede these
 * values whenever a session is actually opened.
 * @param meta - the listed session's header (identity witness; no log read).
 * @returns the cut (`asOfSeq` = lowest served-row watermark), or
 *   `undefined` when no usable row exists for this lifecycle.
 */
cachedSnapshot(meta: SessionHeader): ProjectionSnapshot | undefined

/**
 * Durably checkpoint one live session NOW (both mandatory points call
 * this; tests and carriers may too). The registry cut is snapshotted at
 * this boundary (states are live references), then the whole record is
 * replaced. NOT fail-soft — callers on the fail-soft paths contain it.
 * @param session - the live session to checkpoint.
 * @returns resolution after durability and event emission.
 */
async write(session: Session): Promise<void>

/**
 * Cold-read one persisted session's projections with zero full-log load:
 * cached rows + a persistence `readFrom` tail from the registry's restore
 * floor, refolded by the registry and written back (fail-soft) so the next
 * cold read starts closer. A cache row invalidated by a shrunk log
 * (crash-repair truncation) triggers one full re-read from seq 0 — the
 * ladder's slow rung, still no crash. Rejects when the session has no
 * persisted log (`not found` from the persistence seam).
 * @param id - the persisted session to read.
 * @param signal - optional cancellation for the persistence reads.
 * @returns the snapshot cut at the stored log end.
 */
async coldSnapshot(id: SessionId, signal?: AbortSignal): Promise<ProjectionSnapshot>
```

Types: [Session](session.md) · [SessionHeader](persistence.md) · [SessionId](core.md)

Source: [`packages/session/session-projection-cache/src/index.ts:71`](../../packages/session/session-projection-cache/src/index.ts)

<a id="ctxsessionprojections--sessionprojectionregistry"></a>

### `ctx.sessionProjections` — `SessionProjectionRegistry`

`ctx.sessionProjections`: the projection unit table and its drive. The service subscribes to `session/event` once; every committed event passes every registered unit's `apply` (eager drive), and a changed state reference notifies the change feed with the schema-validated view. Cells build lazily — a unit registered after events flowed, or a session older than the registry, folds `init` over the in-memory log on first touch (event or read). Registration is an effect (disposer rides the calling fiber): an unloaded domain plugin's key disappears from snapshots and clients read it as capability absence. Domain plugins register under `ctx.inject(['sessionProjections'], …)` so headless assemblies without the registry stay unaffected. Registrants sharing a key share one unit and are counted: the same tool package mounted in N agent presets registers N times, and the key survives until the last one unloads.

```ts cordis-catalog
/**
 * Register one domain's unit. The registration is an effect on the calling
 * context's fiber: disposing the fiber (or calling the returned disposer)
 * removes the key — and the unit's cached cells — from subsequent drives
 * and snapshots.
 * @param definition - key, state schema, pure unit functions, and stateVersion.
 * @returns the exact disposer that unregisters this unit.
 */
register<K extends keyof SessionProjectionMap, S>(definition: ProjectionDefinition<K, S>): () => void

/**
 * Subscribe to the change feed. The registration is an effect on the
 * calling context's fiber.
 * @param listener - called once per unit whose state reference changed, per committed event.
 * @returns the exact disposer that unsubscribes.
 */
onChanged(listener: ProjectionChangeListener): () => void

/**
 * One consistent cut over every registered unit for one session, read from
 * the watermark cache (missing cells fold lazily over the in-memory log).
 * Fully synchronous — every value and `asOfSeq` reflect the same log
 * position. Each value passes its unit's schema before leaving.
 * @param session - the session whose projection values are read.
 * @returns the snapshot; `values` is empty when no unit is registered.
 */
snapshot(session: Session): ProjectionSnapshot

/**
 * State-level checkpoint of every registered unit for one session, read
 * from the watermark cache (missing cells fold lazily over the in-memory
 * log). This is the write side of the persisted projection cache: the
 * returned rows are the `(key → {ver, seq, val})` part of the durable
 * `(sessionId, key, ver, seq, val)`
 * rows. Every `val` is a DETACHED structured clone — never the live
 * cell reference: the watermark cache is this registry's authoritative
 * mutable state, and a caller reaching the live reference could corrupt
 * every subsequent snapshot and frame through it (plain JSON by the unit
 * contract, so the clone is total).
 * @param session - the session whose unit states are checkpointed.
 * @returns one row per registered key; empty when no unit is registered.
 */
checkpoint(session: Session): ProjectionCheckpoint

/**
 * The stored seq a {@link restore} tail read over `checkpoint` must start
 * at: one event BELOW the lowest usable watermark (a row is usable when
 * its `ver` matches the live unit's `stateVersion`; an absent or mismatched row
 * pulls the floor to `0` — that key must refold the full log). The
 * one-below anchor is load-bearing: the tail then proves how far the
 * stored log still extends, so {@link restore} can detect a log that
 * shrank below a row's watermark (crash-repair truncation) instead of
 * serving the stale row as current — an empty tail read from the anchor
 * yields an end below every watermark and the restore rejects for a full
 * re-read.
 * @param checkpoint - persisted rows for one session (possibly stale or empty).
 * @returns the seq to hand the persistence `readFrom`, or `undefined`
 *   when no unit is registered (no read needed — {@link restore} would
 *   serve empty values regardless).
 */
restoreFloor(checkpoint: ProjectionCheckpoint): number | undefined

/**
 * View a checkpoint's rows without any log read: for every registered
 * unit whose row's `ver` matches, serve the schema-validated
 * `view` of the stored state; mismatched or absent rows leave their key
 * absent (a cold or listing consumer treats it as not-yet-available and a
 * fuller read path refolds it). The zero-I/O rung of the read ladder —
 * values are as stale as their rows, never wrong.
 * @param checkpoint - persisted rows for one session (possibly stale or empty).
 * @returns whole values per key with a usable row; empty when none.
 */
viewCheckpoint(checkpoint: ProjectionCheckpoint): Partial<SessionProjectionMap>

/**
 * Cold read: fold every registered unit over a stored log suffix, seeding
 * each from its checkpoint row when usable — the one read recipe (cached
 * state + forward tail replay + `view`) applied without a live `Session`.
 * Call with the events returned by a persistence
 * `readFrom(id, restoreFloor(checkpoint))` and that same floor as
 * `baseSeq`; the floor's one-below anchor makes the supplied end honest,
 * so a shrunk log is detected here. A row is usable iff its
 * `ver` matches the live unit's `stateVersion`, it does not predate `baseSeq`
 * (`seq >= baseSeq - 1`), and it does not claim events past the
 * supplied end (`seq <= endSeq`); an unusable row is discarded
 * and its key refolds from `init` — which is only sound over the full
 * log, so a discarded row with `baseSeq > 0` throws (the caller re-reads
 * from seq 0, e.g. after a crash-repair truncation shrank the log below
 * a row's watermark).
 * @param checkpoint - persisted rows for one session (possibly stale or empty).
 * @param events - the stored events with `seq >= baseSeq`, in seq order.
 * @param baseSeq - the seq `events` starts at (its first event's seq when non-empty).
 * @returns the snapshot cut at the supplied log end (`asOfSeq` is the last
 *   supplied event's seq, `baseSeq - 1` for an empty tail) plus the
 *   refreshed checkpoint rows at that cut, ready for a durable write-back.
 */
restore(checkpoint: ProjectionCheckpoint, events: readonly SessionEvent[], baseSeq: number): { snapshot: ProjectionSnapshot; checkpoint: ProjectionCheckpoint }
```

Types: [Session](session.md) · [SessionEvent](session.md)

Source: [`packages/session/session-projection/src/index.ts:171`](../../packages/session/session-projection/src/index.ts)
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