# 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-session-controller](../../packages/api/session-controller)'s history tail page and `session/projection` push frame). Host state readers require the registry during activation. 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)

A `ProjectionHandle` retains its synchronous reader after unregistration. `stateOf(session)` folds newly committed events with the original unit; it does not rejoin global snapshots, checkpoints, or notifications. Registration must still finish before the handle can be used. See the [registration lifecycle](../../packages/session/session-projection/README.md).

```ts type-equiv
/** An effect-owned registration with a reader retained independently of registration. */
interface ProjectionHandle<S> {
  /**
   * Read the current state, including after unregistering, by folding new
   * Session events with this handle's original unit. Do not mutate the live value.
   * @param session - Session whose committed events are folded.
   * @returns the projection state at the Session's current cursor.
   */
  stateOf(session: Session): S
  /**
   * Create a reader for one exact Session without reading state or preparing it.
   * The reader retains this unit after unregistration and owns no disposal.
   * @param session - Session whose committed events the bound reader folds.
   * @returns a synchronous callback with the same state and failures as `stateOf(session)`.
   */
  readerFor(session: Session): () => S
  /**
   * Unregister from global drives and snapshots without invalidating this reader.
   * @returns a promise that resolves when this registration is released.
   */
  dispose(): Promise<void>
}
```

The [compatibility rules](../../packages/session/session-projection/README.md#deprecated-plugin-compatibility) define legacy disposal, keyed reads, and their once-per-key deprecation warnings.

`ProjectionHandle.readerFor(session)` returns a `() => S` callback for one exact Session. Calling it reads the original unit and preserves borrowed state, readiness requirements and synchronous failures. Owners can compose several readers in a typed object; domain services keep them private. See [binding several states](../../packages/session/session-projection/README.md#bind-several-states-to-one-session).

## The unit

`ProjectionDefinition<K, S>` carries each unit's host state type locally; registration infers `S` from `stateSchema` and returns `ProjectionHandle<S>`. Only client-visible keys merge into `SessionProjectionMap`, which types their whole values. A `wire` block makes a unit client-visible, and rendering belongs to the slot system, never this layer:

```ts type-equiv
/**
 * One domain's state-driven computation unit: a pure synchronous fold plus
 * declarations and an optional client view — never an opaque getter. The framework drives
 * `apply` on every committed session event; the domain holds no
 * subscriptions and owns only the computation. All 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 string, S> {
  /** The projection key this unit owns. */
  key: K
  /** Validates persisted state before it seeds a fold. */
  stateSchema: ZodType<S>
  /**
   * State for the empty log and its immutable Session metadata.
   * @param header - immutable metadata for the Session being projected.
   * @param inheritedEventCount - exact fork-inherited prefix length.
   * @returns the initial state.
   */
  init(header: SessionHeader, inheritedEventCount: SessionLogOffset): NoInfer<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: NoInfer<S>, event: SessionEvent): NoInfer<S>
  /** Client view. Omit for host-only units. */
  wire?: K extends keyof SessionProjectionMap ? {
    /** Validates the wire payload before it leaves the host. */
    viewSchema: ZodType<SessionProjectionViewValue<SessionProjectionMap[K]>>
    /**
     * State → wire payload (the read-side projection). The live drive keeps
     * the two latest raw results and compares them with `Object.is`; an
     * object-valued view must reuse its reference to suppress publication
     * across internal-only state changes.
     * @param state - the current state.
     * @returns the whole current value for this unit's key.
     */
    view(state: NoInfer<S>): SessionProjectionViewValue<SessionProjectionMap[K]>
    /**
     * Whether a Session summary carries this unit; omitted means `true`.
     * Declare `false` on a key whose map entry carries the
     * `SessionProjectionSummaryExcluded` mark: the runtime key set follows this
     * field, while the client view type follows the mark.
     * @default true
     */
    summary?: boolean
  } : never
  /**
   * 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
}
```

Every served projection value is a complete read model. A source event may carry a whole value or a domain-owned operation; the unit's deterministic `apply` owns replay, and checkpoint plus forward tail replay reconstructs the same state.

## The snapshot and the change feed

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

```ts type-equiv
/**
 * Change-feed listener: one unit's raw `view` result changed by `Object.is`
 * for one session. `value` is the schema-validated 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: SessionSeq,
) => 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. It returns only client views, and every value passes its unit's `viewSchema` before return. `handle.stateOf(session)` reads one live host state without computing unrelated views; callers must not mutate the borrowed reference. A state-reference change computes one cached raw view, and the change feed fires only when that result changes by `Object.is`; an object-valued view must preserve its reference to suppress publication across internal-only state changes.

## Summary reads

A Session summary carries every registered client view except the keys whose `SessionProjectionMap` entry carries the `SessionProjectionSummaryExcluded<T>` mark. The mark has no runtime representation: it decides the client view type, while `wire.summary` decides the runtime key set, with omission meaning `true`. A key whose entry carries the mark declares `wire.summary: false` so the two agree; the field is a plain boolean, so the type does not enforce it.

`summaryKeys()` derives the key set from the live registrations and keeps returning the same instance until a registration or its disposal changes the key table. A summary consumer passes it to the existing key-taking faces — `cachedSnapshot(session, keys)` for cached cells and `viewCheckpoint(rows, keys)` for checkpoint rows, both keeping their watermark rules and both using a set without copying — so there is no summary-specific read face and no second definition of the set.

A list row carries that summary projection, never the full projection: it promises the summary key set and nothing more. `snapshot`, `cachedSnapshot`, and `viewCheckpoint` remain complete when called without a key subset, which is how an opened Session, its follow baseline, and the control baseline keep every projected value; the `@` reference reads only its label keys through the same key-taking faces.

### Summary vocabulary

The declarations below state the summary rule. The mark, its stripping, the excluded-key test, and the summary value type are declared in the pure-type outlet; the key selection both read faces accept is declared in the service module.

```ts type-equiv
/**
 * One `SessionProjectionMap` entry excluded from Session summary payloads
 * (the Session list row and any other summary read). The mark is type-only;
 * `T` keeps its exact runtime value and wire representation. Wrap only the
 * excluded contributor's own map entry, and place a nullable entry's `null`
 * outside the wrapper (`SessionProjectionSummaryExcluded<Item[]> | null`) so
 * the required symbol cannot absorb that member.
 */
type SessionProjectionSummaryExcluded<T> = T & {
  readonly [sessionProjectionSummaryExcluded]: true
}
```

```ts type-equiv
/**
 * One map entry's client view value: the summary mark stripped. The
 * conditional distributes over a nullable entry's union, so a `| null` member
 * written outside the wrapper survives unchanged.
 */
type SessionProjectionViewValue<T> =
  T extends SessionProjectionSummaryExcluded<infer Value> ? Value : T
```

```ts type-equiv
/** Every client view value, keyed by projection key, with no summary mark. */
type SessionProjectionViewValues = {
  [K in keyof SessionProjectionMap]: SessionProjectionViewValue<SessionProjectionMap[K]>
}
```

```ts type-equiv
/**
 * Keys a Session summary omits: map entries whose non-nullish value carries
 * the summary mark. Nullish members are ignored, so a nullable entry is still
 * recognized when its `null` lives outside the wrapper.
 */
type SessionProjectionSummaryExcludedKey = {
  [K in keyof SessionProjectionMap]:
  NonNullable<SessionProjectionMap[K]> extends SessionProjectionSummaryExcluded<unknown> ? K : never
}[keyof SessionProjectionMap]
```

```ts type-equiv
/**
 * The client values a Session summary carries: every client view value except
 * {@link SessionProjectionSummaryExcludedKey}. A summary read returns these
 * keys only; an opened Session's full snapshot keeps every view value.
 */
type SessionProjectionSummaryValues =
  Omit<SessionProjectionViewValues, SessionProjectionSummaryExcludedKey>
```

```ts type-equiv
/**
 * One read's client-visible key selection: an explicit key list, or a derived
 * set such as {@link SessionProjectionRegistry.summaryKeys}. A set is used as
 * given; only a list is copied into one.
 */
type ProjectionKeySelection =
  | readonly Extract<keyof SessionProjectionMap, string>[]
  | ReadonlySet<Extract<keyof SessionProjectionMap, string>>
```

## 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). Domains await `register(definition)` during activation and retain its handle. Registration belongs to the calling fiber: its last release removes the key from global drives and snapshots, while retained handles remain readable. Same-key, same-version registrants share one unit; incompatible versions reject registration.

<!-- BEGIN GENERATED cordis-surface (gen-cordis-catalog.ts) — do not edit between markers -->

<a id="cordis-surface"></a>

## Cordis API

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`) — the language sides differ only in locale-specific paired document paths. 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` API 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 three mandatory points — session creation, `turn/end`, and session disposal (the live-to-cold moment) — and serves the cached rows for a session header. Every durable write is fail-soft: failures log a warning and the cache self-heals on the next write.

```ts cordis-catalog
/**
 * The zero-I/O listing read: whole values viewed straight from the stored
 * rows (version-matching keys only) of the record bound to the caller's
 * lifecycle. The header is the only identity witness a listing holds, so
 * this face matches the lifecycle identity (`formatVersion`, `createdAt`,
 * `cwd`, `isSeeded`) and not the inherited cut: within one format
 * generation the cut is fixed at fork time, so it distinguishes no
 * lifecycle the other fields do not, and a viewed value never seeds a fold.
 * The view is as stale as the last durable checkpoint but never wrong and
 * never from an unrelated log. Its `asOfSeq` is the lowest watermark among
 * the served rows: the stored record's own position, which the header
 * cannot relate to the log the caller later opens. The Session list
 * therefore labels the block as cached, and the client lets every value the
 * connected Session produces supersede it whatever this number says.
 * @param meta - the listed session's header (identity witness; no log read).
 * @param keys - optional projection keys required by the caller's audience.
 * @returns the viewed block, or `undefined` when no usable row exists for
 *   this lifecycle at the current Session format.
 */
cachedSnapshot( meta: SessionHeader, keys?: ProjectionKeySelection, ): ProjectionSnapshot | undefined

/**
 * Read only a predecessor checkpoint's title as a zero-I/O listing hint.
 *
 * The authoritative Session header supplies the lifecycle identity. A cache
 * checkpoint can lag that log but cannot lead it because writes flush the
 * log first, so a matching predecessor title is a genuine (possibly stale)
 * fact from this Session. The registry still requires the current title
 * projection's row version and schema. No other predecessor projection is
 * exposed: format normalization can change their current meaning, and the
 * {@link cachedSnapshot} / hydration paths continue to reject them.
 * @param meta - authoritative listed Session header.
 * @returns a title-only block at the stored title row's watermark, or
 *   `undefined` when the record is current, newer, unrelated, missing, or
 *   incompatible with the title unit.
 */
cachedPredecessorTitle(meta: SessionHeader): ProjectionSnapshot | undefined

/**
 * Hydrate projection cells for an already-prepared Session without another
 * persistence read. The cache seeds matching rows; the supplied exact log
 * advances every unit to the observation cut. No checkpoint is written
 * because the logical observation may contain recovery events not yet durable.
 * @param session - exact unpublished Session retained by persistence.
 * @param events - exact logical event prefix represented by the observation.
 * @returns all projection values at the event cut.
 */
hydratePrepared( session: Session, events: readonly SessionEvent[], ): ProjectionSnapshot

/**
 * Durably checkpoint one live session NOW (all mandatory points call
 * this; tests and carriers may too). The registry cut is snapshotted at
 * this boundary (states are live references), then the session's record is
 * replaced on the domain's write chain. 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 session's projections from its complete log. Each unit is
 * seeded from the identity-checked cached rows — the registry skips `apply`
 * for the already-folded prefix (events at or below the row's `seq`) — and
 * the refreshed checkpoint is written back (fail-soft, fire-and-forget), so
 * the first cold read creates the cache row and later ones seed from it.
 * The caller supplies the complete log in seq order: this service never
 * consults the persistence layer.
 * @param meta - the stored session header (identity witness).
 * @param inheritedEventCount - exact inherited prefix length for projection initialization and identity.
 * @param events - the session's complete log, in seq order.
 * @returns the projection cut at the log end.
 */
coldSnapshot( meta: SessionHeader, inheritedEventCount: SessionLogOffset, events: readonly SessionEvent[], ): ProjectionSnapshot
```

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

Source: [`packages/session/session-projection-cache/src/index.ts`](../../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). A changed state reference computes the next client view; the change feed is notified only when its raw result changes by `Object.is`. 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). Consumers declare `sessionProjections` as a required injection, await registration during activation, and retain its ready handle. Registrants sharing a key share one unit and are counted. The last release removes the unit from live drives, checkpoints, and client snapshots; retained handles still fold new events on demand using the original unit.

```ts cordis-catalog
/**
 * Register one domain's unit on the calling context's fiber. Fiber or handle
 * disposal releases one reference; the final release removes live participation.
 * Retained handles remain readable after release.
 * Callers must await registration before publishing consumers; registration may perform asynchronous work.
 * @param definition - key, state schema, pure unit functions, and stateVersion.
 * @returns an awaitable yielding a ready handle; the callable runtime value is deprecated.
 */
register<K extends keyof SessionProjectionMap, S>( definition: Omit<ProjectionDefinition<K, S>, 'wire'> & { wire: NonNullable<ProjectionDefinition<K, S>['wire']> }, ): Promise<ProjectionHandle<S>>

/**
 * Register one host-only unit. Its state is omitted from client snapshots
 * and always checkpointed like every other unit.
 * Callers must await registration before publishing consumers; registration may perform asynchronous work.
 * @param definition - key, state schema, pure unit functions, and stateVersion.
 * @returns an awaitable yielding a ready handle; the callable runtime value is deprecated.
 */
register<K extends string, S>( definition: ProjectionDefinition<K, S> & { key: Exclude<K, keyof SessionProjectionMap> }, ): Promise<ProjectionHandle<S>>

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

/**
 * One consistent cut over every registered client-visible 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 `viewSchema` before leaving.
 * @param session - the session whose projection values are read.
 * @param keys - optional client-visible outputs; state materialization remains complete.
 * @returns the snapshot; `values` is empty when no selected client-visible unit is registered.
 */
snapshot( session: Session, keys?: ProjectionKeySelection, ): ProjectionSnapshot

/**
 * Read only already-materialized client-visible cells without folding history.
 * Values may trail the live Session and are therefore hints, not a complete
 * baseline. Missing cells are omitted.
 * @param session - attached Session whose cached cells are inspected.
 * @param keys - optional wire keys to view.
 * @returns the lowest common cached cut, or `undefined` when no wire cell exists.
 */
cachedSnapshot( session: Session, keys?: ProjectionKeySelection, ): ProjectionSnapshot | undefined

/**
 * The keys a Session summary carries: every registered client-visible key
 * whose unit does not declare `summary: false`. The returned set is stable —
 * the same instance on every call — until a registration or its disposal
 * changes the key table, so a consumer may hold it across rows. Callers pass
 * it to a key-taking read face such as `cachedSnapshot(session, summaryKeys())`
 * or `viewCheckpoint(rows, summaryKeys())`; those faces use the set as-is.
 * @returns the current summary key set; do not mutate it.
 */
summaryKeys(): ReadonlySet<Extract<keyof SessionProjectionMap, string>>

/**
 * State-level checkpoint of every persisted 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.
 */
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 offset for the stored-log suffix read (`SessionHandle.read`),
 *   or `undefined` when no unit is registered (no read needed —
 *   {@link restore} would serve empty values regardless).
 */
restoreFloor(checkpoint: ProjectionCheckpoint): SessionLogOffset | undefined

/**
 * View a checkpoint's rows without any log read: for every registered
 * client-visible unit whose row's `ver` matches, serve the schema-validated
 * `view` of the schema-validated stored state; mismatched, malformed, 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).
 * @param keys - optional wire keys to view.
 * @returns whole values per key with a usable row; empty when none.
 */
viewCheckpoint( checkpoint: ProjectionCheckpoint, keys?: ProjectionKeySelection, ): Partial<SessionProjectionMap>

/**
 * Cold read: fold every persisted 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 stored events at or past `restoreFloor(checkpoint)` (a
 * `SessionHandle.read` slice) 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).
 * @param header - immutable metadata for the Session being restored.
 * @param inheritedEventCount - exact fork-inherited prefix length supplied to unit initialization.
 * @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: SessionLogOffset, header: SessionHeader, inheritedEventCount: SessionLogOffset, ): { snapshot: ProjectionSnapshot; checkpoint: ProjectionCheckpoint }

/**
 * Restore an exact cut and install its states on the supplied prepared Session.
 * A later publication reuses these cells; ordinary live reads and event drive
 * advance any constructor-owned suffix exactly once.
 * @param session - exact prepared Session that owns the restored log prefix.
 * @param checkpoint - persisted rows for this Session lifecycle.
 * @param events - exact events at the observation cut.
 * @param baseSeq - first supplied event sequence.
 * @returns all projection values at the supplied cut.
 */
hydrate( session: Session, checkpoint: ProjectionCheckpoint, events: readonly SessionEvent[], baseSeq: SessionLogOffset, ): ProjectionSnapshot
```

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

Source: [`packages/session/session-projection/src/index.ts`](../../packages/session/session-projection/src/index.ts)
<!-- END GENERATED cordis-surface -->