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deepseek-harness/docs/subsystems/session-projection.md
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Magolorand_Kerman fcca540841 refactor(session): unify runtime projection reads (#7347)
* refactor(session): bind runtime projection reads to one session

* refactor(session): expose session-bound readers on projection handles

---------

Co-authored-by: _Kerman <[email protected]>
2026-10-27 09:50:22 +00:00

471 lines
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Markdown

# 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 -->