Session Projections
The session-projection seam — a capability seam 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, ctx.sessionProjections), domain contributors (each registering one pure unit), and carriers (dsh-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; drive/cache/feed contracts: the package README.
Source: 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:
/**
* 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
/**
* 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>
}/**
* 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,
) => voidsnapshot(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) 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.
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) — 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, and the framework-inherited ctx API lives in cordis-api/inherited.md.
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.
/**
* 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 · SessionHeader · SessionId
Source: packages/session/session-projection-cache/src/index.ts:71
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.
/**
* 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 · SessionEvent
Source: packages/session/session-projection/src/index.ts:171