# Agent Note: Domain KV storage capability seam and the workspace entity

Status: proposed

English | [中文](2026-07-24-domain-kv-storage-and-workspace.zh.md)

## Problem

The host's only persistence surface is the session event log (`packages/session-persistence`: append-only, one file per session). Anything that does not belong to a single session has nowhere to live, and two real needs exist today:

- **The workspace entity.** The GUI needs workspace as a real object: path, title, and the list of owned sessions. Ownership belongs to the workspace — "which sessions belong to this workspace" is not any single session's fact, so writing it into the session log is semantically wrong. Until now workspace was only a sidebar visual grouping derived from cwd, with no entity (that conclusion has been overturned).
- **Dynamic session metadata** (the foreseeable second consumer). Cold session listings read only the first log line (an immutable creation-time snapshot); title, terminal status, and anything that evolves with the session is unavailable. The fix direction is a sidecar metadata table — exactly a KV table with high-frequency per-key updates.

Separately, Session deletion needs a `SessionPersistence` delete primitive and a `session.delete` endpoint. That gap's design is settled in this note, but its implementation remains future work.

The later [Workspace registration deletion decision](../../implemented/feature/2026-07-27-workspace-registration-deletion.md) supersedes only that coupling: deleting a Workspace registration preserves its Sessions and their logs, while Session deletion remains separate future work. The cascade design below is therefore not the Workspace GUI delete semantic.

## Proposal

Create the `packages/storage/` group — the `ctx.storage` hub (backend registry + data-form mounts), two backends, the domain data form — plus the workspace consumer package; extend `SessionPersistence` with a delete primitive.

| Package | Path | ctx surface | This phase |
| --- | --- | --- | --- |
| `@deepseek-ai/dsh-storage` | `packages/storage/storage/` | `ctx.storage` (the hub) | ✓ |
| `@deepseek-ai/dsh-storage-json` | `packages/storage/storage-json/` | registers backend `json` | ✓ |
| `@deepseek-ai/dsh-storage-sqlite` | `packages/storage/storage-sqlite/` | registers backend `sqlite` | ✓ |
| `@deepseek-ai/dsh-storage-domain` | `packages/storage/storage-domain/` | mounts `ctx.storage.domain` | ✓ |
| `@deepseek-ai/dsh-workspace` | `packages/workspace/workspace/` | `ctx.workspace` | ✓ |
| `SessionPersistence.delete` extension + cascade orchestration | `packages/session-persistence/*` | new method on the existing seam | ✗ future work (session side untouched this phase) |
| `workspace.*` / `session.delete` RPC, GUI wiring, boot assembly | — | — | ✗ next phase |

(workspace lives in its own group rather than `packages/host/`: the host group's naming rule requires the `dsh-host-*` prefix while this package is named `dsh-workspace`; and the workspace entity is a domain concept, not bound to the host assembly tier. Unrelated to the existing `workspace-context` package — that is an AGENTS.md instruction loader.)

Dependency direction: `dsh-workspace` → `dsh-domain` → `dsh-storage` ← the two backends. `dsh-workspace` additionally depends on the read-only face of `ctx.sessionPersistence` (attach's cwd check reads the session header; when the service is absent, attach rejects outright — no verification, no bookkeeping). The `ctx.sessions` running-check for session deletion moves into future work together with the cascade.

### `dsh-storage`: the storage hub

A pure registration hub, no IO of its own, no Config. The `Storage` service mounts at `ctx.storage` with two faces: `backend` (a `BackendRegistry`: `register(name, backend)` returns the disposer, duplicate names throw; `get(name)` throws `backend-not-found` for unknown names) and data-form mounting (`mount(form, facility)` over the merge-extensible `StorageForms` map, into which `dsh-domain` merges the `domain` key; unmounted access throws `form-not-mounted`). The signature text lives in `packages/storage/storage/src/index.ts` and `src/registry.ts`.

**Multiple backends stay mounted side by side**; which backend serves a domain is `dsh-domain`'s configuration (below), never a global either-or. Disposer semantics = remove the name from the table; closing the backend itself belongs to the backend package's effect closure, unregister first then close.

A backend is one **medium owner** (a file-tree root / one db file) exposing primitives through **data-shape facets** — only `kv` this phase; the session migration adds `log` (see the migration section). A facet is an optional member: absence means the backend cannot serve that shape, and resolution fails loud. The `kv` facet's primitive surface: `open(descriptor)` (descriptor = name/version/table list/global flag, with names and table names restricted to `^[a-z][a-z0-9_]*$` doubling as file-name and SQL-identifier segments) returns a unit exposing `loadAll` / `putRecord` / `deleteRecord` (missing key is a no-op) / `setGlobal` / `close` (idempotent); values are opaque JSON to the backend. The normative text (with per-method JSDoc) is `packages/storage/storage/src/backend.ts`.

The backend contract (asserted clause by clause by the shared conformance suite, one suite for both backends):

1. `open` creates when the medium holds nothing (lazy materialization allowed: may defer to the first write, but `loadAll` must immediately serve empty tables); loads when the medium exists.
2. A stored version ≠ descriptor.version → `StorageError('version-mismatch')`; no migration, no rebuild.
3. Durability: after a write primitive resolves, a process crash followed by a re-open must observe the write in `loadAll`.
4. The backend does not promise write ordering within a unit — **the caller serializes**; the backend only guarantees each single call is atomic (JSON whole-file replace / SQLite single statement).
5. `deleteRecord` is idempotent; `putRecord` overwrites.
6. Any string key / any JSON value is safe (keys never reach file paths, a structural property).
7. `close` is idempotent; any operation after close → `StorageError('closed')`.

The error vocabulary is `StorageError` with a code discriminant: `backend-not-found` / `form-not-mounted` / `duplicate-backend` / `duplicate-mount` / `version-mismatch` / `malformed-medium` / `closed` (`packages/storage/storage/src/error.ts`).

### `dsh-storage-json`

Config is `root` only (required, no default, schemastery); apply registers backend `json` inside `ctx.effect()`, and the disposer unregisters the name before `backend.close()`.

- Layout `<root>/<unitName>.json`, one file per unit; directory 0o700, files 0o600.
- File format (version stamp in the header; the file is always the current net state, `JSON.stringify(…, null, 2)` human-readable — that legibility is this backend's reason to exist):

```json
{
  "unit": { "name": "workspace", "version": 1 },
  "global": null,
  "tables": { "workspaces": { "<key>": {} } }
}
```

- Writes: every write primitive = full serialization of the in-memory state → temp write + fsync → atomic rename publish (the Windows variant follows session-persistence-jsonl's win32 path). Memory is authoritative, disk is its projection.
- `loadAll`: parse the whole file at open; a missing `unit` header, non-object tables, etc. → `malformed-medium`. A missing file = an empty unit, materialized on first write.

### `dsh-storage-sqlite`

Config is `path` (required, `':memory:'` allowed) plus `journalMode` (enum, default `wal`); apply mirrors json, registering backend `sqlite`.

- `node:sqlite` `DatabaseSync`; the open sequence follows session-persistence-sqlite: mkdir 0o700 → `open(path,'wx',0o600)` exclusive create when missing → `PRAGMA foreign_keys=ON` → journal_mode → version check → create tables.
- Physical layout version `STORAGE_SQLITE_SCHEMA_VERSION = 1` in `PRAGMA user_version`: 0 → stamp; ≠ → `version-mismatch`.
- DDL (all STRICT; table names concatenated from the restricted character set with the `u_` prefix, no external input ever reaches DDL):

```sql
CREATE TABLE IF NOT EXISTS units (name TEXT PRIMARY KEY, version INTEGER NOT NULL) STRICT;
CREATE TABLE IF NOT EXISTS unit_globals (
  unit TEXT PRIMARY KEY REFERENCES units(name), value TEXT NOT NULL) STRICT;
-- 每 unit 每表：
CREATE TABLE IF NOT EXISTS "u_<unit>_<table>" (
  key TEXT PRIMARY KEY, value TEXT NOT NULL) STRICT;             -- value = 记录 JSON 文档
```

- Unit versions live in `units` rows; a descriptor mismatch → `version-mismatch`. Row granularity is document-per-row, preserving precise per-key durable updates (the path left open for high-frequency point-update tables like the session sidecar); when query needs appear, JSON1 reads the value column directly.
- Write primitives are single statements and thus atomic; no cross-statement transactions needed (the domain layer has no cross-table transactions, see the out-of-scope list).

### `dsh-domain`: the domain data form

A single implementation, not abstracted; consumers depend on this layer only and never touch backends directly.

```ts ignore-check
export const Config = z.object({
  backend: z.string().required(),                // 默认后端名，必填
  routes: z.dict(z.string()).default({}),        // per-domain 覆盖：{ workspace: 'sqlite' }
})

export function apply(ctx: Context, config: Config) {
  ctx.effect(() => ctx.storage.mount('domain', new DomainFacility(ctx, config)))
}
```

(Facility unmount order: dispose each domain first (drain its write chain), then remove the name from the hub — in-flight writes still emit `domain/changed` during the drain, and the event-consistency invariant resolves domains back through the facility, so the name must stay resolvable at that point.)

Domain declarations (the spec object is defined and exported by the package that owns the domain — the single source of type and runtime truth; schemas use zod with `z.infer` deriving the types without re-declaration — the record model projects into RPC wire schemas next phase and the wire boundary is all zod; schemastery still owns plugin Config only):

```ts ignore-check
export interface DomainGlobalSpec<G> { readonly schema: ZodType<G>; readonly initial: G }
export interface DomainTableSpec<K extends string, V> { readonly valueSchema: ZodType<V> }

export interface DomainSpec {
  readonly name: string                          // ^[a-z][a-z0-9_]*$
  readonly version: number
  readonly global?: DomainGlobalSpec<unknown>
  readonly tables: Record<string, DomainTableSpec<string, unknown>>
}

export function defineDomain<S extends DomainSpec>(spec: S): S
export function domainTable<K extends string, V>(schema: ZodType<V>): DomainTableSpec<K, V>
```

`DomainFacility.open(spec)` exact semantics (sequential; any failing step fails the whole open):

1. A domain with this name already open → `DomainError('already-open')`.
2. Backend name = `config.routes[spec.name] ?? config.backend`; `ctx.storage.backend.get(name)` (an unmounted name propagates `backend-not-found` — misconfiguration fails loud).
3. Backend lacks the `kv` facet → `DomainError('facet-unsupported')`.
4. `kv.open(descriptorOf(spec))` (the descriptor is a direct projection of the spec).
5. `loadAll()`; every record passes `valueSchema.parse`, the global passes its schema (null takes `initial`, not persisted — first write materializes). A failure → `DomainError('invalid-record', { table, key })` (the durable boundary must validate; the write side does not re-validate).
6. Construct the `Domain` and register `ctx.effect()`: the disposer drains the write chain → `unit.close()`.

```ts ignore-check
export interface Domain</* 由 spec 推导 */> {
  readonly name: string
  readonly global: { get(): G; set(value: G): Promise<void> }   // 仅当 spec.global 声明
  table<N extends keyof S['tables']>(name: N): KvTable<KeyOf<N>, ValueOf<N>>
}

export interface KvTable<K extends string, V> {
  get(key: K): V | undefined                     // 内存快照，同步
  entries(): IterableIterator<[K, V]>
  keys(): IterableIterator<K>
  readonly size: number
  put(key: K, value: V): Promise<void>
  delete(key: K): Promise<boolean>               // false = 本就不存在
  /** Atomic read-modify-write on the domain's single write chain; fn is sync-pure. */
  update(key: K, fn: (current: V) => V): Promise<V>   // 缺 key → DomainError('missing-key')
}
```

Rules:

- **Single-level mapping**: key → record, no nested tables; hierarchical needs use composite keys or fields inside the value. The two backends stay isomorphic as a result (one JSON object level ↔ one SQLite row).
- **Records are plain data**: immutable, directly JSON-serializable POJOs; values returned by `get`/`entries` must not be mutated in place (TypeScript readonly projection, no runtime freezing). Behavior-carrying domain objects belong to consumer packages.
- **Serialized writes**: one promise chain per domain; `put`/`delete`/`update`/`global.set` all queue on it; `update`'s fn runs on the chain, so concurrency cannot interleave. No active-record (pulling out a mutable object that auto-persists — uncontrollable persist timing, in conflict with the whole-unit atomic-rewrite model).
- **Version fails loud**: a stored version differing from the spec throws outright; no migration, no rebuild (the data is not regenerable; pre-release rejects old formats).
- **Change events**: after each write's durability resolves, emit `domain/changed` (`@mode emit`), one per record, no old value (matching the repository's "new snapshot + operation discriminant" convention, template `goal/changed`); the payload `DomainChanged` is a put/deleted discriminated union — domain + table + key (both `''` for global changes) + operation, with the put branch carrying the new snapshot value and the deleted branch carrying none (`packages/storage/storage-domain/src/events.ts`). This is next phase's RPC push-frame event source. The error vocabulary is `DomainError`, codes: `already-open` / `facet-unsupported` / `invalid-record` (with `{ table, key }`) / `missing-key` / `closed`.

### Future work: session-side deletion (design settled, not implemented this phase)

This section is the settled construction spec; the implementation phase changes code only, not semantics. No session-persistence file is modified this phase.

```ts ignore-check
export abstract class SessionPersistence extends Service {
  /**
   * Permanently delete one session's stored log.
   * Queued on the per-id write chain (serialized with in-flight appends).
   * Unknown id → reject; un-materialized create intent → cancel it and resolve.
   * After deletion the id behaves as unknown for every subsequent operation.
   */
  abstract delete(id: SessionId): Promise<void>
}
```

- JSONL backend: unlink the session's file (including the `.zstd` variant); neither file nor intent → reject.
- SQLite backend: one transaction `DELETE FROM events…; DELETE FROM sessions…`; zero rows hit and no intent → reject.
- After a successful delete, emit `'session-persistence/deleted'(id: SessionId)` (`@mode emit`; the session-persistence event surface, unrelated to `domain/changed`). Derived data (the session-query full-text index and the like) subscribes and cleans itself; the persistence layer never reaches into indexes, and the crash window is covered by derived indexes being droppable-and-rebuildable.

Orchestration rules (implemented together with the cascade; the `session.delete` RPC and the workspace cascade reuse the same rules):

| Check (in order) | On failure |
| --- | --- |
| No target (the whole subtree when recursive) is running in `ctx.sessions` | throw, delete nothing; callers cancel first then delete — the persistence layer never reaches back into the runtime |
| Non-recursive: the target has no descendants (descendants = the `parentSessionId` transitive closure, derived from `list()` headers) | throw: by default only leaves are deletable; `recursive: true` opts into recursion |
| Recursive order is bottom-up (leaves → root) | — a mid-way crash leaves only "half the subtree deleted, ancestors intact"; re-running the same delete converges, and no dangling parent exists at any moment |
| Some id in the cascade is already gone from disk | skip (idempotent resumption); any other error aborts |

### `dsh-workspace`

The package owns the `WorkspaceId` brand and exposes `ctx.workspace`. The record key is a generated uuid — path is not the key: normalization rewrites it, and reference anchors must be stable.

```ts ignore-check
export type WorkspaceId = Branded<'WorkspaceId'>
export function WorkspaceId(id: string): WorkspaceId

const workspaceRecord = z.object({
  path: z.string(),                              // realpath，见下
  title: z.string(),
  sessionIds: z.array(z.string().transform(SessionId)),
  createdAt: z.string(),                         // ISO
  updatedAt: z.string(),
})
export type WorkspaceRecord = z.infer<typeof workspaceRecord>

export const workspaceDomainSpec = defineDomain({
  name: 'workspace', version: 1,
  tables: { workspaces: domainTable<WorkspaceId, WorkspaceRecord>(workspaceRecord) },
})

declare module 'cordis' { interface Context { workspace: WorkspaceRegistry } }

export interface Workspace {
  readonly id: WorkspaceId
  readonly path: string
  readonly title: string
  readonly sessionIds: readonly SessionId[]      // 唯一真相且有序：数组序即展示序
  setTitle(title: string): Promise<void>
  /** Record a session under this workspace (idempotent). Rejects when the session
   *  header's cwd (realpath) differs from this workspace's path. */
  attachSession(sessionId: SessionId): Promise<void>
  detachSession(sessionId: SessionId): Promise<void>
  /** Live directory check, uncached. */
  status(): Promise<'ok' | 'missing-dir'>
}

export class WorkspaceRegistry extends Service {
  constructor(ctx: Context)                      // super(ctx, 'workspace')
  // start(): this.domain = await ctx.storage.domain.open(workspaceDomainSpec)
  //          实体缓存 Map<WorkspaceId, WorkspaceEntity> 重建
  create(path: string, title?: string): Promise<Workspace>   // realpath 后撞已有 → reject
  get(id: WorkspaceId): Workspace | undefined
  list(): Workspace[]
  resolveByPath(path: string): Promise<Workspace | undefined> // 同 realpath 口径，故 async
  delete(id: WorkspaceId): Promise<boolean>      // 只删注册记录；目录与 session 日志保留
}
```

- **Path canon**: the stored value = `fs.realpath(input)` (trailing slashes, `..`, and symlinks all resolved); uniqueness = string equality after normalization (a symlink resolving to the same directory counts as a collision). A missing directory makes create reject outright (realpath fails — a workspace must point at an existing directory; "Create new = make the directory" is upper-layer interaction: mkdir first, then create). The session cwd in attach checks follows the same canon. Single-valued cwd + unique path ⇒ one session structurally belongs to at most one workspace; double bookkeeping is impossible on the write side.
- **Title**: a display name, defaults to `basename(path)`, mutable, duplicates allowed. Ownership is never derived from cwd as a fallback — cwd cannot express ordering, and ownership is a workspace-side fact; sessions started headless belong to no workspace.
- Consumers see only the `Workspace` interface; `WorkspaceEntity` stays inside the package (a single implementation does not pre-split a seam). Entities are unique per id (registry cache); the record snapshot is swapped in place after each write, and the outside sees getters only. Every write funnels through the entity's internal `mutate(fn)` → `table.update`, with `updatedAt` refreshed inside mutate. Domain objects never cross RPC; next phase the wire layer projects records into zod wire schemas.
- **Session deletion remains future work.** The later [Workspace registration deletion decision](../../implemented/feature/2026-07-27-workspace-registration-deletion.md) ships `ctx.workspace.delete(id)` as a metadata-only operation that preserves Sessions and logs. Recursive Session deletion, running checks, and crash-rerun convergence belong to a separate `session.delete` capability.

Consistency doctrine (the ledger = the only ownership authority; the implementation and test baseline):

| Situation | Behavior |
| --- | --- |
| A ledger id has no session on disk | filtered at `list()`/entity projection; pruned by the next mutate; no error (a normal product of deletion crash-consistency) |
| A session's cwd matches a workspace but is not in the ledger | not owned: no merging, no adoption. The GUI may later build an "orphan sessions" area (orphans = the complement of all ledgers) |
| One session in two ledgers | structurally blocked on the write side (attach check); detected at load → throw (externally hand-edited data, never masked) |
| The workspace directory does not exist | record and ledger stay; `status()` = `'missing-dir'`; the storage layer never auto-deletes (the directory may only be temporarily moved) |

### Reuse and the session-backend migration outlook

**Long-term direction**: the pure medium operations inside session-persistence's JSONL/SQLite backends sink into `dsh-storage` backends (the session packages stay; the `SessionPersistence` seam and coordinator semantics do not move — only the file/db operation layer beneath them does). The motive for reuse: the medium layer is all filesystem operations, database calls, and cross-platform grit (Windows permission and atomic-publish variants, fsync semantics, exclusive file creation…), which should be written once; business semantics (how a session appends, when, and what) stay above — while "did this append complete correctly underneath" (durability/atomicity/platform correctness) is the lower layer's responsibility, and the responsibility boundary is the facet primitive contract. The backend interface is therefore designed as **medium owner + data-shape facets**: a session log is an append-only stream, a different shape from KV — forcing them into one set of primitives would deform both, so facets split them (`kv` this phase, `log` at migration) while sharing the medium and its lifecycle.

The current reuse audit (an account already legible before the migration):

| Existing session-persistence logic | Nature | Disposition |
| --- | --- | --- |
| JSONL: temp write + fsync + link/unlink atomic publish, 0o700/0o600 permissions, Windows variant (win32.ts) | pure medium | copied by `dsh-storage-json` this phase (whole-file atomic rewrite is the same protocol); becomes the shared implementation at migration |
| JSONL: line-append, first-line header fast read, zstd per-frame compression | log shape | stays put; moves into the `log` facet at migration |
| SQLite: openDatabase (mkdir/exclusive create/PRAGMA sequence/user_version check) | pure medium | copied by `dsh-storage-sqlite` this phase — the two openDatabase copies are already near line-identical and this group is the third user; copy now, extract at migration |
| SQLite: events/sessions schema, same-transaction materialization | log shape | stays put; moves into the `log` facet at migration |
| coordinator (per-id write chain, lazy materialization, crash repair, flush barrier) | session semantics | never sinks — event-log domain logic whose counterpart here is the domain layer's write chain; each owns its own |
| encodeSegment (id-to-path escaping) | medium utility | unused on the domain side (keys never reach paths); sinks together with the `log` facet (one file per session) at migration |

**This phase does not touch session-persistence's medium code** (only the delete primitive is added); the table above is the migration-phase work list and the design evidence that the backend interface must accommodate the log shape.

### Test matrix

| Suite | Coverage | Backends |
| --- | --- | --- |
| backend contract (shared suite, written once, run on both) | the seven contract clauses + version rejection + close idempotence | json, sqlite (`:memory:` + temp dirs) |
| registry/mount | duplicate registration, unmounted access, disposer removal | — |
| domain layer | the six open steps, schema rejection, update serialization (concurrent interleaving stress), `domain/changed` per record, global initial-value lazy materialization, routing and `facet-unsupported` | either (json) |
| workspace | create/uniqueness/realpath, attach checks (including rejection when sessionPersistence is absent), the four consistency-doctrine cases | mock domain or json |
| session delete contract (future work, joins runPersistenceContract at implementation) | unknown id, deleted-id reuse, un-materialized intent, serialization with in-flight appends, the deleted event | jsonl, sqlite |

Snapshots: no model-visible or assembly surface this phase, none added; next phase's RPC wiring brings them with the `workspace.*` domain.

### Out-of-scope list

| Not doing | Trigger | Rework point | Groundwork |
| --- | --- | --- | --- |
| Session deletion (`SessionPersistence.delete`, the deleted event, recursive delete, running checks) | a destructive Session-delete product flow starts | implement the session primitive plus `session.delete`; keep it independent from Workspace registration deletion | orchestration rules and rejection table above remain groundwork; Workspace deletion preserves Sessions and logs |
| The `log` facet and the session-backend migration | any phase after this one | sink the medium operations (the reuse audit table is the work list) | the facet structure is in place; both backends' medium code is organized in sinkable shape already |
| Multi-process write protection | two host processes writing one medium | JSON backend file locks; SQLite WAL is natively multi-process | all writes already funnel through the domain's single point; locking touches backends only |
| Cross-process change observation | GUI reconnect awareness | the revision pattern (copy session-persistence) | `domain/changed` already exists in-process |
| Data migration | model changes after the first tagged release | version-driven per-domain migration | versions are on the medium from day one |
| Large-table performance | a thousand-record domain routed to json | point `routes` at sqlite, migrate the data by hand once | routing is configuration; consumers unchanged |
| Multi-segment keys | a real two-segment consumer appears (per-workspace per-session dimension data) | key generics become tuples, SQLite composite primary keys, JSON nested levels | single-level tables are the one-segment special case; no arbitrary-depth nesting; no string-concatenated keys |
| The scope dimension | a "one per workspace" domain appears and composite keys cannot express it | DomainSpec gains a scope declaration + a scope segment in file names (encodeSegment) | the name character set is already restricted; file names cannot collide |
| Cross-table atomic transactions | one business operation touching two tables of one domain atomically | `domain.transact(fn)`; JSON whole-unit rewrite is naturally atomic, SQLite wraps a transaction | — |
| Secondary indexes / conditional queries | in-memory filtering stops scaling (tens of thousands of records) | SQLite JSON1 over the value column, a read-only query facet on the seam | the JSON backend does not follow |
| Moving a session across workspaces | a product need appears | relax the attach check into a "detach first, then attach" orchestration | — |
| Session-delete RPC/GUI | a destructive Session-delete product flow starts | `session.delete` endpoint, wire schema, and explicit confirmation UI | Workspace RPC/GUI is shipped separately; no cascade coupling remains |

## Alternatives considered

- **Reusing session-persistence's coordinator/backends**: event-log semantics (append-only, turn crash repair, lazy materialization) do not match KV overwrite semantics; only the layering idea is borrowed (a coordination layer owns write ordering, backends implement minimal primitives).
- **A workspace-specific storage package, seam extracted later**: the second consumer (the session sidecar) is already foreseeable; generalizing later means touching the interface twice.
- **Merging domain and storage into one layer**: backends would be forced to touch schema validation, change events, and write serialization — domain concerns; split apart, storage backends implement only opaque primitives (the smallest replaceable surface) while the single domain implementation concentrates all domain logic (zod/events/serialization written once, not doubled per backend).
- **JSON backend as jsonl append + tombstones + compaction**: temp+fsync+rename crash safety is equivalent to append; rewriting keeps the file the net current state, human-readable, with no folding/compaction/torn-line tolerance; at domain scale a full rewrite costs the same as appending a line.
- **JSON one file per table**: under whole-file rewrites the file granularity does not affect write cost; merging per domain means fewer files and gives the global singleton a home.
- **SQLite storing a whole domain as one blob row**: any single-record change rewrites the whole domain, forfeiting per-key precise updates — SQLite's only edge over JSON reduced to zero.
- **SQLite generating typed columns from the schema**: a DDL generator is over-engineering; document-per-row suffices, revisit when real query needs appear.
- **One sqlite db file per domain**: contrary to the repository's one-database-many-tables convention.
- **A single whole-store backend choice (the session-persistence single-slot pattern)**: the initial design; changed to coexisting backends + configured routing because the hub will carry multiple data forms whose backend preferences (human-readable vs high-frequency point updates) are bound to diverge — a single slot forces the coarse "swap everything + hand-migrate data" move. The cost is one extra name lookup, backed by fail-loud.
- **path as the workspace key**: normalization/symlink resolution rewrites the path; reference anchors must be stable.
- **Ownership derived from cwd (or merged with the ledger)**: two sources of truth; cwd cannot express ordering; ownership is a workspace-side fact to begin with.
- **Change events carrying the old value**: the repository's change-event convention is "new snapshot + operation discriminant" (the sole exception, fs's before/after, is a method return value rather than an event, because the old value is unrecoverable afterwards and has a diff consumer); consumers needing diffs hold their own previous snapshot.
- **Delete auto-cancelling a running session**: the persistence/orchestration layer reaching back into the runtime dirties the layering; cancel already exists, callers compose it.

## Acceptance criteria

- This phase's four test suites all green: the shared backend contract suite on both json/sqlite, registry/mount disposer semantics, the domain layer (including the six open steps and fail-loud routing), and full workspace semantics (create/attach checks/consistency doctrine).
- `ctx.workspace` completes the create → attach → list → metadata-only delete lifecycle under a test assembly.
- Zero diff in the session-persistence packages (the acceptance line for not touching the session side this phase).
- No new snapshots this phase (no model-visible or assembly surface); added next phase with the RPC wiring.

## Risks

- **The repository's first push-mode change event on a persistence surface** (session-persistence polls revisions): the shape has the `goal/changed` template, but "the storage layer emits events" is a new precedent, validated only when next phase's RPC consumes it.
- **The JSON backend's whole-unit rewrite scale premise**: if the second consumer (the session sidecar) lands on the JSON backend at thousand-record scale before being routed to SQLite, the rewrite cost surfaces earlier than expected; the mitigation is exactly `routes` pointing at sqlite.
- **The deletion orchestration's weak dependency on `ctx.sessions`**: a headless assembly without the runtime registry treats it as "no hot sessions", leaving a window (an external process running the session); multi-process is already out of scope, accepted.
- **Facet generalization designed against the future `log` facet without implementing it this phase**: a "reserved shape does not fit" risk; mitigated by organizing both backends' medium code in the sinkable shape from the reuse audit, so when the `log` facet lands only the facet layer moves.
