# persist-on-sx: Durable state on the SX kernel

> **DRAFT outline.** Foundation subsystem — the durable substrate the other five
> currently fake with in-memory mutable lists. Build this first.
>
> **"persist" = persistence / data store, NOT the shop.** The shop/commerce vertical
> is `commerce-on-sx`.

rose-ash needs durable state: every subsystem (feed log, flow store, mod audit,
search index, acl grants, sessions) today hand-rolls an in-memory structure that
vanishes on restart. `persist-on-sx` is the one durable substrate they share. It
lives directly on the SX kernel's IO-suspension primitives (`perform`/`cek-resume`
— the third CEK phase) so a read/write `perform`s and the kernel persists at the
boundary. Concrete storage backends are injected.

## Does it cover ALL persistence? No — and on purpose.

Event-sourcing-everything is a known trap (replay cost, event schema evolution,
awkward ad-hoc queries, 5MB images in a log). So persist owns the **durable
source-of-truth substrate**, exposed as **two facets over one backend protocol**,
with two things explicitly delegated out:

| Shape | Owner | Notes |
|-------|-------|-------|
| **Event streams** (append-only, history matters) | persist — **log facet** | feed activities, mod audit, order ledger, flow state, content edits |
| **Current-state values** (KV / document, no history) | persist — **kv facet** | profiles, stock counts, config, session blobs; also where projections materialize |
| **Snapshots / read models** (derived, queryable) | persist — projections → kv/log | rebuildable from the log; persisted so you don't replay to answer a query |
| **Blobs / large objects** (images, media) | **delegated** → content-addressed store (artdag/IPFS already) | persist stores the *reference/CID*, never the bytes |
| **Cache** (ephemeral, evictable) | **out of scope** | not persistence — different lifecycle (Redis-shaped) |
| **Ad-hoc relational query** | the subsystem, over a projected read model | the log is bad at "all orders by X in March"; project into a queryable kv/SQL backend |

So: persist is the **single durable substrate** for state that's either a stream of
changes or a current value — but it does **not** force everything into an event
log, it does **not** hold blobs (only their content-addressed refs), and it does
**not** do caching. Those boundaries are the whole point of calling it a substrate
rather than "the database."

End-state: `log` (append/read streams) + `kv` (get/put/delete by key) facets, an
injectable backend protocol (mem → file → Postgres → IPFS-ref), pure projections
with incremental snapshots, optimistic concurrency, and a subscription hook so
read models (feeds, indices, audit logs) update incrementally.

## Status (rolling)

`bash lib/persist/conformance.sh` → **122/122** (Phases 1–4 complete + extensions)

## Ground rules

- **Scope:** only `lib/persist/**` and `plans/persist-on-sx.md`. May **import** the
  kernel's IO-suspension surface (`perform`, platform IO ops) — verify what's
  exported first. Do not add host primitives; a missing durable IO op is a Blockers
  entry (it belongs in `hosts/`, out of scope).
- **Architecture:** an event is `{:stream :seq :type :at :data}`; the log is an
  ordered append-only vector; a projection is `(fold step seed events)`; a kv value
  is `(get/put/delete key)`. Both facets sit on one injected backend
  `{:append :read :kv-get :kv-put :snapshot-read :snapshot-write}`. The in-memory
  backend is the test default; real backends wire in unchanged.
- **Determinism:** replay is pure — same log → same state, always. No clocks or
  randomness inside projections; time lives on the event.
- **Blobs:** store the content-address/CID and metadata; never the bytes. The blob
  backend is a separate injected dependency.
- **Commits:** one feature per commit. Progress log + tick boxes.

## Architecture sketch

```
Command / write                         Read model / value
  (append stream type data)               (project stream step seed)
  (kv-put key value)                       (kv-get key)
        │                                       ▲
        ▼                                       │
lib/persist/event.sx                    lib/persist/project.sx
  — {:stream :seq :type :at :data}        — fold step seed; incremental from snapshot
        │                                       ▲
        ▼                                       │
lib/persist/log.sx   lib/persist/kv.sx   lib/persist/snapshot.sx
  — append/read       — get/put/delete    — checkpoint; replay = snapshot + tail
  — optimistic seq    — current-state
        │                  │                    ▲
        └──────────────────┴── (perform → backend) ───┘
                            │
lib/persist/backend.sx              lib/persist/api.sx
  — injected protocol                 — (persist/append) (persist/project)
  — mem | file | pg | ipfs-ref        — (persist/kv-get/put) (persist/subscribe)
        │
        └── blobs → content-addressed store (artdag/IPFS), by reference only
```

## Phase 1 — Log + kv + in-memory backend
- [x] `event.sx` — event record, stream/seq helpers
- [x] `backend.sx` — injectable protocol + in-memory impl (log + kv)
- [x] `log.sx` — `append` (optimistic seq), `read`, `read-from`
- [x] `kv.sx` — `get`/`put`/`delete` current-state
- [x] `api.sx` + tests + scoreboard + conformance.sh

## Phase 2 — Projections + subscriptions
- [x] `project.sx` — `(project stream step seed)`, incremental fold
- [x] subscription hook — projection / kv read model re-runs on append
- [x] concurrency conflict surfaced as a real result, not a crash

## Phase 3 — Snapshots + replay
- [x] `snapshot.sx` — checkpoint a projection; replay = snapshot + tail
- [x] compaction policy; replay-determinism tests

## Phase 4 — Durable backends via kernel IO
- [x] file/log backend driven through `perform` (IO-suspension boundary)
- [x] blob backend interface (store ref/CID; bytes live in artdag/IPFS)
- [x] crash/restart replay test (mock IO platform)
- [x] migration notes for swapping mem → durable under a live subsystem

### Migration notes — mem → durable under a live subsystem

The facet API takes the backend as its first argument and never names a concrete
backend, so swapping storage is a one-line change at the open site:

```
(persist/open)                              ; in-memory (test / ephemeral)
(persist/mock-durable (persist/mem-backend)); durable protocol, in-process disk
(persist/durable-backend)                   ; production: ops cross perform → host
```

Everything above the backend — `append`/`read`/`project`/`subscribe`/`snapshot`
/`compact` — is byte-identical across all three. A subsystem migrates by:

1. **Pick the seam.** The subsystem holds one backend value (today an in-memory
   list). Replace its construction with `persist/open`/`durable-backend`; leave
   every call site untouched.
2. **Backfill.** For an existing in-memory store, replay its current state into
   the durable backend once (append historical events / `kv-put` current
   values) before cutting reads over. New writes go to durable from then on.
3. **Read models rebuild themselves.** A projection is pure `(fold step seed)`;
   after cutover, `persist/replay` (snapshot + tail) reconstructs every read
   model from the durable log — no bespoke migration of derived state.
4. **Blobs first, by reference.** Move large payloads into the content store and
   store only `persist/blob-ref`s; the log/kv stay small, so the backfill in (2)
   never copies bytes.
5. **Concurrency is already handled.** Two writers racing a stream get a
   `persist/conflict?` result, not corruption — the same on mem or durable, so
   no new code is needed at cutover.

The only behavioural difference durable introduces is that each op crosses the
kernel IO-suspension boundary (`perform`): under the real kernel the call
suspends and the host resumes it transparently, so the facet code is unaware.
Tests prove this by routing the identical request shapes through `persist/serve`
over an in-process disk (the mock-IO harness).

## Extensions (post-roadmap)
- [x] `view.sx` — materialized views: bundle stream + fold + snapshot name;
  `view-attach` keeps the snapshot current on every publish so `view-peek` is an
  O(1) read. The consumer-facing read-model abstraction (feed indices, audit
  rollups, search counters).

## Consumers (post-foundation, not in scope here)
feed/-log, flow store, mod/audit, search index, acl grants, identity sessions all
become `persist` log or kv. Track each migration in that subsystem's plan.

## Progress log
- **Ext: materialized views (122/122).** `view.sx` — `persist/view` bundles
  stream + step + seed + snapshot name; `view-attach` subscribes it to a hub so
  every publish refreshes the snapshot incrementally; `view-peek` is then an
  O(1) current read (no fold), `view-value` always folds the tail so it's never
  stale. 11 tests incl. on durable backend + a sum-over-data view.
- **Phase 4c+4d (111/111) — Phase 4 complete, roadmap done.** `recovery.sx` — a
  6-test crash/restart integration: an order ledger (event log + subscription
  kv read model + snapshot + compaction + invoice blob ref) over the durable
  backend, where "crash" drops every in-process object and "restart" rebuilds
  over the same disk + content store. Log, read model, snapshot, compacted
  replay, and blob ref all survive; seq continues; two restarts converge
  (determinism). Migration notes (mem → durable under a live subsystem) added
  inline above.
- **Phase 4b (105/105).** `blob.sx` — large objects stay out of persist. A blob
  ref is `{:cid :size :mime}`; the blob store is a SEPARATE injected dependency
  (`persist/blob-io` over an injectable transport, perform in prod / mock
  content store in tests). `persist/blob-store` puts bytes and returns ONLY the
  ref; `persist/blob-fetch` retrieves bytes via the ref. Mock store is
  content-addressed (same bytes dedupe). 14 tests assert the invariant: a ref in
  the log/kv carries the CID, never the bytes (`has-key? :bytes` is false).
- **Phase 4a (91/91).** `durable.sx` — a backend whose every op crosses the
  kernel IO boundary via `(perform {:op "persist/..." :args (...)})`. The
  transport is injectable: `persist/durable-backend` uses the kernel's
  `perform` (suspends; host resumes); `persist/mock-durable` uses
  `persist/serve` over an in-memory disk. `persist/serve` is the reference host
  + the mock-IO harness. Because the request shapes are identical, the ENTIRE
  facet stack (log/kv/project/snapshot/compaction) runs unchanged on
  mock-durable — verified. Crash/restart (drop backend, keep disk) recovers log
  + kv + snapshot by replay; seq counter continues. 15 tests. See Blockers for
  why end-to-end perform suspension isn't exercised under sx_server.exe.
- **Phase 3b (76/76) — Phase 3 complete.** Backend refactor: `last-seq` is now
  a monotonic per-stream high-water mark (backend `seqs` dict), not physical
  length, so a compacted log keeps assigning climbing seqs. Added backend
  `:truncate-through` + `persist/truncate`. `compaction.sx` — `persist/compact`
  checkpoints then drops events with seq <= snapshot seq; `should-compact?`/
  `maybe-compact` give an explicit "compact every N tail events" policy. 11
  tests: post-compaction replay value == uncompacted full replay (determinism),
  seq continuity after truncation, idempotence. `persist/count` = physical
  stored count (shrinks on compaction) vs `persist/last-seq` = logical.
- **Phase 3a (65/65).** `snapshot.sx` — a snapshot is a projection state
  `{:value :seq}` stored in the kv facet under `snapshot/<name>`.
  `persist/checkpoint` replays + saves; `persist/replay` = snapshot + tail.
  11 tests assert the headline both ways: snapshot+tail == full replay (value
  and whole state), plus replay determinism.
- **Phase 2c (54/54) — Phase 2 complete.** `concurrency.sx` — optimistic
  concurrency: `persist/append-expect b stream expected ...` refuses the append
  if the stream advanced past `expected`, returning a conflict VALUE
  `{:conflict true :expected :actual}` (never a crash, never a silent
  overwrite). `persist/conflict?` + accessors; caller re-reads actual and
  retries. 8 tests incl. two-writer race + retry.
- **Phase 2b (46/46).** `subscribe.sx` — `persist/hub` wraps a backend with
  per-stream callbacks. `persist/publish` appends then fires subscribers
  `(backend stream event)`; direct `persist/append` bypasses them by design
  (bulk load/replay). Canonical use: callback re-runs `project-resume` or bumps
  a kv counter so read models update on write. 9 tests.
- **Phase 2a (37/37).** `project.sx` — projection state `{:value :seq}`;
  `persist/project` folds whole stream from seed, `persist/project-resume`
  folds only the tail (seq > prior seq) so read models update incrementally.
  step is pure `(value event) -> value`. 9 tests incl. resume==full-from-zero.
- **Phase 1 complete (28/28).** `event.sx` (event record + accessors),
  `backend.sx` (injectable protocol + in-memory log/kv impl, closure state via
  set!), `log.sx` (append/read/read-from, sequential per-stream seq, stream
  isolation), `kv.sx` (get/put/delete/has?/keys/get-or/update), `api.sx`
  (`persist/open` — mem default, backend injectable). conformance.sh + three
  suites (event/log/kv). Gotcha logged in Blockers: `map` returns an
  array-backed list not `equal?` to a `(list ...)` literal — assertions build
  compared lists with list/nth.

## Blockers
- **Phase 4 perform-suspension not exercised end-to-end under sx_server.exe (by
  design, not a bug).** The CEK suspension primitives (`cek-step-loop`,
  `cek-resume`, `cek-suspended?`, `cek-io-request`) and a settable SX-level IO
  hook are only bound by the `run_tests` OCaml binary (out of scope: hosts/, and
  sx_build is forbidden). Under `sx_server.exe`, an unhandled `perform` resolves
  through the OCaml io-request/io-response stdin bridge (production path) — not
  callable from the pure-eval conformance harness. Resolution: the durable
  backend's transport is injectable, so the production path is one line
  `(perform req)` (kernel-handled) and ALL durable logic is tested through the
  mock transport (`persist/serve` over an in-memory disk). The single untested
  line is the kernel primitive itself. No host primitive needed; nothing to fix.
- **Not a blocker, a testing convention:** `map` returns an array-backed list
  that is NOT `equal?` to a `(list ...)` cons-literal (two `map` results do
  compare equal to each other). When asserting list-shaped results against a
  `(list ...)` literal, build the compared value with `list`/`nth`/`cons`, not
  `map`. `into`/list-coercion needs the IO bridge and is unusable in the
  pure-eval harness.