# Design — @suaveplan/storage-mux

Non-normative companion to `proposal.md` and `specs/storage-mux/spec.md`.

## 1. Role in the family

```text
@suaveplan/storage                 AbstractStorage core (key/value, in-memory)
  ├─ @suaveplan/storage-cdn        CdnAdapter contract
  │    ├─ @suaveplan/storage-cdn-bunnycdn
  │    └─ @suaveplan/storage-cdn-rackspace
  ├─ @suaveplan/storage-s3
  ├─ @suaveplan/storage-http
  ├─ @suaveplan/storage-file
  └─ @suaveplan/storage-mux        Multi-backend composition (THIS PACKAGE)
                                    └─ wraps N StorageLike, exposes 1 StorageLike
```

`storage-mux` is a **composition** package, not a transport. It accepts any list of `StorageLike` (cdn / s3 / http / file / memory / other mux instances) and produces a single `StorageLike`. Recursion is intentional — a sharded-then-failed-over deployment is `Mux(round-robin, [Mux(failover, shard1), Mux(failover, shard2)])`.

## 2. File layout

```text
src/
  index.ts                                # explicit named re-exports
  error-codes.ts                          # defineCodes("STORAGE_MUX", [...]) + StorageMuxError
  types/
    backend-entry.types.ts                # BackendEntry interface
    mux-options.types.ts                  # StorageMuxOptions, MuxPolicy union
    mux-options.schemas.ts                # Zod boundary schemas
  mux/
    mux.ts                                # StorageMux extends AbstractStorage
    mux.md
    mux.test.ts
    mux.spec-conformance.test.ts          # runStorageConformance × 4 policies
  policies/
    selector.types.ts                     # BackendSelector interface
    failover/
      failover.ts
      failover.md
      failover.test.ts
    round-robin/
      round-robin.ts
      round-robin.md
      round-robin.test.ts
    sticky-by-key/
      sticky-by-key.ts                    # stable hash → backend index
      sticky-by-key.md
      sticky-by-key.test.ts
    shield/
      shield.ts                           # reads cascade, writes fan-out
      shield.md
      shield.test.ts
  health/
    health.ts                             # BackendHealth tracker + cooldown
    health.md
    health.test.ts
  observability/
    events.ts                             # event topic constants + emit helpers
    events.md
    events.test.ts
    otel.ts                               # span attribute helpers
    otel.md
    otel.test.ts
  error-codes/
    error-codes.test.ts                   # reachability sweep
```

## 3. Policy state machines

### 3.1 `failover`

```text
            ┌──────────────────────────────────────────┐
            ▼                                          │
[idx=0] ─ call backend[idx] ── ok ──► return           │
            │                                          │
            ├── transient error ─────► idx++ ──────────┘
            │
            ├── non-transient error ──► throw (surface as-is)
            │
            └── idx == backends.length ─► throw STORAGE_MUX.ALL_BACKENDS_DOWN
                                          (cause = last transient error)
```

Per-attempt: `runWithRetry(retryStrategy, () => backend[idx].op(...))` so transient errors get the configured retry inside a single backend before falling through.

### 3.2 `round-robin`

Atomic counter modulo `healthyBackends.length`. On `ALL_BACKENDS_DOWN`, returns the failover error. Down backends skipped via `BackendHealth.isUp(name)`.

### 3.3 `sticky-by-key`

```ts
const h = stableHash(key) % healthyBackends.length;
return healthyBackends[h];
```

Hash is FNV-1a over UTF-8 bytes (universal, no crypto dep). When a backend goes down, the modulo redistributes — same key may move to a different backend; this is acceptable for cache workloads and documented.

### 3.4 `shield`

```text
read:  backend[0] (origin) ?? backend[1..n] (edge mirrors) ── first success wins
write: Promise.allSettled(backend[0..n].set(...))
       ├── all ok                      ─► return
       ├── primary ok, mirror failed   ─► return + emit WRITE_FANOUT_PARTIAL event
       └── primary failed              ─► throw (cause = primary error)
```

Distinct from `failover` because `shield` reads **must** hit origin first (origin is canonical), and writes **must** fan out (mirrors only serve what was written to them). `failover` is symmetric — any backend can serve any operation.

## 4. Error classification

Which errors trigger failover vs surface directly:

| Error shape | Classification | Triggers fall-through? | Code surfaced |
|---|---|---|---|
| `AbortError` / `signal.aborted` | non-transient | NO | `STORAGE_MUX.BACKEND_ABORTED` |
| `STORAGE.KEY_NOT_FOUND` (from `get`/`stat`) | semantic | NO | rethrow as-is |
| `STORAGE.INVALID_KEY` | semantic | NO | rethrow as-is |
| Network timeout (configurable `backendTimeoutMs`) | transient | YES | wrapped, last in cause chain |
| `ECONNRESET` / `ETIMEDOUT` / fetch network error | transient | YES | wrapped |
| HTTP 5xx from backend | transient | YES | wrapped |
| HTTP 4xx (except 408/429) | non-transient | NO | rethrow |
| HTTP 408 / 429 | transient | YES | wrapped |
| `STORAGE_CDN.CREDENTIAL_EXPIRED` | non-transient | NO | rethrow (caller must rotate) |
| Unknown thrown value (non-Error) | transient | YES | wrapped |

Classification lives in `src/policies/classify.ts`, pure function over `(error: unknown) => "transient" | "semantic" | "abort"`. Tested independently.

## 5. Backend-isolation guarantees

A slow primary MUST NOT block a fallback. Mechanism:

1. **Per-backend timeout** — every backend call wrapped in `Promise.race([call, timeoutPromise(backendTimeoutMs)])`. Default 30 s, configurable per-mux.
2. **AbortSignal forwarding** — the per-attempt signal is linked (`AbortSignal.any([userSignal, timeoutSignal])`) and passed to the backend. A well-behaved backend respects it; a misbehaved backend will eventually reject via the timeout race.
3. **`shield` reads cascade sequentially with timeout** — origin gets `backendTimeoutMs`; on timeout/error, the mux moves to mirror[0], etc. The next backend's call starts before the previous backend's underlying promise has settled (it is left to its own cleanup).
4. **`shield` writes are `Promise.allSettled`** — one slow mirror cannot starve the others. Caller sees the result when all settle OR `backendTimeoutMs * 2` elapses (guard against a permanently-hung mirror).

The mux NEVER waits for a hung promise indefinitely — `Promise.race` with a clock-driven timeout is the contract.

## 6. Health tracking

`BackendHealth` per-backend state:

```ts
interface BackendState {
  status: "up" | "down";
  consecutiveFailures: number;
  cooldownUntil: number;      // Clock.now() millis
  lastError?: unknown;
}
```

Transitions:

- **up → down**: triggered by any classified-transient error. `cooldownUntil = now + cooldownMs * 2 ** min(consecutiveFailures, 6)` (exponential capped at 64× base). Emits `storage-mux.backend.down`.
- **down → up**: triggered when `now >= cooldownUntil` AND the next selection call probes the backend AND the probe succeeds. Resets `consecutiveFailures`. Emits `storage-mux.backend.recovered`.
- **active healthcheck**: if `BackendEntry.healthcheck` is provided AND `healthcheckIntervalMs` is set, the mux schedules `Clock.setTimeout` to invoke it; failure marks the backend down without waiting for a real request.

`Clock` is injected (`@suaveplan/types#Clock`), so tests use `FakeClock` for deterministic cooldown / healthcheck advancement.

## 7. Tier purity

Universal — no `node:*`, no `bun:*`, no DOM. Hash uses pure-JS FNV-1a. Timeouts use injected `Clock`. AbortSignal is standard Web API. Conformance harness runs under Node, Bun, browsers, Workers.

## 8. Trade-offs

- **Strategy classes vs. switch statement.** Chose strategy classes (`BackendSelector` interface). Each policy is a separate file ≤ 250 lines, individually tested, and new policies (e.g. `weighted-random` later) plug in without editing the core `StorageMux`.
- **FNV-1a vs. xxhash for `sticky-by-key`.** FNV-1a is 20 lines of pure JS with no dep; xxhash needs a wasm or noble dep. For backend selection (low-cardinality output), FNV's distribution is sufficient. If a future workload needs better distribution, the hash function is injectable via options.
- **`shield` writes fan out vs. primary-only.** Fan-out is the right default because mirrors that don't see writes can't serve reads. Callers who want primary-only writes should use `failover` instead — the policy choice is the API.
- **Recursive Mux.** Allowed because `StorageMux` is itself a `StorageLike`. The conformance harness verifies recursion does not break invariants. The cost — debugging a deeply-nested mux tree — is mitigated by per-call OTel span attributes naming every backend hop.