# Cluster coordination through Spark Control (v0.25.0)

Spark Control is the **GPU arbiter, not a job runner.** Your recurring pipelines
(model-warming crons, "daily X" generators, batch jobs) live in your own
services and *drive Spark Control's swap API*. This page documents the safety
layer around that: a **swap reservation lock**, a **swap-event webhook**, and a
**read-only schedule registry**.

If only the dashboard ever swaps models, you don't need any of this — it's for
when something automated also swaps.

All endpoints are on the Spark Control host (same LAN/VPN URL as the LLM, audio,
and embeddings proxies). There is no API-token auth by design (LAN + split-tunnel
VPN only); a non-browser client passes the same-origin guard automatically.

---

## 1. Swap reservation lock

A short, TTL-bounded reservation of the swap path. While a lock is held, **any
real swap that doesn't present the holder's token is refused with `423 Locked`**
— including the dashboard's manual swap. The holder *name* is descriptive; the
returned **token** is the secret that authorises swaps and the release.

The lock is in-memory: it resets to *unlocked* if Spark Control restarts (the
safe-for-availability default), and the swap engine's own in-progress guard
still prevents two swaps running at once.

### `POST /api/swap/lock` — acquire (or extend)

```json
// request
{ "holder": "openclaw-daily-vol", "ttl_seconds": 900, "note": "daily vol run" }

// 200 response
{
  "held": true,
  "holder": "openclaw-daily-vol",
  "acquired_at": "2026-06-17T12:00:00+00:00",
  "expires_at":  "2026-06-17T12:15:00+00:00",
  "seconds_remaining": 900,
  "note": "daily vol run",
  "token": "a1b2c3…"        // SECRET — store it; needed to swap and to release
}
```

- `ttl_seconds` is optional (default 900) and clamped to `[1, 86400]`.
- **`409`** if a *different* holder already holds it (body includes the current
  `lock` state). To **extend** your own lock, POST again with the same `holder`
  **and** your `token` — the token is preserved and the window slides forward.

### `GET /api/swap/lock` — status (no token)

```json
{ "held": true, "holder": "openclaw-daily-vol", "expires_at": "…", "seconds_remaining": 612, "note": "…" }
// or
{ "held": false }
```

### `DELETE /api/swap/lock` — release

Send your token in the `X-Swap-Lock-Token` header (or `?token=`):

```
DELETE /api/swap/lock
X-Swap-Lock-Token: a1b2c3…
```

- **`403`** if the token doesn't match. The dashboard's human override is
  `DELETE /api/swap/lock?force=true` (no token).

### Swapping while you hold the lock

Pass the token on the swap call; the dashboard (no token) is then blocked:

```
POST /api/swap
X-Swap-Lock-Token: a1b2c3…
{ "model_key": "gemma-3-27b" }
```

Recommended scheduler flow: **acquire → swap (with token) → poll `/api/swap/{id}`
→ release**. Always release in a `finally`; if you crash, the TTL frees it.

> `POST /api/swap/{key}/validate` (pre-flight) and dry-run swaps are **not**
> blocked by the lock — they don't touch the cluster.

---

## 2. Swap-event webhook

Configure a URL in **Configure Sparks → "Swap webhook URL"**. After every real
swap, Spark Control POSTs:

```json
{
  "event": "swap_complete",   // or "swap_failed"
  "job_id": "1a2b3c4d",
  "model_key": "gemma-3-27b",
  "state": "ready",           // or "failed"
  "returncode": 0,
  "started_at": "2026-06-17T12:00:00+00:00",
  "finished_at": "2026-06-17T12:03:11+00:00",
  "dry_run": false
}
```

Headers: `X-Spark-Event: swap_complete`. If you set a **webhook secret**, the
body is signed: `X-Spark-Signature: sha256=<hmac>` (HMAC-SHA256 of the raw body
with the shared secret). Verify it like:

```python
import hmac, hashlib
expected = "sha256=" + hmac.new(secret.encode(), raw_body, hashlib.sha256).hexdigest()
assert hmac.compare_digest(expected, request.headers["X-Spark-Signature"])
```

Delivery is best-effort and fire-and-forget (5 s timeout, no retries) — a
webhook failure never affects the swap itself. Dry runs don't fire.

---

## 3. Schedule registry (read-only display)

So the dashboard can show *what's scheduled to touch the GPU and when*, your
schedulers register their jobs here. **Spark Control only displays these — it
never executes them.**

### `POST /api/schedule` — register / update

```json
// request (pass a stable `id` to update in place on re-register)
{ "id": "daily-vol", "name": "Daily Vol", "owner": "openclaw",
  "cron": "0 6 * * *", "next_run": "2026-06-18T06:00:00Z",
  "description": "Swaps to the big model, generates the vol report" }

// response: the stored entry (generates an id if you omit one)
```

`name` is required; `id` (if given) must match `[A-Za-z0-9_.-]` (≤64 chars).

### `GET /api/schedule` — list

```json
{ "schedules": [ { "id": "daily-vol", "name": "Daily Vol", "owner": "openclaw",
  "cron": "0 6 * * *", "next_run": "…", "description": "…",
  "registered_at": "…", "updated_at": "…" } ] }
```

### `DELETE /api/schedule/{id}` — deregister

```json
{ "deleted": true }
```

The registry is in-memory — re-register your schedules on your own startup so
they survive a Spark Control restart.