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perf(mtconnect): read through the live pump index; instance-id check on the /current fallback (R06:T-4, T-2)

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- T-4: every ReadAsync batch fetched and parsed the whole device /current and
  built a fresh index even while the /sample pump held a live one. A read is now
  answered from the pump's index (read only - the pump stays its single writer)
  while the current-generation pump is running, has delivered since it started and
  is not degraded; /current stays the fallback, and its per-batch index is built
  over a tag map frozen once per binding instead of per call. A served read clears
  the read-path degradation a failed /current left.
- T-2: the /current fallback checks the answer's instanceId; a change (an Agent
  restart seen by a browse-only driver) drops the cached /probe model and raises
  rediscovery once per distinct id, only while the read's session is still installed.
- The pump also acts on no verdict (Degrade / latch) once its session generation
  is gone, so an abandoned pump cannot write health over the new session (T-1 follow-through).

Refs #743

Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
This commit is contained in:
Joseph Doherty
2026-10-22 15:39:58 -04:00
parent 77bfadbdda
commit b548cc46e6
4 changed files with 293 additions and 5 deletions
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docs/drivers/MTConnect.md
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@@ -87,7 +87,7 @@ request/response handshake — is a different mechanism and remains deferred; se
| Capability | Path | Notes |
|------------|------|-------|
| `ITagDiscovery` | `DiscoverAsync` — streams `/probe`'s Device→Component→DataItem tree into the address-space builder | `SupportsOnlineDiscovery = true`, `RediscoverPolicy = Once` |
| `IReadable` | `ReadAsync` → one `/current` per call | **Not the production data path** — see below |
| `IReadable` | `ReadAsync` → the live `/sample` pump's index while it is delivering and healthy, else one `/current` per call | **Not the production data path** — see below |
| `IWritable` | `WriteAsync` → `POST {agent}/{device}?{dataItemId}={value}` on the Agent's `AllowPut` surface | **Off by default** — needs `Write.Enabled` **and** the Agent's `AllowPut`; CONDITION items are refused |
| `ISubscribable` | `SubscribeAsync`/`OnDataChange` — the shared `/sample` long-poll pump | **The production data path** |
| `IHostConnectivityProbe` | periodic `/probe` under `Probe.*` | No consumer wires it today — see [Known limitations](#known-limitations) |
@@ -113,6 +113,15 @@ subscribes once and lives off `OnDataChange`. `ReadAsync` exists for the
Client CLI (`... read -n ...`) and for the unit/integration suites; it is
correct and tested, just not on the hot path.
**Read path (review 2026-10-21 T-4 / T-2).** While the current-generation `/sample` pump is running, has
delivered since it started (a chunk, heartbeat or re-baseline) and its stream is not degraded, a read is answered
from the pump's observation index — **read only**; the pump stays its single writer — with no Agent round trip.
Otherwise (no subscription, a pump still catching up after a resubscribe, a degraded stream) it falls back to one
`/current`, whose per-batch index is now built over a tag map frozen once per binding. A served read also clears a
read-path degradation an earlier failed `/current` left (the read path is answering). The `/current` fallback now
checks the answer's `instanceId`: a changed one (an Agent restart seen by a browse-only driver with no pump) drops the
cached `/probe` model and raises `OnRediscoveryNeeded` once per distinct id — it used to be detected only by the pump.
## Minimum deployment
```jsonc