# AB Legacy diagnostic counters

Per-device diagnostic counters surface as auto-generated read-only OPC UA
variables under each device's synthetic `_Diagnostics/` folder. HMIs can bind
directly without going through a separate diagnostics RPC. Mirrors the AB CIP
`_System/` pattern from PR abcip-4.3.

Closes #253 (PR ablegacy-10).

## The seven counters

Each device managed by the `AbLegacyDriver` exposes seven read-only nodes under
`AbLegacy/<host>/_Diagnostics/<name>`:

| Name | Type | Semantics |
|---|---|---|
| `RequestCount` | Int64 | Total `ReadAsync` requests issued against this device. One increment per non-diagnostic reference per call, success or failure. |
| `ResponseCount` | Int64 | Successful read responses. |
| `ErrorCount` | Int64 | Failed read responses (any non-Good status). |
| `RetryCount` | Int64 | Retry attempts beyond the first per the PR 9 retry loop. A single read with two retries adds two. |
| `LastErrorCode` | Int32 | Most recent libplctag status code on a failed read; `0` when no error has been seen since the last reset. |
| `LastErrorMessage` | String | Most recent libplctag error message on a failed read; empty when no error has been seen since the last reset. |
| `CommFailures` | Int64 | Count of read failures mapped to `BadCommunicationError`. Spans transient libplctag throws + retried-out chains so operators see a single "wire fell off" counter. |

**Address shape**: `_Diagnostics/<deviceHostAddress>/<name>` —
e.g. `_Diagnostics/ab://10.0.0.5/1,0/RequestCount`.

The `<deviceHostAddress>` segment is the canonical `ab://host[:port]/cip-path`
string from `AbLegacyDeviceOptions.HostAddress`. The browse path looks like
`AbLegacy/<deviceHostAddress>/_Diagnostics/<name>` — the same shape as a
user-config tag node, just under a reserved sibling folder.

## Reset behaviour

| Trigger | Effect |
|---|---|
| `ReinitializeAsync` | Every counter for every device resets to zero, plus `LastErrorMessage` clears to empty. |
| `ShutdownAsync` | Same as Reinitialize — counters drop with the device map. |
| Driver process restart | Counters start at zero. |
| Probe transition Stopped→Running | **No automatic reset** — counters are cumulative across reconnect events so operators can spot intermittent links by watching `CommFailures` keep climbing. |

There is no in-process "reset" RPC at the time of writing. If you need to
clear counters without a redeploy, kick a `ReinitializeAsync` from the Admin
RPC surface — the driver re-EnsureDevice's each host so the freshly registered
counters start at zero.

## What does *not* increment counters

Reads against `_Diagnostics/<host>/<name>` are **driver-local observability**,
not field traffic — they short-circuit before the libplctag dispatch and do
NOT increment `RequestCount` or any other counter. Otherwise a 1 Hz HMI poll
of `RequestCount` would make the counter chase its own tail.

Writes against `_Diagnostics/*` are rejected with `BadNotWritable` because
every diagnostic node is `SecurityClassification.ViewOnly` — a misbehaving
SCADA template can't accidentally clobber the diagnostic surface.

## Collision with user tags

User-config tags must not shadow the seven reserved diagnostic names and
must not live under the synthetic `_Diagnostics/` folder. Both shapes are
rejected at `InitializeAsync` time with a clear `InvalidOperationException`:

- A tag named `RequestCount` (or any of the other six reserved names) is
  rejected because it would silently never resolve at read time — the
  diagnostics short-circuit wins.
- A tag whose `Address` starts with `_Diagnostics/` is rejected because the
  whole prefix is owned by the auto-emitted counters.

Pick a different name (`SiteRequestCount`, `MachineRequestCount`) or a
different address path (real PCCC files like `N7:0`).

## HMI binding examples

### OPC UA Client CLI

```powershell
dotnet run --project src/ZB.MOM.WW.OtOpcUa.Client.CLI -- read `
  -u opc.tcp://localhost:4840 `
  -n "ns=2;s=AbLegacy/ab://10.0.0.5/1,0/_Diagnostics/RequestCount"
```

### AB Legacy CLI (driver-direct, no OPC UA layer)

```powershell
dotnet run --project src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Cli -- read `
  -g "ab://10.0.0.5/1,0" -P Slc500 `
  --address "_Diagnostics/RequestCount"
```

The driver-direct path lets you sanity-check the counter without standing up
an OPC UA server — useful when triaging a wire-level issue on the bench.

### Subscription pattern

Subscribe to all seven counters at a slow rate (e.g. 5–10 s) on a long-lived
overview dashboard, plus a faster rate (1 s) on `LastErrorMessage` /
`LastErrorCode` when actively debugging a flapping link. The diagnostics
short-circuit makes every read O(1) — there's no penalty for fast polling
of the counter itself, only the OPC UA subscription bookkeeping.

## Cross-references

- [`AbLegacyDiagnosticTags.cs`](../../src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDiagnosticTags.cs)
  — counter store + read short-circuit
- [`AbLegacyDriver.cs`](../../src/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs)
  — increment sites in `ReadAsync`, discovery emission in `DiscoverAsync`
- [`AbLegacy-Test-Fixture.md`](AbLegacy-Test-Fixture.md) — `AbLegacyDiagnosticsTests`
  + collision-rejection contract
- [AB CIP `_System/` parallel](../../src/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipSystemTagSource.cs)
  — same pattern with the CIP-specific six entries (incl. writeable
  `_RefreshTagDb` trigger)