lmxopcua/docs/drivers/TwinCAT-Test-Fixture.md

# TwinCAT test fixture

Coverage map + gap inventory for the Beckhoff TwinCAT ADS driver.

**TL;DR:** Integration-test suite lives at
`tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.IntegrationTests/`. `TwinCATXarFixture`
probes TCP 48898 on an operator-supplied runtime; the suite runs **14
`[TwinCATFact]` methods + one 16-case `[TwinCATTheory]` = 30 test cases** end-to-end
through the real ADS stack when the runtime is reachable, skips cleanly
otherwise. The runtime can be a Hyper-V XAR VM or a TCBSD VM
(`TwinCatProject/README.md` covers both). Unit tests via `FakeTwinCATClient`
still carry the exhaustive contract coverage alongside.

TwinCAT is the only driver outside Galaxy that uses **native notifications**
(no polling) for `ISubscribable`. The integration suite verifies that path on
the wire; the fake exposes a fire-event harness so notification routing is
also contract-tested rigorously at the unit layer.

## What the fixture is

**Integration layer**: `tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.IntegrationTests/`
— `TwinCATXarFixture` TCP-probes ADS port 48898 on the host supplied by
`TWINCAT_TARGET_HOST` (defaults to `localhost`) + requires
`TWINCAT_TARGET_NETID` (AmsNetId of the runtime). Optionally takes
`TWINCAT_TARGET_PORT` (default `851` = TC3 PLC runtime 1). No fixture-owned
lifecycle — XAR / TCBSD can't run in Docker because they bypass the host
kernel scheduler, so the runtime stays operator-managed.
`TwinCatProject/README.md` documents the required project state; the tests
gate on `[TwinCATFact]` / `[TwinCATTheory]` and skip cleanly when
`TWINCAT_TARGET_NETID` is unset or the probe fails.

**Unit layer**: `tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/` remains the
primary contract coverage. `FakeTwinCATClient` fakes the
`AddDeviceNotification` flow so tests can trigger callbacks without a running
runtime.

## What it actually covers

### Integration (live runtime)

Every capability the driver implements is exercised on the wire:

- **Read** — `Driver_reads_seeded_DINT_through_real_ADS` (AMS handshake +
  symbolic read of `GVL_Fixture.nCounter`)
- **Write + read round-trip** — `Driver_write_then_read_round_trip_on_scratch_REAL`
  on `GVL_Fixture.rSetpoint`
- **Array element round-trip** — `Driver_round_trips_array_element_write_and_read`
  on `GVL_Arrays.aReal1D[5]` (exercises `TwinCATSymbolPath` subscript
  rendering)
- **Subscribe (native ADS notifications)** —
  `Driver_subscribe_receives_native_ADS_notifications_on_counter_changes`;
  observes `OnDataChange` firing within 10 s of subscribe
- **Symbol browse (direct client path)** —
  `Driver_browses_committed_symbol_hierarchy_via_real_ADS` via
  `ITwinCATClient.BrowseSymbolsAsync`
- **Symbol browse (through DiscoverAsync + `IAddressSpaceBuilder` pipeline)** —
  `DiscoverAsync_renders_declared_tags_and_controller_browse_hits_address_space_builder`
  verifies the real `TwinCAT/ → device/ → Discovered/` folder tree
- **Auto-reconnect** — `Driver_auto_reconnects_after_underlying_client_is_disposed`
  disposes the `AdsClient` mid-flight; next read must re-establish
- **Primitive type coverage** — `Driver_reads_every_primitive_type_with_correct_mapping`
  runs as a `[Theory]` against the 16 primitives in `GVL_Primitives`
  (Bool, SInt, USInt, Int, UInt, DInt, UDInt, LInt, ULInt, Real, LReal,
  String, Time, TimeOfDay, Date, DateTime) — asserts status + CLR type +
  seed value where ergonomic
- **Bit-indexed BOOL** — `Driver_reads_bit_indexed_BOOL_from_word` against
  `GVL_Primitives.vWord.3` + `.4` (bits of `0x​BEEF`)
- **Nested UDT navigation** — `Driver_reads_deeply_nested_UDT_path` reads
  `GVL_Plant.Line1.Stations[1].Axes[1].Motor.Temperature` (LREAL) + `.Running` (BOOL)
- **Multi-device routing + isolation** —
  `Driver_routes_reads_per_device_and_isolates_unreachable_peers` pairs the
  real runtime with a bogus AmsNetId; healthy device reads still succeed
- **Probe loop + `IHostConnectivityProbe`** —
  `Probe_loop_raises_host_status_transition_to_Running_on_reachable_target`
  asserts `OnHostStatusChanged → Running` and snapshot parity
- **Negative error mappings** —
  `Driver_reports_errors_for_unknown_tag_and_nonexistent_symbol_and_readonly_write`
  covers `BadNodeIdUnknown`, ghost-symbol communication errors, and the
  `BadNotWritable` short-circuit

All tests gate on `TWINCAT_TARGET_NETID` (required) via `[TwinCATFact]` /
`[TwinCATTheory]`; `TWINCAT_TARGET_HOST` (default `localhost`) and
`TWINCAT_TARGET_PORT` (default `851`) are optional overrides.

### Unit

- `TwinCATAmsAddressTests` — `ads://<netId>:<port>` parsing + routing
- `TwinCATCapabilityTests` — data-type mapping (primitives + declared UDTs),
  read-only classification
- `TwinCATReadWriteTests` — read + write through the fake, status mapping
- `TwinCATSymbolPathTests` — symbol-path routing for nested struct members
- `TwinCATSymbolBrowserTests` — `ITagDiscovery.DiscoverAsync` via
  `BrowseSymbolsAsync` + system-symbol filtering
- `TwinCATNativeNotificationTests` — `AddDeviceNotification` registration,
  callback-delivery-to-`OnDataChange` wiring, unregister on unsubscribe
- `TwinCATDriverTests` — `IDriver` lifecycle

Capability surfaces whose contract is verified at the unit layer: `IDriver`,
`IReadable`, `IWritable`, `ITagDiscovery`, `ISubscribable`,
`IHostConnectivityProbe`, `IPerCallHostResolver`. The integration suite now
verifies `ITagDiscovery` + `IHostConnectivityProbe` on the wire as well.

## Bugs caught by live runs

The integration suite surfaced three driver defects that `FakeTwinCATClient`
couldn't, since each lived below the abstraction boundary:

1. **Notification cycle time unit** — `NotificationSettings(cycleTime, maxDelay)`
   takes **milliseconds** per Beckhoff InfoSys
   (`tcadsnetref/7313319051`), but the driver was multiplying by `10_000`
   under a "100 ns units" assumption. A requested 250 ms cycle was being set
   to ~41 minutes — subscribe never fired. Fix in `AdsTwinCATClient.AddNotificationAsync`.
2. **`STRING(N)` / `WSTRING(N)` type mapper** — `MapSymbolTypeName` only
   matched bare `"STRING"` / `"WSTRING"`, so sized strings (the common case)
   fell off `BrowseSymbolsAsync` entirely. Fix: strip the `(…)` bound before
   the switch.
3. **Bit-indexed BOOL path** — driver was sending `"GVL.vWord.3"` to ADS as
   a BOOL read. TwinCAT's symbol table doesn't expose bit-access paths; the
   read returned `DeviceSymbolNotFound`. Fix: strip the `.N` suffix, read
   the parent word as `uint`, extract the bit locally via `ExtractBit`.

All three paths are now pinned by live-wire tests.

## What it does NOT cover

### 1. AMS / ADS wire framing

No raw AMS packet is inspected. Beckhoff's `TwinCAT.Ads` NuGet (their own
.NET SDK, not libplctag-style OSS) has no in-process fake at the frame
level; tests run against a real router.

### 2. Multi-route AMS

ADS supports chained routes (`<localNetId> → <routerNetId> → <targetNetId>`)
for PLCs behind an EC master / IPC gateway. Parse coverage exists; wire-path
coverage is single-hop only.

### 3. Notification coalescing under jitter

`AddDeviceNotification` delivers at the runtime's cycle boundary; under
sustained CPU load or network jitter real notifications can coalesce. The
live test only asserts at-least-one delivery within a generous window —
coalescing behavior under stress isn't verified.

### 4. TC2 vs TC3 variant handling

TwinCAT 2 (ADS v1) and TwinCAT 3 (ADS v2) have subtly different
`GetSymbolInfoByName` semantics + symbol-table layouts. Driver + tests target
TC3; TC2 compatibility is not exercised.

### 5. Alarms / history

Driver doesn't implement `IAlarmSource` or `IHistoryProvider` — not in scope
for this driver family. TwinCAT 3's TcEventLogger could theoretically back
an `IAlarmSource`, but shipping that is a separate feature.

## When to trust TwinCAT tests, when to reach for a rig

| Question | Unit tests | Real TwinCAT runtime |
| --- | --- | --- |
| "Does the AMS address parser accept X?" | yes | - |
| "Does notification → `OnDataChange` wire correctly?" | yes (contract) | yes |
| "Does symbol browsing filter TwinCAT internals?" | yes | yes |
| "Does a real ADS read return correct bytes?" | no | yes (required) |
| "Does auto-reconnect work on router restart?" | no (contract only) | yes (required) |
| "Do notifications coalesce under sustained load?" | no | yes (required) |
| "Does a TC2 PLC work the same as TC3?" | no | yes (required) |

## Follow-up candidates

Deferred to v3 — see [`docs/v3/twincat-backlog.md`](../v3/twincat-backlog.md).
Covers TC2 coverage, notification-coalescing-under-load, multi-hop AMS,
license-rotation automation, and a dedicated lab IPC.

## Key fixture / config files

- `tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.IntegrationTests/TwinCATXarFixture.cs`
  — TCP probe + skip-attributes + env-var parsing
- `tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.IntegrationTests/TwinCAT3SmokeTests.cs`
  — wire-level test suite (14 `[TwinCATFact]` + 16-case `[TwinCATTheory]`)
- `tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.IntegrationTests/TwinCatProject/README.md`
  — project spec + VM setup + license-rotation notes
- `tests/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT.Tests/FakeTwinCATClient.cs` —
  in-process fake with the notification-fire harness
- `src/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs` — ctor is
  `(TwinCATDriverOptions, string driverInstanceId, ITwinCATClientFactory? = null)`