# `Calculation` pseudo-driver — mini-design (v3 task #11)

**Date:** 2026-07-15
**Status:** design complete (resolves v3 open item "Calculation driver mini-design").
**Parent:** `docs/plans/2026-07-15-raw-uns-two-subtree-v3-design.md` §"Calculated tags (two-tier)".

Signal-level calculated tags are ordinary **raw tags** bound to a `Calculation` driver. They
inherit the entire raw pipeline (folders, tag-groups, CSV, historization, native alarms, UNS
references) with no special-casing. This doc pins the runtime mechanism, storage, triggers,
gates, and error semantics.

## 1. Driver shape

`CalculationDriver : IDriver, ISubscribable, IReadable` in a new
`src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Calculation/` project.

- **Not `IWritable`** — calc tags are computed sources; author them read-only
  (`AccessLevel` enforced at authoring + CSV validation).
- **Not `ITagDiscovery`** — tags are authored, never discovered
  (`SupportsOnlineDiscovery` default false ⇒ no Browse button, manual/CSV entry only —
  consistent with the universal-browser gate).
- `IReadable.ReadAsync` returns the last computed snapshot per ref
  (`BadWaitingForInitialData` before first evaluation).
- Registration: `CalculationDriverFactoryExtensions.DriverTypeName = "Calculation"` + one
  line in `DriverFactoryBootstrap.AddOtOpcUaDriverFactories` and one `TryAddEnumerable` in
  `AddOtOpcUaDriverProbes` — the standard pattern. *(Note: the live registration seam is
  `DriverFactoryRegistry`; `DriverTypeRegistry` (metadata/JSON-schema) is vestigial — nothing
  in `src/` calls its `Register`.)*
- Devices: one auto-created default device (`Engine`), empty `DeviceConfig`. No PollGroups.
- `GetHealth()` = always Connected (no backend); memory footprint = compiled-cache size.

## 2. Per-tag `TagConfig`

```json
{ "scriptId": "…", "changeTriggered": true, "timerIntervalMs": 5000 }
```

- `scriptId` (required) — logical FK to the **shared `Script` entity** (same convention as
  `VirtualTag.ScriptId`; generation-scoped, `SourceHash` compile-cache key). The AdminUI tag
  editor reuses the VirtualTagModal's script dropdown + "New script" + inline Monaco panel
  components; the standalone `/scripts/{id}` page works unchanged.
- Trigger: `changeTriggered` (default **true**) and/or `timerIntervalMs` (≥ 50), at least one
  required — the authored shape mirrors `VirtualTag`, but see §4 for the runtime reality.

## 3. Value production — how inputs reach the driver

**The gap:** drivers are host-blind; nothing feeds one driver another driver's values. The
per-node `DependencyMuxActor` already receives **every** `AttributeValuePublished` (via
`DriverHostActor.ForwardToMux`), keyed by wire-ref — under v3, RawPath.

**Mechanism — a new composable capability interface** (in the spirit of `IDriver`'s
capability set):

```csharp
/// <summary>Optional capability: the driver consumes other tags' live values.
/// The host registers the declared refs with its dependency mux and forwards changes.</summary>
public interface IDependencyConsumer
{
    /// <summary>Wire-refs (RawPaths) this driver needs, derived from its authored tags.
    /// Re-read by the host after Initialize/Reinitialize.</summary>
    IReadOnlyCollection<string> DependencyRefs { get; }

    /// <summary>Host push of a dependency value change. Called from an actor context —
    /// implementations must be non-blocking.</summary>
    void OnDependencyValue(string rawPath, object? value, uint statusCode, DateTime timestampUtc);
}
```

`DriverHostActor` wiring (~30 lines): when a spawned driver implements `IDependencyConsumer`,
spawn a tiny mux-adapter child that `RegisterInterest(driver.DependencyRefs, Self)` on the
existing `DependencyMuxActor` and forwards `DependencyValueChanged` into the driver callback.
Re-register on every apply (refs change when tags/scripts change).

**Output path is the ordinary driver path** — evaluation raises `ISubscribable.OnDataChange`
→ `DriverInstanceActor` → `AttributeValuePublished` → node fan-out **and** back into the mux.
Two things fall out for free:

1. **Calc-of-calc chains work with no extra machinery** — calc A's published output re-enters
   the mux, calc B (subscribed to A's RawPath) re-evaluates. This is exactly why the
   deploy-time **cycle gate** (§5) is mandatory: an undetected A→B→A cycle is a live
   oscillation loop, not just a stale value.
2. Historization, native alarms, and UNS references attach to the calc tag's raw node exactly
   like any driver tag.

`DependencyRefs` = the union of `EquipmentScriptPaths.ExtractDependencyRefs(source)` over the
driver's authored tags' scripts (literal-only `ctx.GetTag` paths, enforced by
`DependencyExtractor`).

## 4. Evaluation engine + triggers

- **Evaluator:** a `CalculationEvaluator` with the same construction as
  `RoslynVirtualTagEvaluator` — `CompiledScriptCache<VirtualTagContext, object?>` keyed by
  source, `TimedScriptEvaluator` (2 s default timeout), passthrough fast-path, single-tag
  mode (`ctx.SetVirtualTag` dropped + logged). Reuses `VirtualTagContext` so the Monaco
  editor, sandbox, and diagnostics pipeline apply verbatim. Catalog keys for completions are
  RawPaths; the `{{equip}}` branch is already conditional in `ScriptAnalysisService` and
  simply never triggers for raw-level scripts.
- **Change trigger:** evaluate on `OnDependencyValue` for any declared dep of that tag,
  gated like `VirtualTagActor`: publish nothing until all declared deps have arrived at least
  once; dedupe equal results.
- **Timer trigger:** owned **inside the driver** by reusing the dormant-but-tested
  `TimerTriggerScheduler` (`Core.VirtualTags`) — one timer per distinct interval group,
  per-group in-flight skip. *(Fact: the live VirtualTag runtime is change-trigger only —
  `VirtualTagActor` has no timer handler and `EquipmentVirtualTagPlan` drops the trigger
  fields. The Calc driver revives the scheduler without touching the VT runtime; whether the
  VT plan seam later gains timer support is out of scope here.)*
- **Concurrency:** evaluations run inline on the driver's dispatch (same "fast enough to run
  inline" contract as the VT evaluator); the compile cache makes steady-state evaluation
  cheap. Compiled scripts are dropped on `ReinitializeAsync` when the source set changed
  (`SourceHash` comparison, mirroring `IScriptCacheOwner` usage).

## 5. Deploy gates (new — `DraftValidator`)

Today `DraftValidator` checks only name uniqueness; `DependencyGraph` (Tarjan SCC) exists but
is dormant. v3 adds, for `Calculation`-bound tags:

1. **`scriptId` existence** — the referenced `Script` row must exist in the draft generation.
2. **Cycle gate (hard error)** — build the calc→calc edge set (tag → deps that are themselves
   Calculation tags) and run `DependencyGraph.DetectCycles`; any SCC ⇒ deploy error naming
   the cycle members. Cross-driver refs (calc reading a Modbus tag) are terminal nodes,
   never edges.
3. **Compile: deliberately NOT a hard gate** — parity with VirtualTags. The editor's live
   diagnostics mirror the compile exactly, the `StartDeployment` compile-cost advisory stays
   warn-only, and a runtime compile failure lands as Bad quality + script-log (§6). A hard
   compile gate would make deploys O(scripts) slow and diverge Calc tags from VT behavior.

## 6. Error semantics (mirrors `VirtualTagActor` 02/S13)

- Evaluator throw / compile error / timeout ⇒ publish **Bad** quality carrying the last-known
  value, once per Good→Bad transition, only after all deps have arrived; emit a
  `ScriptLogEntry` on the `script-logs` DPS topic per failure (visible on the script-log
  panel). Recovery to Good is force-published. No retry — re-evaluation on next trigger.
- Historized Bad results record `BadInternalError` (0x80020000), matching VT historization.

## 7. Probe

`CalculationDriverProbe : IDriverProbe` (`DriverType = "Calculation"`). `ProbeAsync` =
validate the driver config parses; there is no backend to connect. (Per-script compile
verification belongs to the editor diagnostics + the tag editor's inline panel, not the
driver probe — the probe receives only `DriverConfig`, which for Calculation is empty.)
Returns Ok with negligible latency; never throws.

## 8. Testing

- **Unit:** evaluator parity tests vs the VT evaluator (passthrough, timeout, sandbox
  violation, SetVirtualTag drop); dependency-ref extraction from authored tags; change-gate
  (no publish until all deps seen); dedupe; timer-group scheduling via
  `TimerTriggerScheduler`; Bad-transition + script-log emission; cycle gate (self-cycle,
  2-cycle, cross-driver terminal).
- **Integration:** deploy a Calc tag reading a fixture-driver tag on the 2-node harness;
  assert the raw node publishes computed values, a UNS reference sees them, and calc-of-calc
  chains propagate.
- **Live `/run` (Batch 2 gate):** author a Calc driver + tag on docker-dev via `/raw`,
  script it against a Modbus fixture tag, watch the value compute; break the script, watch
  Bad + script-log row; fix, watch recovery.

## 9. Out of scope

- Multi-tag scripts / `ctx.SetVirtualTag` fan-out (single-tag mode, as VT today).
- Timer support in the UNS VirtualTag runtime (dormant there; revived only inside this driver).
- Write-through calc tags (inverse calculations).