# Structured Multi-Value Attribute — Design

**Date:** 2026-06-16
**Status:** Implemented (MV-1 … MV-15, branch `feature/multivalue-attribute`) — full solution builds clean; feature-targeted tests green across Commons, TemplateEngine, SiteRuntime, DataConnectionLayer, Communication, Transport, ManagementService, CLI, and CentralUI.
**Branch:** `feature/multivalue-attribute`

## Problem

ScadaBridge's `DataType` enum (`Boolean·Int32·Float·Double·String·DateTime·Binary`) has no
collection type. An attribute is persisted as a single `string? Value` plus a `DataType`
discriminator. So a Galaxy/object attribute that is conceptually a list of values — e.g.
`MoveInWorkOrderNumbers`, `MoveInPartNumbers` (string arrays arriving via the
`IpsenMESMoveIn` inbound-API method) — can only be stored by collapsing it to a single
`DataType.String` blob. We want these stored as a **first-class structured multi-value
attribute** that round-trips through every path.

## Scope (decided)

- **Element types:** a *homogeneous* list of any existing **scalar** `DataType` —
  `String, Int32, Float, Double, Boolean, DateTime`. **Excluded:** `Binary` elements and
  nested lists (no lists-of-lists, no lists-of-objects).
- **Lifecycle paths — all four in scope:**
  1. Script writes the attribute at runtime and reads it back (the `IpsenMESMoveIn` case);
     persists as a static attribute (site SQLite, survives failover).
  2. Statically authored default in a template, optionally overridden per instance, via
     Central UI and CLI.
  3. Read in from an OPC UA array node (DCL subscription → live attribute value).
  4. Written out to an OPC UA array node via the DCL write path.

## Decision summary (defaults locked in)

| Decision | Choice |
|---|---|
| Type modeling | One `DataType.List` member + nullable `ElementDataType` companion |
| Value encoding | JSON array, invariant culture (round-trippable) |
| Display encoding | `ValueFormatter.FormatDisplayValue` stays comma-joined, display-only |
| Element types | 6 scalars; no `Binary`, no nesting |
| Override granularity | Whole-list replacement (no per-element override) |
| Element type mutability | Fixed by base attribute; not overridable on derived/instance |
| Bad OPC UA element | Coerce/validate; mismatch → Bad quality + log (non-fatal) |
| Empty vs unset | `null` = unset/cleared; `"[]"` = explicit empty list |
| gRPC wire | Existing `string value` field carries the canonical JSON (additive) |

## Architecture

The key insight from the blast-radius survey: most of the runtime is already
collection-capable. `ScriptParameters` already converts to typed `List<T>`/`T[]`; the OPC UA
write path wraps values in a `Variant` (which supports arrays natively);
`AttributeValueChanged` already carries `object?`. The real work concentrates in **(a)** the
type model, **(b)** a canonical encode/decode codec, **(c)** the InstanceActor / script-accessor
boundary, **(d)** validation, and **(e)** the UI/CLI editors.

### 1. Type model

- Add **one** enum member: `DataType.List`
  (`Commons/Types/Enums/DataType.cs`).
- Add nullable `ElementDataType` (`DataType?`) to:
  - `Commons/Entities/Templates/TemplateAttribute.cs`
  - `Commons/Entities/Instances/InstanceAttributeOverride.cs`
  - `Commons/Types/Flattening/FlattenedConfiguration.cs` → `ResolvedAttribute`
- **Invariant:** `ElementDataType` required when `DataType == List`, null otherwise; element
  type must be one of the 6 allowed scalars.

### 2. Canonical value codec — new `AttributeValueCodec`

One round-trippable codec used everywhere a value is **stored or transmitted**:

- `Encode(object? value)`:
  - scalar → invariant-culture string (**identical to today's behavior**; existing scalars
    unchanged)
  - non-string `IEnumerable` → JSON array, invariant culture
    (`["WO-1","WO-2"]`, `[1,2,3]`, `["2026-06-16T00:00:00Z"]`)
- `Decode(string? value, DataType dataType, DataType? elementType)`:
  - `List` → typed `List<T>` (reuse the existing `ScriptParameters` element-conversion
    machinery where practical)
  - scalar → string (unchanged)
- `null` value = unset/cleared; `"[]"` = explicit empty list.

`ValueFormatter.FormatDisplayValue` (comma-joined) remains **display-only** — diff text,
log lines, KPI strings — and is *not* the persisted/wire form.

### 3. Persistence & migration

- One EF Core migration, **written idempotent** (per the open follow-up #70 lesson —
  guard inserts/alters so re-running against a partially-migrated DB is safe):
  - add `ElementDataType nvarchar(50) NULL` to `TemplateAttributes` and
    `InstanceAttributeOverrides`
  - widen `Value` / `OverrideValue` from `nvarchar(4000)` → `nvarchar(max)` (lists can
    exceed 4000)
- Site **SQLite** static-attribute store: no schema change (already a string column) — it
  simply stores the canonical JSON.
- EF config: `TemplateConfiguration.cs`, `InstanceConfiguration.cs` map the new column
  (`HasConversion<string>().HasMaxLength(50)` for `ElementDataType`).

### 4. Flatten / resolve

- `TemplateEngine/Flattening/FlatteningService.cs` carries `ElementDataType` into
  `ResolvedAttribute` next to `DataType`.
- Inheritance / compose / override shape unchanged.
- **Override replaces the whole list** (no per-element merge).
- A derived template or instance override **cannot change `ElementDataType`** — it is fixed
  by the base attribute, enforced like the existing `LockedInDerived` path.

### 5. Site runtime (the core change)

- **`SiteRuntime/Scripts/ScopeAccessors.cs:56,73`** — `AttributeAccessor` set / `SetAsync`:
  replace `value?.ToString()` with `AttributeValueCodec.Encode(value)`. (Today a
  `List<string>` would `.ToString()` to `"System.Collections.Generic.List`1[...]"` — the
  central bug this fixes.) Scalars produce the same string as before.
- **`SiteRuntime/Actors/InstanceActor.cs:116-121`** — on load, for `List` attributes
  `Decode` the JSON `Value` into a typed `List<T>` and store *that* in `_attributes`
  (scalars unchanged).
- **`HandleSetStaticAttribute`** — for `List` attributes: decode the canonical string →
  typed list for `_attributes`, validate element types, persist the canonical JSON to
  SQLite, stream the canonical JSON.
- **`HandleGetAttribute`** — unchanged; returns the live `_attributes` object, now a real
  `List<T>` for list attributes. Scripts read a first-class list.

### 6. Data Connection Layer (OPC UA read + write)

- **Read:** an OPC UA array node yields a CLR array → attribute value object. Coerce /
  validate element type against `ElementDataType`; on mismatch → **Bad quality** + log,
  never crash the actor.
- **Write:** `DataConnectionLayer/Adapters/RealOpcUaClient.cs:619-628` already wraps values
  in `new Variant(value)`, which supports arrays natively; a `List<T>`/`T[]` writes as an
  OPC UA array. Minimal change beyond passing the typed list through.

### 7. Validation (pre-deploy semantic)

`TemplateEngine/Validation/SemanticValidator.cs` + `ValidationService.cs`:

- `ElementDataType` present and a valid scalar when `DataType == List`; absent otherwise.
- Authored default `Value` (if present) must parse as a JSON array whose elements match
  `ElementDataType`.
- List attributes **rejected as operands** in numeric alarm triggers (HiLo /
  RangeViolation) and binary triggers — extends the existing `NumericDataTypes` guard.

### 8. Streaming / DebugView

- `Communication/Actors/StreamRelayActor.cs:48` — encode via `AttributeValueCodec` (JSON for
  lists) instead of `FormatDisplayValue`. Additive: lists are a new type, so no existing wire
  consumer breaks; the proto `string value` field (`sitestream.proto:57`) is unchanged.
- Central DebugView renders the canonical string; for lists it shows the JSON array.
  *Optional polish:* chip rendering of list elements.

### 9. Central UI

- **`CentralUI/Components/Pages/Design/TemplateEdit.razor`** — attribute form: when
  type = `List`, reveal an `ElementDataType` dropdown and a **list editor** (repeatable
  add/remove rows) bound to the JSON value; inline per-element validation.
- **`CentralUI/Components/Pages/Deployment/InstanceConfigure.razor`** — override panel: the
  same list editor for overriding a list attribute.

### 10. CLI

- **`CLI/Commands/TemplateCommands.cs`** (and the instance-override command): add
  `--element-type <Scalar>`; `--value` accepts a JSON array (`'["a","b"]'`). Optional
  ergonomic alternative: repeatable `--value`. Validate element type before submit.
- `ManagementService/ManagementActor.cs` add/update attribute handlers: parse + validate
  `ElementDataType`, validate `Value` against it before persisting.

### 11. Transport (import/export)

- `Transport/Serialization/EntityDtos.cs` `TemplateAttributeDto` already carries the
  `DataType` enum + `string? Value`; add `ElementDataType`. Enum/value round-trip is
  otherwise automatic. `BundleImporter` carries the new field through.

## Error handling

- Malformed JSON in a persisted `Value`: caught at deploy-time validation; if it ever
  reaches runtime decode, log + treat the attribute as Bad quality — never crash the
  Instance Actor (audit/best-effort principle).
- OPC UA element-type mismatch on read: Bad quality + log (non-fatal).
- Script writes a non-list to a list attribute (or vice versa): rejected at
  `HandleSetStaticAttribute` validation, surfaced to the script as an error.

## Testing strategy

- **Codec round-trip:** every element type; empty list; embedded commas/quotes/escaping;
  `DateTime`; culture-invariance.
- **Flatten:** override replaces whole list; element-type lock enforced.
- **Validation:** good/bad authored defaults; trigger-operand rejection; missing/extra
  `ElementDataType`.
- **InstanceActor:** set/get typed list; SQLite persistence survives simulated failover.
- **DCL:** OPC UA array read coercion; Bad quality on element mismatch; `Variant` array
  write.
- **Streaming:** `StreamRelayActor` emits canonical JSON for a list attribute.
- **CLI:** `--element-type` + JSON `--value` parse/validate.
- **UI:** list-editor component behavior (if harness present).

## Out of scope (deferred)

- Nested objects / lists-of-records; heterogeneous lists.
- `Binary` element lists.
- Per-element overrides; element-level alarm triggers.

## Blast-radius reference (file:area)

| Area | Key locations | Change size |
|---|---|---|
| Enum + entities | `DataType.cs`, `TemplateAttribute.cs`, `InstanceAttributeOverride.cs`, `FlattenedConfiguration.cs` | small |
| Codec (new) | `Commons/Types/AttributeValueCodec.cs` (new) | small–med |
| EF + migration | `TemplateConfiguration.cs`, `InstanceConfiguration.cs`, new migration | small |
| Flatten | `FlatteningService.cs:177` | small |
| Runtime boundary | `ScopeAccessors.cs:56,73`, `InstanceActor.cs:116-121,246-315` | **med (core)** |
| DCL | `RealOpcUaClient.cs` read coercion (write already array-capable) | small–med |
| Validation | `SemanticValidator.cs:18-21,130-193`, `ValidationService.cs` | small–med |
| Streaming | `StreamRelayActor.cs:48` | small |
| UI | `TemplateEdit.razor`, `InstanceConfigure.razor` | **med (largest)** |
| CLI + mgmt | `TemplateCommands.cs`, `ManagementActor.cs:1441-1461` | small–med |
| Transport | `EntityDtos.cs:77-83`, `BundleImporter.cs:2302` | small |

**Note:** the Inbound API type system (`ParameterDefinition` with `Object`/`List`,
`InboundApiSchema`) is genuinely separate from attribute `DataType` and is **not** modified;
its `ScriptParameters.ConvertToList/ConvertToArray` conversion helpers are reusable by the
codec.