# Component: Data Connection Layer

## Purpose

The Data Connection Layer provides a uniform interface for reading from and writing to physical machines at site clusters. It abstracts protocol-specific details behind a common interface, manages subscriptions, and delivers live tag value updates to Instance Actors. It is a **clean data pipe** — it performs no evaluation of triggers, alarm conditions, or business logic.

## Location

Site clusters only. Central does not interact with machines directly.

## Responsibilities

- Manage data connections defined centrally and deployed to sites as part of artifact deployment (OPC UA servers, LmxProxy endpoints). Data connection definitions are stored in local SQLite after deployment.
- Establish and maintain connections to data sources based on deployed instance configurations.
- Subscribe to tag paths as requested by Instance Actors (based on attribute data source references in the flattened configuration).
- Deliver tag value updates to the requesting Instance Actors.
- Support writing values to machines (when Instance Actors forward `SetAttribute` write requests for data-connected attributes).
- Report data connection health status to the Health Monitoring component.

## Common Interface

Both OPC UA and LmxProxy implement the same interface:

```
IDataConnection : IAsyncDisposable
├── Connect(connectionDetails) → void
├── Disconnect() → void
├── Subscribe(tagPath, callback) → subscriptionId
├── Unsubscribe(subscriptionId) → void
├── Read(tagPath) → value
├── ReadBatch(tagPaths) → values
├── Write(tagPath, value) → void
├── WriteBatch(values) → void
├── WriteBatchAndWait(values, flagPath, flagValue, responsePath, responseValue, timeout) → bool
└── Status → ConnectionHealth
```

Additional protocols can be added by implementing this interface.

### Concrete Type Mappings

| IDataConnection | OPC UA SDK | LmxProxy SDK (`LmxProxyClient`) |
|---|---|---|
| `Connect()` | OPC UA session establishment | `ConnectAsync()` → gRPC `ConnectRequest`, server returns `SessionId` |
| `Disconnect()` | Close OPC UA session | `DisconnectAsync()` → gRPC `DisconnectRequest` |
| `Subscribe(tagPath, callback)` | OPC UA Monitored Items | `SubscribeAsync(addresses, onUpdate)` → server-streaming gRPC (`IAsyncEnumerable<VtqMessage>`) |
| `Unsubscribe(id)` | Remove Monitored Item | `ISubscription.DisposeAsync()` (cancels streaming RPC) |
| `Read(tagPath)` | OPC UA Read | `ReadAsync(address)` → `Vtq` |
| `ReadBatch(tagPaths)` | OPC UA Read (multiple nodes) | `ReadBatchAsync(addresses)` → `IDictionary<string, Vtq>` |
| `Write(tagPath, value)` | OPC UA Write | `WriteAsync(address, value)` |
| `WriteBatch(values)` | OPC UA Write (multiple nodes) | `WriteBatchAsync(values)` |
| `WriteBatchAndWait(...)` | OPC UA Write + poll for confirmation | `WriteBatchAndWaitAsync(values, flagAddress, flagValue, responseAddress, responseValue, timeout)` |
| `Status` | OPC UA session state | `IsConnected` property + keep-alive heartbeat (30-second interval via `GetConnectionStateAsync`) |

### Common Value Type

Both protocols produce the same value tuple consumed by Instance Actors. Before the first value update arrives from the DCL, data-sourced attributes are held at **uncertain** quality by the Instance Actor (see Site Runtime — Initialization):

| Concept | ScadaLink Design | LmxProxy SDK (`Vtq`) |
|---|---|---|
| Value container | `{value, quality, timestamp}` | `Vtq(Value, Timestamp, Quality)` — readonly record struct |
| Quality | good / bad / uncertain | `Quality` enum (byte, OPC UA compatible: Good=0xC0, Bad=0x00, Uncertain=0x40) |
| Timestamp | UTC | `DateTime` (UTC) |
| Value type | object | `object?` (parsed: double, bool, string) |

## Supported Protocols

### OPC UA
- Standard OPC UA client implementation.
- Supports subscriptions (monitored items) and read/write operations.

### LmxProxy (Custom Protocol)

LmxProxy is a gRPC-based protocol for communicating with LMX data servers. An existing client SDK (`LmxProxyClient` NuGet package) provides a production-ready implementation.

**Transport & Connection**:
- gRPC over HTTP/2, using protobuf-net code-first contracts (service: `scada.ScadaService`).
- Default port: **5050**.
- Session-based: `ConnectAsync` returns a `SessionId` used for all subsequent operations.
- Keep-alive: 30-second heartbeat via `GetConnectionStateAsync`. On failure, the client marks itself disconnected and disposes subscriptions.

**Authentication & TLS**:
- API key-based authentication (sent in `ConnectRequest`).
- Full TLS support: TLS 1.2/1.3, mutual TLS (client cert + key in PEM), custom CA trust, self-signed cert allowance for dev.

**Subscriptions**:
- Server-streaming gRPC (`IAsyncEnumerable<VtqMessage>`).
- Configurable sampling interval (default: 1000ms; 0 = on-change).
- Wire format: `VtqMessage { Tag, Value (string), TimestampUtcTicks (long), Quality (string: "Good"/"Uncertain"/"Bad") }`.
- Subscription disposed via `ISubscription.DisposeAsync()`.

**Additional Capabilities (beyond IDataConnection)**:
- Built-in retry policy via Polly: exponential backoff (base delay × 2^attempt), configurable max attempts (default: 3), applied to reads. Transient errors: `Unavailable`, `DeadlineExceeded`, `ResourceExhausted`, `Aborted`.
- Operation metrics: count, errors, p95/p99 latency (ring buffer of last 1000 samples per operation).
- Correlation ID propagation for distributed tracing (configurable header name).
- DI integration: `AddLmxProxyClient(IConfiguration)` binds to `"LmxProxy"` config section in `appsettings.json`.

**SDK Reference**: The client SDK source is at `LmxProxyClient` in the ScadaBridge repository. The DCL's LmxProxy adapter wraps this SDK behind the `IDataConnection` interface.

## Subscription Management

- When an Instance Actor is created (as part of the Site Runtime actor hierarchy), it registers its data source references with the Data Connection Layer.
- The DCL subscribes to the tag paths using the concrete connection details from the flattened configuration.
- Tag value updates are delivered directly to the requesting Instance Actor.
- When an Instance Actor is stopped (due to disable, delete, or redeployment), the DCL cleans up the associated subscriptions.
- When a new Instance Actor is created for a redeployment, subscriptions are established fresh based on the new configuration.

## Write-Back Support

- When a script calls `Instance.SetAttribute` for an attribute with a data source reference, the Instance Actor sends a write request to the DCL.
- The DCL writes the value to the physical device via the appropriate protocol.
- The existing subscription picks up the confirmed new value from the device and delivers it back to the Instance Actor as a standard value update.
- The Instance Actor's in-memory value is **not** updated until the device confirms the write.

## Value Update Message Format

Each value update delivered to an Instance Actor includes:
- **Tag path**: The relative path of the attribute's data source reference.
- **Value**: The new value from the device.
- **Quality**: Data quality indicator (good, bad, uncertain).
- **Timestamp**: When the value was read from the device.

## Connection Actor Model

Each data connection is managed by a dedicated connection actor that uses the Akka.NET **Become/Stash** pattern to model its lifecycle as a state machine:

- **Connecting**: The actor attempts to establish the connection. Subscription requests and write commands received during this phase are **stashed** (buffered in the actor's stash).
- **Connected**: The actor is actively servicing subscriptions. On entering this state, all stashed messages are unstashed and processed.
- **Reconnecting**: The connection was lost. The actor transitions back to a connecting-like state, stashing new requests while it retries.

This pattern ensures no messages are lost during connection transitions and is the standard Akka.NET approach for actors with I/O lifecycle dependencies.

**LmxProxy-specific notes**: The LmxProxy connection actor holds the `SessionId` returned by `ConnectAsync` and passes it to all subsequent operations. On entering the **Connected** state, the actor starts the 30-second keep-alive timer. Subscriptions use server-streaming gRPC — the actor processes the `IAsyncEnumerable<VtqMessage>` stream and forwards updates to Instance Actors. On keep-alive failure, the actor transitions to **Reconnecting** and the client automatically disposes active subscriptions.

## Connection Lifecycle & Reconnection

The DCL manages connection lifecycle automatically:

1. **Connection drop detection**: When a connection to a data source is lost, the DCL immediately pushes a value update with quality `bad` for **every tag subscribed on that connection**. Instance Actors and their downstream consumers (alarms, scripts checking quality) see the staleness immediately.
2. **Auto-reconnect with fixed interval**: The DCL retries the connection at a configurable fixed interval (e.g., every 5 seconds). The retry interval is defined **per data connection**. This is consistent with the fixed-interval retry philosophy used throughout the system. **Note on LmxProxy**: The LmxProxy SDK includes its own retry policy (exponential backoff via Polly) for individual operations (reads). The DCL's fixed-interval reconnect owns **connection-level** recovery (re-establishing the gRPC session after a keep-alive failure or disconnect). The SDK's retry policy handles **operation-level** transient failures within an active session. These are complementary — the DCL does not disable the SDK's retry policy.
3. **Connection state transitions**: The DCL tracks each connection's state as `connected`, `disconnected`, or `reconnecting`. All transitions are logged to Site Event Logging.
4. **Transparent re-subscribe**: On successful reconnection, the DCL automatically re-establishes all previously active subscriptions for that connection. Instance Actors require no action — they simply see quality return to `good` as fresh values arrive from restored subscriptions.

## Write Failure Handling

Writes to physical devices are **synchronous** from the script's perspective:

- If the write fails (connection down, device rejection, timeout), the error is **returned to the calling script**. Script authors can catch and handle write errors (log, notify, retry, etc.).
- Write failures are also logged to Site Event Logging.
- There is **no store-and-forward for device writes** — these are real-time control operations. Buffering stale setpoints for later application would be dangerous in an industrial context.

## Tag Path Resolution

When the DCL subscribes to a tag path from the flattened configuration but the path does not exist on the physical device (e.g., typo in the template, device firmware changed, device still booting):

1. The failure is **logged to Site Event Logging**.
2. The attribute is marked with quality `bad`.
3. The DCL **periodically retries resolution** at a configurable interval, accommodating devices that come online in stages or load modules after startup.
4. On successful resolution, the subscription activates normally and quality reflects the live value from the device.

Note: Pre-deployment validation at central does **not** verify that tag paths resolve to real tags on physical devices — that is a runtime concern handled here.

## Health Reporting

The DCL reports the following metrics to the Health Monitoring component via the existing periodic heartbeat:

- **Connection status**: `connected`, `disconnected`, or `reconnecting` per data connection.
- **Tag resolution counts**: Per connection, the number of total subscribed tags vs. successfully resolved tags. This gives operators visibility into misconfigured templates without needing to open the debug view for individual instances.

## Dependencies

- **Site Runtime (Instance Actors)**: Receives subscription registrations and delivers value updates. Receives write requests.
- **Health Monitoring**: Reports connection status.
- **Site Event Logging**: Logs connection status changes.

## Interactions

- **Site Runtime (Instance Actors)**: Bidirectional — delivers value updates, receives subscription registrations and write-back commands.
- **Health Monitoring**: Reports connection health periodically.
- **Site Event Logging**: Logs connection/disconnection events.