scadalink-design/Component-DataConnectionLayer.md

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 at the site level (OPC UA servers, custom protocol endpoints).
• 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 the custom protocol implement the same interface:

IDataConnection
├── Connect(connectionDetails) → void
├── Disconnect() → void
├── Subscribe(tagPath, callback) → subscriptionId
├── Unsubscribe(subscriptionId) → void
├── Read(tagPath) → value
├── Write(tagPath, value) → void
└── Status → ConnectionHealth

Additional protocols can be added by implementing this interface.

Supported Protocols

OPC UA

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

Custom Protocol

• Proprietary protocol adapter.
• Implements the same subscription-based model as OPC UA.

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.

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.
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.