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.

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.

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