Subscription

The Subscription controls the delivery cadence of notifications through two key intervals:

• Publishing Interval: The cycle time at which the Server checks for queued notifications and sends them to the Client. This is a Server-side timer.
• Sampling Interval: The rate at which a Monitored Item samples its source value. This can be set independently or inherited from the Publishing Interval.
• Queueing: If multiple changes occur between publishing cycles, the Server queues them up to a configurable Queue Size, ensuring no data loss during bursts.

Subscriptions are stateful objects that require active management to prevent resource leaks:

• Lifetime Count: The number of consecutive Publishing Intervals that can elapse without a Client acknowledgment before the Subscription is automatically deleted by the Server.
• Keep-Alive Count: The number of Publishing Intervals after which the Server sends an empty notification if no data changes have occurred. This allows the Client to detect a broken connection even during silent periods.
• A Client must call the Publish service to acknowledge received notifications and reset the Lifetime counter.

A Subscription acts as a container for one or more Monitored Items, allowing logical grouping and shared configuration:

• All Monitored Items within a Subscription share the same Publishing Interval.
• Each Monitored Item can have its own Sampling Interval and Filter (e.g., a Deadband Filter).
• Grouping related data points (e.g., all sensors on a single machine) into one Subscription reduces the overhead of managing multiple independent notification streams.

When a Publishing Interval elapses, the Server assembles a Notification Message containing:

• Sequence Number: A monotonically increasing integer that allows the Client to detect missed or out-of-order messages.
• Publish Time: The exact Server timestamp when the message was assembled.
• Data Change Notifications: An array of value updates, each containing the current value, status code, and source timestamp for a specific Monitored Item.
• Event Notifications: If the Subscription monitors Alarms and Conditions, event payloads are included in the same message.

A Subscription exists entirely within the scope of a single Session:

• If a Session is closed or times out, all its Subscriptions are automatically deleted by the Server.
• A single Session can host multiple Subscriptions, each with different Publishing Intervals for different data priority levels.
• The Transfer Subscriptions service allows a Client to move a Subscription from one Session to another, enabling high-availability failover without losing configuration.

Subscriptions include metadata for resource management and troubleshooting:

• Priority: A relative byte value (0-255) assigned by the Client. When Server resources are constrained, lower-priority Subscriptions may be throttled or dropped first.
• Max Notifications Per Publish: Limits the number of queued notifications sent in a single message to prevent overwhelming the Client.
• Diagnostic Info: The Server can return detailed operational metrics, such as the number of late publishing cycles or discarded notifications, enabling proactive health monitoring.