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AVEVA Enterprise SCADA

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Understanding subscribers

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  • Last UpdatedApr 24, 2025
  • 2 minute read

There are two kinds of subscribers that connect directly to a PubSub Engine:

  1. Most commonly, the subscriber is running on the same computer as the PubSub Engine.

  2. A small number of subscribers are configured to connect to a PubSub Engine running on a different computer in the system, usually the Hot RealTime server. The configuration setting for enabling this behavior is the PubSub Transmission Control Protocol (TCP)/Internet Protocol (IP) override.

    For more information around installation, please contact AVEVA Customer Support.

Both of these subscribers use the Local Delivery Layer (LDL) for communications.

Subscribers subscribe to data by specifying the topic name in which they are interested. When subscribing, they can specify one of two integrity modes for the first update:

  • With no integrity - the subscriber receives an update the next time a publisher publishes on the subscribed topic.

  • With integrity - if the topic has been cached previously by the connected PubSub Engine (due to a pre-existing subscription) or if the topic is published by a persistence store, the subscriber receives an update immediately. Otherwise, the subscription is fulfilled as if it were subscribed with no integrity.

Subscribers are free to subscribe to a topic that currently has no publishers. The PubSub system tracks the subscription until a publisher finally publishes something. Multiple subscribers can subscribe to the same topic, or a single subscriber can subscribe to the same topic multiple times.

Multiple subscriptions from a subscriber to the same topic result in multiple updates for every published update. If a subscriber subscribes to the same topic multiple times, it should unsubscribe a matching number of times.

Subscribers should not make assumptions about how often a topic is published, or whether consecutive publications contain different data. It is valid to republish the same value, or to choose not to publish a value, because it has not changed enough. Publication is solely at the discretion of the publisher.