Understanding the PubSub system

The PubSub system is designed to match the needs of a supervisory control system. It offers high performance, as well as a guarantee that subscribers to the same topic always receive publications in the same chronological order. It also supports subscribing to topics with a persistent backing data store and requesting (and receiving) an immediate integrity update. These features differentiate the PubSub system from most commercial Pub/Sub software, where publications are purely event driven.

The PubSub architecture is loosely coupled and topic based. Messages are delivered to subscribers who have registered an interest in a published topic. It is not necessary for the publisher to know of the subscriber’s existence. Subscribers can also register an interest in a topic without knowing whether a publisher exists or not. This ability is possible because subscribers and publishers never communicate directly with each other. Instead, both parties communicate through the PubSub Engine, which acts as a message broker.

The PubSub system consists of multiple communication layers, supporting delivery of PubSub messages to any destination within a group of physically connected AVEVA Enterprise SCADA systems. These layers include the following:

• Local Delivery Layer (LDL), which supports delivery within the same computer; communicating using Transmission Control Protocol (TCP)/Internet Protocol (IP).

• Network Delivery Layer (NDL), which supports delivery to different computers within the same AVEVA Enterprise SCADA system; using User Datagram Protocol (UDP) for broadcast communications.

• Inter-System Publish/Subscribe (ISPS), which supports delivery between different AVEVA Enterprise SCADA systems; using TCP/IP and secure sockets.