Understanding topic hierarchy

The structure of a topic name reflects a tree-like hierarchy. The following image is an example of a topic hierarchy:

The top of the hierarchy, as indicated by the square, is known as the root topic. By convention, the root topic is often the name of the SCADA system, but that is not a requirement. The root topic will have child topics underneath it, and those child topics may have siblings and child topics of their own.

The circles indicate the leaf topics. Leaf topics do not have child topics. Leaf topics often match a topic that has been published, but they may also represent a subscription for which data has not yet been published.

The triangles represent topic branches connecting the root to the leaves. Like the leaf topics, they may or may not have data published to them yet.

The tree hierarchy can be conceptually divided into multiple topic spaces, where a topic space is defined as a parent topic plus all of its descendants. The topic space provides flexibility to subscribers in that:

• A subscriber may subscribe to a leaf topic (a.b.e.h), in which case, it will receive publications for just that topic, or

• A subscriber may subscribe to a parent topic (a.b) to receive publications for the entire topic space. This topic space may include the parent (a.b), the child topics of the parent (a.b.d, a.b.e, a.b.f), the topics of the child topics (a.b.e.h), and so on until the last leaf topic is reached.