Topic Strings

Topics

A topic string, or topic name, is a character string that identifies the topic of a publish/subscribe message. Topic strings are encoded in UTF-8 and can be constructed from any Unicode character. The topic string is the identifier that is used by the publisher to publish messages. A subscriber can subscribe to that same identifier. A subscriber can subscribe to multiple topics. Wildcards can be used to allow the subscriber to subscribe to a group of topics that have a similar topic string.

Three characters have special meaning in publish/subscribe. These characters are the forward slash (/), the number sign (#), and the plus sign (+).

The use of the special characters number sign (#), and plus sign (+) is explained in Wildcards. The uses of the special character forward slash (/), is explained in the following section.

The forward slash (/)

The topic level separator. Use the '/' character to structure the topic into a topic tree.

For performance reasons, avoid empty topic levels, '//', if you can. Empty topic levels correspond to nodes in the topic hierarchy with no topic string. A leading or trailing '/' in a topic string corresponds to a leading or trailing empty node and should be avoided too.