aq13290_.htm |
Last updated Friday, 21 July 2017A topic can contain any character in the Unicode character set, but some characters have a special meaning.
The topic level separator is used to introduce structure into the topic, and can therefore be specified within the topic for that purpose.
The multilevel wildcard and single-level wildcard can be used for subscriptions, but they cannot be used within a topic by the publisher of a message.
However, if a publisher uses the characters "+" or "#" together with other characters in any topic level within a topic, these characters are not treated as wildcards, and they do not have any special meaning.
The topic level separator character "/" is used to provide a hierarchical structure to the topic space. It must be used by applications to separate levels within a topic tree. The use of the topic level separator is significant when the two wildcard characters are encountered in topics specified by subscribers.
Topic hierarchy is important in the administration of access control.
The multilevel wildcard character "#" is used to match any number of levels within a topic. For example, using the example topic tree shown above, if you subscribe to "USA/Alaska/#", you receive messages on topics "USA/Alaska" and "USA/Alaska/Juneau".
The multilevel wildcard can represent zero or more levels. Therefore, "USA/#" can also match the singular "USA", where # represents zero levels. The topic level separator is meaningless in this context, because there are no levels to separate.
The multilevel wildcard can be specified only on its own or next to the topic level separator character. Therefore, "#" and "USA/#" are valid topics where the "#" character is treated as a wildcard. However, although "USA#" is also a valid topic, the "#" character is not regarded as a wildcard and does not have any special meaning. See When wildcards are not wild for more information.
The single-level wildcard character "+" matches one, and only one, topic level. For example, "USA/+" matches "USA/Alabama", but not "USA/Alabama/Auburn". Also, because the single-level wildcard matches only a single level, "USA/+" does not match "USA".
The single-level wildcard can be used at any level in the topic tree, and in conjunction with the multilevel wildcard. The single-level wildcard must be specified next to the topic level separator, except when it is specified on its own. Therefore, "+" and "USA/+" are valid topics where the "+" character is treated as a wildcard. However, although "USA+" is also a valid topic, the "+" character is not regarded as a wildcard and does not have any special meaning. See When wildcards are not wild for more information.
The wildcard characters "+" and "#" have no special meaning when they are mixed with other characters (including themselves) in a topic level.
This means that topics that contain "+" or "#" together with other characters in a topic level can be published.
For example, consider the following two topics:
In the first example, the characters "+" and "#" are treated as wildcards and are therefore not valid in a topic that is to be published.
In the second example, the characters "+" and "#" are not treated as wildcards and therefore the topic can be both published and subscribed to.
Last updated Friday, 21 July 2017