Content Filtered Topic -- String Filter

Concept

A Content Filtered Topic is a Topic with filtering properties. It makes it possible to subscribe to topics and at the same time specify that you are only interested in a subset of the Topic's data. For example, suppose you have a Topic that contains a temperature reading for a boiler, but you are only interested in temperatures outside the normal operating range. A Content Filtered Topic can be used to limit the number of data samples a DataReader has to process and may also reduce the amount of data sent over the network.

A ContentFilteredTopic creates a relationship between a Topic, also called the related topic, and user-specified filtering properties. The filtering properties consist of an expression and a set of parameters.

1. The filter expression evaluates a logical expression on the Topic content. The filter expression is similar to the WHERE clause in a SQL expression.
2. The parameters are strings that give values to the "parameters" in the filter expression. There must be one parameter string for each parameter in the filter expression.

Example Description

After this topic is created, a Content Filtered Topic is created based on the normal topic. In this case, we will use a parameterized STRINGMATCH filter expression: "name MATCH %0", but it could be a static string as well, such as "name MATCH 'RTI'". The filter parameters, represented by %k (where k = 0 to 99), are passed in as a DDS_StringSeq when the content filtered topic is created.

To create this kind of topic, we need to use create_contentfilteredtopic_with_filter() as follows:

create_contentfilteredtopic_with_filter("ContentFilteredTopic", 
                                        topic, 
                                        "name MATCH %0", 
                                        parameters, 
                                        DDS_STRINGMATCHFILTER_NAME);

Parameters can be changed at runtime via append_to_expression_parameter() and remove_from_expression_parameter(). In the example, we modify the expression parameters to three different values.

1. When the application starts, the expression parameters are set to %0="SOME_STRING". After that, we will append to expression the parameter "EVEN".
2. After 10 seconds, we will append again the parameter "ODD". So the DataReader will receive the EVEN and ODD numbers (all).
3. Finally, after 20 seconds, we will remove the parameter "EVEN" and the DataReader will receive just the ODD numbers.

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