Custom Content Filters

Submitted by angel on Wed, 05/21/2014 - 15:42

Concept

Unlike Content Filter Topics, Custom Content Filters allow the use of non-relational operations between topic members, i.e. a computation involving topic members. Similarly to what happens with the Content Filtered Topics, after applying a custom content filter you will get a subset of all samples that fulfill an expression.

When creating your custom content filter, you can use two kinds of filter expressions:

To create a Custom Content Filter we need to create three functions for the three stages that it needs.

  • Compile function: compiles a filter expression and parameters. Connext will call this function when a ContentFilteredTopic is created and when the filter parameters are changed. This parameter cannot be NULL. See "Compile Function" section in the User's Manual.
  • Evaluate function: will be called by Connext each time a sample is received. Its purpose is to evaluate the sample based on the filter. Its parameter cannot be NULL. See "Evaluate Function" section in the User's Manual.
  • Finalize function: will be called by Connext when an instance of the custom content filter is no longer needed. Its parameter may be NULL. See "Finalize Function" section in the User's Manual.

Example Description

Our Custom Content Filter uses a STRINGMATCH-based filter. It takes the expression "%0 %1 x", where %0 is an integer, %1 is "divides" or "greater-than", and x is the field we are comparing in the sample. The Filter lets the sample through if the parameter[0] divides or is greater than x, respectively.

In this example, most of the work occurs in the compile stage. We check that we have a valid expression, and initialize compile_data to a structure containing the current parameter and a pointer to the evaluation function. When evaluate() is called for each sample, it merely returns the result of that function applied to the sample data.

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