Module parameters are a form of data used to control the pipeline: either the way that Ziggy executes the processing, or the way that the algorithms themselves function. For example, module parameters can tell an algorithm module to enable or disable a particular processing step; set convergence criteria for model fitting algorithms; set the number of data points to use in a median filter; or, really, anything of that nature.
Module parameters are organized into groups known as "parameter sets." When Ziggy provides parameters to a pipeline module, it's in the form of one or more parameter sets, rather than individual parameters. There's no limit on the number of parameter sets that can be provided to a pipeline or a pipeline module, so don't feel that you need to cram a hundred parameters into one parameter set; it's probably the case that the parameters can be logically grouped into sets, and you should feel free to group them that way and then pass all the needed parameter sets to the modules that need them.
Ziggy expects its parameter set XML files to start with "pl-" (for "Parameter Library"). In the case of the sample pipeline, take a look at config/pl-sample.xml. Note that you also don't need to confine yourself to a single parameter library file; you can have as many as you like.
Because the sample pipeline is extremely simple, we have only one parameter library file and it, in turn, has only one parameter set:
<parameter-set name="Algorithm Parameters">
<parameter name="throw exception subtask 0" value="false" type="boolean"/>
<parameter name="produce output subtask 1" value="true" type="boolean"/>
<parameter name="dummy array parameter" value="1, 2, 3" type="intarray"/>
<parameter name="execution pause seconds" value="5" type="int"/>
</parameter-set>Each parameter set must have a unique name, in this case "Algorithm Parameters". Each parameter must have a name that is unique within the parameter set (i.e., I could have another parameter named "dummy array parameter" in a different parameter set, but obviously I can't have 2 parameters with the same name in the same set).
Each individual parameter in a set has a name, a value, and a type. The allowed types the usual ones you would expect: boolean, byte, short, int, long, float, double, String. The parameters can be scalar or they can be arrays: to specify an array, append array after the type name, as shown by the dummy array parameter.
Note that both the parameter set name and the parameter name can have whitespace in them. However, when the parameter sets are provided to the algorithm, as shown in the article on Configuring a Pipeline, it's done in a context that doesn't allow for whitespace. As a result, the parameter set names and parameter names will have any whitespace replaced with underscore ("_") characters. In my opinion, this is a recipe for confusion and annoying software bugs: you have two different names for parameters and/or parameter sets, but they're almost the same, and you need to remember exactly what the difference is and which part of the system uses each of the names. Still, if you want to do this, you can, which is why we're showing you how to do it (and then telling you not to).