Reducer Configuration

Configuring reducers can be tricky because they are flexible in so many different ways.

Extractor Keys

Sometimes multiple extractors will be defined but a particular reducer only cares about or can only work with a particular type of extract. In this case, you can use the extractor keys property to restrict the extracts that are sent to this reducer. The format of this value is either a string (for a single extractor key) or an array of strings (for multiple extractors). The default, a blank string or a nil, sends all extracts.

Topic

Extracts are always implicitly grouped before being combined. There are two different ways of doing this, whose names are hopefully self-explanatory. The default is reduce_by_subject

• reduce_by_subject
• reduce_by_user

Grouping

This is a confusing setting because extracts are already obviously grouped according to the topic. This allows an additional grouping pass, which, crucially, can be done on the basis of the value of a specified field. So to configure this, you need to set the name of the field to group by (in format extractor_key.field_name) and then a flag indicating how to handle when the extracts for a given classification are missing that field.