The fcrepo-message-consumer expects nodes to have a type of "http://fedora.info/definitions/v4/indexing#Indexable" and to specify an indexing transformation. The following steps can be used to set up the namespace and create and register that mixin with your repository.
Define Indexing Properties
External indexing relies upon the resources you wish to have indexed to have an indexing:Indexable mixin property. This property can be added to a resource in the same way as any other property. See: RESTful HTTP API - Containers (PATCH, PUT, or POST).
Where the file "indexing.rdf" contains: