A role is a responsibility to satisfy a particular piece of logic and/or task in the communication flow required for SitecoreDXG. A fully-functional SitecoreDXG ecosystem consists of the following roles:
Serializer: responsible for serializing the Sitecore template architecture and making it available to the middleman for retrieval.
Middleman: responsible for retrieving the serialized architecture from the serializer and passing it along to the trigger
Trigger: responsible for executing the generation process on the generator with the serialized architecture received from the middleman
Generator: responsible for performing the generation process on the serialized architecture when called by the trigger
Completion Handler: an optional role responsible for performing post-generation logic on the generated output when called by the generator after it finishes generating
The following diagram shows the general communication workflow between these roles. Note that this workflow may change slightly depending on how you choose to use SitecoreDXG and implement your Middleman and Trigger.