Projector Capabilities about riddl

[reid-spencer]

This raises questions about how we should model projections in RIDDL. Assuming the intended target is a relational database makes it too rigid, in my view. I will try to synthesize a more abstract solution.

[reid-spencer]

@AlexWeinstein92 - some conclusions I've made from considering your suggestion:

• Projectors cannot be queried. They have no data. They are merely transformation services. Consequently, I've removed the ability for Projectors to have "on query <query-path_id>" clauses in a recent push. Queries should be on repositories
• All of the items in your on event bullet points can/should be done by sending messages to the repository.
• There needs to be a clause in the projector to indicate which repository is the backing store that the projector updates. This is not yet implemented, but the syntax will be like:

updating repository MyRepository