Function project

Projects events (materialized view).

• Projectors reduce events by returning state patches. A patch with an empty record {id} represents a delete operation.
• Handles event batches (starting from the subscription watermark), reducing patches in a projection map before the final commit.
• The cardinality of the projection is driven by the input stream (can map one or many aggregates in one record).
• The map can contain record-level patches (by id) and filtered patches (updates and deletes).
• Commits are executed in the following order:
  • Filtered deletes
  • Filtered updates
  • Record level patches