Discussion: Projections Features

Hi @johnbywater! I hope this is an appropriate place to discuss features. 😃 If not, maybe create a Wiki page?

I am interested in your plans for these forthcoming features:

• Base class for event sourced projections or views (forthcoming)
  • In memory event sourced projection, which needs to replay entire event stream when system starts up (forthcoming)
  • Persistent event sourced projection, which stored its projected state, but needs to replay entire event stream when initialized (forthcoming)
• Event sourced indexes, as persisted event source projections, to discover extant entity IDs (forthcoming)

I’d like to help out.

1. Do you have any existing prototypes, design documents, or notes for these features?
2. How much of the d5-kanban-python projections implementation were you thinking of borrowing?

Thanks!

[johnbywater]

Ok! I’ll see what I can do along those lines 😃

[johnbywater]

@adebisi-fa I just pushed changeset d8f9082 which implements the code snippet above on the EventStore class and ActiveRecordStrategy classes. Please note, I renamed the method get_all_domain_events() in the next changeset to all_domain_events(). As the commit message says, from SQLAlchemy the events will be in order, but that isn’t possible with Cassandra so you’d need to somehow post-process the stream if you use Cassandra and need all events in chronological order. Of course, if you just wanted to identify all entity IDs, you wouldn’t need to sort them. The method all_domain_events() returns an iterator which pulls progressively on the database, so it might scale. Is that what you were looking for?