Build projection after events have been dispatched already

[cjost1988]

Hi,

first thanks for this beautiful library, it works like a charm.

As far as I understand Projections just depend on recently dispatched messages and build up from them. What I do not understand is, how can you make it possible that you add new Projections that base on existing events?

I think this is one of the most common use cases, but I do not find information in the Projections documentation and the code also just allows to read messages per AggregateRoot and Projections can depend on multiple different AggregationRoot events.

Maybe I am just blind :D

Thanks in advance,
Chris

[frankdejonge]

Hi :)

Adding new projections rely on the history to be fed into the consumer. Version 3.0 will ship a new feature that makes replaying history a lot more convenient. Currently you can write custom logic to loop over existing events and forward them to the consumer, this would get the projection up to date with the latest information.

Hope this helps.

[mxkh]

That's actually could be a good feature to support lightweight implementation of "Live projections" to deliver immediate consistency for read models

https://www.eventstore.com/blog/live-projections-for-read-models-with-event-sourcing-and-cqrs

[jsor]

Quoting from the article above

One problem to address is the fact that the write model is live and keeps on publishing events

Is there a recommended way of rebuilding a read model by replaying events and then at one point switching over to regular read model projections without missing events during the switch-over?