Hi Martin, 

I'm currently working on view composition using the brand new akka-stream 
> module. Basic idea is to make views stream producers and to use the 
> akka-stream DSL to merge message/event streams from several producers into 
> whatever you need. See also 
> https://twitter.com/mrt1nz/status/457120534111981569 for a first running 
> example.
>
> WDYT?
>

First of all Nice stuff !, I think this is useful for the system at my 
hands (real-time patient monitoring based on medical data)
I've seen the streams announcements but did not dive into that yet. Looking 
at your code StreamExample.scala it more or less 'clicks' in concept. (and 
hopefully in the right way)

>From a 'View' perspective as currently is available in akka-persistence, 
every producing actor needs a view attached to it in order to push the 
events to the streams producer, right ? (when I look at the 
ViewProducer.scala code, this is what is done.)

PersistentFlow.fromProcessor("p1").toProducer(materializer)
Now, I have a sharding cluster with an EventsourcedProcessor  (expect 
10.000ths of these EventsourcedProcessor actor instances) , so I'll need to 
create a line like this for every EventsourcedProcessor in order to get the 
stream of events together. Thereafter, I need to merge them together to get 
a single stream of events. (at least that is one of the features of using 
the streams)

My goal is to have 'Listeners' (that is my interpretation of a 'View' due 
to historic reasons...) that will for instance update a data store, this 
will probably happen on on just a few nodes (maybe 1 and some failover 
stuff). These 'Listeners' need to attach to the sharded Eventsourced system 
and ask to get all event sourced events forwarded. (publish subscribe more 
or less). 

I wonder if the current View (or ViewProducer) fits this situation due to 
the fact you need to create as many views as eventsourcedprocessors are 
created. 
With the merged streams thereafter, it seems a possibility to have just one 
thing per node (I assume actor) that will do the writing to a data store 
(not being the eventstore). 
What would be the way to get these Views 'automagically' attached to the 
proper procesors ? 

And, do you have a pointer how this issue is solved with it's own 
eventstore ? In a sharding cluster, you more or less have the same issue. 
(would streams change your approach there ?) 






 

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