Hi Patrick, Sounds like an interesting approach, storing some meta-data at the view may help to check / show the reliability of the system.
At this moment the events are sent to a processor per node that publishes the event (distributed pub sub) When you talk about view, that's the akka-persistence view ? So more or less, the sub processors could send messages to the View and when there is a Persist() around it, it will be stored. Is that a correct understanding ? Kind regards, Olger On Sunday, April 20, 2014 2:32:07 PM UTC+2, Patrik Nordwall wrote: > > Hi Olger, > > What if you keep the sharded event sourced actors (+10k), but let them > also send the events to one or a few processors. Then you can connect the > views/streams to these processors. > > If you don't like storing the events twice you can instead store some > meta-data (processor-id, seq-no,timestamp) and have a view that creates > sub-views on demand from the replayed meta-data. The sub-views would > forward to the parent aggregated view. > > /Patrik > > 19 apr 2014 kl. 20:46 skrev Olger Warnier <ol...@spectare.nl <javascript:> > >: > > > Hi Martin, > > Had to think about it a little, hereby my follow up. (hope you don't mind > the continues discussion, it helps me a lot in defining the right approach, > thanks for that) > > On Saturday, April 19, 2014 7:11:23 AM UTC+2, Martin Krasser wrote: >> >> Hi Olger, >> >> installing 10k views/producers won't scale, at least not with the current >> implementation. Here are some alternatives: >> > Intresting, what would need to change to have is scaling ? > (Idea is to have the eventsourcedprocessors reflect a DDD style Aggregate > Root instance and have those distributed using cluster sharding) > > >> >> - Maybe a custom journal plugin is what you need: a plugin that delegates >> all write/read requests to the actual journal actor and that additionally >> updates a database with the events to be written. This essentially installs >> a single "listener" per ActorSystem (this is to some extend comparable to a >> database trigger that executes additonal commands. If the backend datastore >> supports that directly, I recommend implementing the trigger there, if >> possible). >> > > I am not sure, if I understand it.. the basic idea is to have the 'events' > stored via the eventsourcedprocessor being published to 'n' views. The > actual number of view that need to listen to these events are not known up > front (people can add their own views... at system startup, it will be > clear) > As every eventsourced actor is actually an AggregateRoot (in DDD terms) > and thereby something of an instance with it's own state, the changes in > these states need to be aggregated (that can be done with the streaming as > you mention) and published to the views that are interested (subscribed). > Doing this by hand in the aggregate root actor is not a problem, > thereafter write your own listener actor and that will populate a view data > store. Still I have the feeling that the actual 'View' (or ViewProducer) > could be implemented in such a way that it's done by the view. > > >> >> - Instead of having thousands of processors, what speaks against >> combining them into a single processor (or only a few) per node? >> > This would mean that I'll have all my aggregate root instances running in > 1 processor meaning that I need to reconstruct state per aggregate root > instance in some way. Using EventsourcedProcessor, I'd expect that I need > to replay everything for all instances and pick the one that I need for > processing at that moment. (this can of course be optimized with snapshots > and something like memcached). This appears to be a performance hit as I > feel it. > > >> >> Further comments inline ... >> >> On 18.04.14 16:10, Olger Warnier wrote: >> >> 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) >> >> >> Every processor instance itself could create such a producer during start >> and send it to another actor that merges received producers. >> > That would not allow me to implement 'View' (as is known in the > persistence package) in order to listen to events within my cluster of > aggregate root instances, I'll need to build something additional for that > (as View is more used for the collection of those events and thereafter > will push them through) > > At this moment, I use an akka extension (gives more or less a singleton) > that is used directly in the EventSourcedProcessor after storage (persist) > of the event. > Thereafter I have listeners that get these events and transform them into > data that needs storage for a certain type of view (CQRS style) (this is > where I expected the 'View' to be used) > >> >> >> 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 ? >> >> >> See above. >> >> >> 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 ?) >> >> >> There's one journal actor per ActorSystem where n journal actors in a >> cluster update a replicated >> journal<http://akka.io/community/#plugins_to_akka_persistence> >> . >> > Thanks, thats not done as an akka extension right ? (why not ?) > >> >> Hope that helps. >> > Certainly, really appreciate your patience. > >> >> Cheers, >> Martin >> > > Kind regards, > > Olger > > > -- > >>>>>>>>>> Read the docs: http://akka.io/docs/ > >>>>>>>>>> Check the FAQ: > http://doc.akka.io/docs/akka/current/additional/faq.html > >>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user > --- > You received this message because you are subscribed to the Google Groups > "Akka User List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to akka-user+...@googlegroups.com <javascript:>. > To post to this group, send email to akka...@googlegroups.com<javascript:> > . > Visit this group at http://groups.google.com/group/akka-user. > For more options, visit https://groups.google.com/d/optout. > > -- >>>>>>>>>> Read the docs: http://akka.io/docs/ >>>>>>>>>> Check the FAQ: >>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html >>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user --- You received this message because you are subscribed to the Google Groups "Akka User List" group. 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