Hi Murali,
I started development of an application based on Akka Persistence to implement CQRS concepts about a year ago. A lot of ideas came from different topics in this group. Recently I started to extract a small library from this application. The approach I took is to redundantly store all events in a global persistent actor in order to recreate views in a journal agnostic way. This also allows for easy in-memory testing. There is a minor risk, in that storing the event twice breaks consistency. The globally stored events I only use for (re)constructing views. Once there is a better solution, the global persistent actor can be deleted and the views can be reconstructed in a different manner. Even though this solution is not perfect, it might help your use case. I will add more tests and documentation over time as well, since currently, all tests remain in the application code. https://github.com/Product-Foundry/akka-cqrs Hope this helps, Andre On Friday, March 27, 2015 at 1:33:43 PM UTC+1, rkuhn wrote: > > Hi Murali, > > the core team at Typesafe cannot work on this right now (we need to finish > Streams and HTTP first and have some other obligations as well), but Akka > is an open-source project and we very much welcome contributions of all > kinds. In this case we should probably start by defining more closely which > queries to (initially) support and how to model them in the various > backends, so that we can get a feel for how we shall change the Journal SPI. > > Regards, > > Roland > > 27 mar 2015 kl. 12:41 skrev Ganta Murali Krishna <gan...@gmail.com > <javascript:>>: > > Hello Roland, > > Any news on this please. When we can expect implementation roughly? Your > response will be really appreciated. > > Regards > Murali > > On Wednesday, 27 August 2014 20:04:30 UTC+5:30, rkuhn wrote: >> >> Dear hakkers, >> >> there have been several very interesting, educational and productive >> threads in the past weeks (e.g. here >> <https://groups.google.com/d/msg/akka-user/SL5vEVW7aTo/KfqAXAmzol0J> and >> here <https://groups.google.com/d/msg/akka-user/4kbYcwWS2OI/hpmAkxnB9D4J>). >> We have taken some time to distill the essential problems as well as >> discuss the proposed solutions and below is my attempt at a summary. In the >> very likely case that I missed something, by all means please raise your >> voice. The intention for this thread is to end with a set of github issues >> for making Akka Persistence as closely aligned with CQRS/ES principles as >> we can make it. >> >> As Greg and others have confirmed, the write-side (PersistentActor) is >> already doing a very good job, so we do not see a need to change anything >> at this point. My earlier proposal of adding specific topics as well as the >> discussed labels or tags all feel a bit wrong since they benefit only the >> read-side and should therefore not be a concern/duty of the write-side. >> >> On the read-side we came to the conclusion that PersistentView basically >> does nearly the right thing, but it focuses on the wrong aspect: it seems >> most suited to track a single PersistentActor with some slack, but also not >> with back-pressure as a first-class citizen (it is possible to achieve it, >> albeit not trivial). What we distilled as the core functionality for a >> read-side actor is the following: >> >> >> - it can ask for a certain set of events >> - it consumes the resulting event stream on its own schedule >> - it can be stateful and persistent on its own >> >> >> This does not preclude populating e.g. a graph database or a SQL store >> directly from the journal back-end via Spark, but we do see the need to >> allow Akka Actors to be used to implement such a projection. >> >> Starting from the bottom up, allowing the read-side to be a >> PersistentActor in itself means that receiving Events should not require a >> mixin trait like PersistentView. The next bullet point means that the Event >> stream must be properly back-pressured, and we have a technology under >> development that is predestined for such an endeavor: Akka Streams. So the >> proposal is that any Actor can obtain the ActorRef for a given Journal and >> send it a request for the event stream it wants, and in response it will >> get a message containing a stream (i.e. Flow) of events and some >> meta-information to go with it. >> >> The question that remains at this point is what exactly it means to “ask >> for a certain set of events”. In order to keep the number of abstractions >> minimal, the first use-case for this feature is the recovery of a >> PersistentActor. Each Journal will probably support different kinds of >> queries, but it must for this use-case respond to >> >> case class QueryByPersistenceId(id: String, fromSeqNr: Long, toSeqNr: >> Long) >> >> with something like >> >> case class EventStreamOffer(metadata: Metadata, stream: >> Flow[PersistentMsg]) >> >> The metadata allows the recipient to correlate this offer with the >> corresponding request and it contains other information as we will see in >> the following. >> >> Another way to ask for events was discussed as Topics or Labels or Tags >> in the previous threads, and the idea was that the generated stream of all >> events was enriched by qualifiers that allow the Journal to construct a >> materialized view (e.g. a separate queue that copies all events of a given >> type). This view then has a name that is requested from the read-side in >> order to e.g. have an Actor that keeps track of certain aspects of all >> persistent ShoppingCarts in a retail application. As I said above we think >> that this concern should be handled outside of the write-side because >> logically it does not belong there. Its closest cousin is the construction >> of an additional index or view within a SQL store, maintained by the RDBMS >> upon request from the DBA, but available to and relied upon by the >> read-side. We propose that this is also how this should work with Akka >> Persistence: the Journal is free to allow the configuration of materialized >> views that can be requested as event streams by name. The extraction of the >> indexing characteristics is performed by the Journal or its backing store, >> outside the scope of the Journal SPI; one example of doing it this way has >> been implemented by Martin >> <https://github.com/krasserm/akka-persistence-kafka/#user-defined-topics> >> already. >> We propose to access the auxiliary streams by something like >> >> case class QueryKafkaTopic(name: String, fromSeqNr: Long, toSeqNr: Long) >> >> Sequence numbers are necessary for deterministic replay/consumption. We >> had long discussions about the scalability implications, which is the >> reason why we propose to leave such queries proprietary to the Journal >> backend. Assuming a perfectly scalable (but then of course not real-time >> linearizable) Journal, the query might allow only >> >> case class QuerySuperscalableTopic(name: String, fromTime: DateTime) >> >> This will try to give you all events that were recorded after the given >> moment, but replay will not be deterministic, there will not be unique >> sequence numbers. These properties will be reflected in the Metadata that >> comes with the EventStreamOffer. >> >> The last way to ask for events is to select them using an arbitrarily >> powerful query at runtime, probably with dynamic parameters so that it >> cannot be prepared or materialized while writing the log. Whether and how >> this is supported by the Journal depends on the precise back-end, and this >> is very much deliberate: we want to allow the Journal implementations to >> focus on different use-cases and offer different feature trade-offs. If a >> RDBMS is used, then things will naturally be linearized, but less scalable, >> for example. Document databases can extract a different set of features >> than when storing BLOBs in Oracle, etc. The user-facing API would be >> defined by each Journal implementation and could include >> >> case class QueryEventStoreJS(javascriptCode: String) >> case class QueryByProperty(jsonKey: String, value: String, since: >> DateTime) >> case class QueryByType(clazz: Class[_], fromSeqNr: Long, toSeqNr: Long) >> case class QueryNewStreams(fromSeqNr: Long, toSeqNr: Long) >> >> The last one should elegantly solve the use-case of wanting to catalog >> which persistenceIds are valid in the Journal (which has been requested >> several times as well). As discussed for the SuperscalableTopic, each >> Journal would be free to decide whether it wants to implement deterministic >> replay, etc. >> >> Properly modeling streams of events as Akka Streams feels like a >> consistent way forward, it also allows non-actor code to be employed for >> doing stream processing on the resulting event streams, including merging >> multiple of them or feeding events into Spark—the possibilities are >> boundless. I’m quite excited by this new perspective and look forward to >> your feedback on how well this helps Akka users implement the Q in CQRS. >> >> Regards, >> >> >> *Dr. Roland Kuhn* >> *Akka Tech Lead* >> Typesafe <http://typesafe.com/> – Reactive apps on the JVM. >> twitter: @rolandkuhn >> <http://twitter.com/#!/rolandkuhn> >> >> > -- > >>>>>>>>>> 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. > > > > > *Dr. Roland Kuhn* > *Akka Tech Lead* > Typesafe <http://typesafe.com/> – Reactive apps on the JVM. > twitter: @rolandkuhn > <http://twitter.com/#!/rolandkuhn> > > -- >>>>>>>>>> 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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