On Wed, Jan 14, 2015 at 5:02 PM, Akka Team <[email protected]> wrote:
> Hi Oleg, > Your understanding of both topics, in the current version of Akka, are > correct. > More specifically: > > Is it possible to configure Akka Cluster with auto-downing so that with >> 100% probability at each moment at most one cluster node thinks that he is >> the leader (or singleton) even in case of cluster partitions? Some >> emails/blog posts imply that requiring cluster partition to be majority >> quorum does the trick? > > The auto-downing strategy currently provided is a rather naive one - just > timeout based. It does not guard against split brains, which is why I would > rather not encourage using auto-downing if your cluster needs any kind of > "single" entity. > The timer based auto downing works well for clusters where you have "many > workers, but no master" for example, since causing a split need not end in > a wrong "leader" being elected (since there is no leader). > > Yes, quorum would help making downing more safe (avoid split brains), > however we have not implemented it yet (we are aware of the need and > possibility of course). > Well, to be precise, everything is there for anyone to implement their own downing strategy, we just not provided a prepackaged solution for example for quorum. Akka cluster is a toolbox, but it seems like there is high demand for pre-packaged "just works" solutions. -Endre > > How to do it with auto-scaling clusters? > > In general it boils down to disallowing situations where the cluster can > get "two majorities" (one side knows only old nodes, new side has more > nodes and didn't reach quorum yet). > One of the methods I'm aware of is described in the raft paper - section > 6: membership changes <https://ramcloud.stanford.edu/raft.pdf>, which > describes how it guarantees that at no point in time there can be "two > majorities". > > We have not looked into implementing a better downing strategy yet but > it's "on the radar" - it's not a given it would be raft based, needs more > thought put into it. > > > Am I correct that if two actors with the same persistence id work >> concurrently on different cluster partitions the sequence numbers of events >> will get mixed up? > > Correct, currently these sequence numbers are "source of truth" and mixing > them up causes problems during replay. > > >> Does it imply that if Akka Persistence is used with Akka Cluster Sharding >> then Akka Cluster must guarantee uniqueness of ShardCoordinator? > > In fact, the cluster sharding *uses* persistence in order to survive the > leader going down - so we can restore the shard allocation information once > the new coordinator boots up. > > Yes, currently there is a hard requirement for "only one writer" in > akka-persistence. This can be facilitated by either cluster-sharding or > cluster-singletons. > > Slightly related "future work": We do have some CRDT work by Patrik > Nordwall stashed (it's public as akka-data-replication) and will want to > move it into Akka main at some point, > those of course do not require any kind leaders in the cluster. > > > Hope this helps! > > -- > Konrad > > On Mon, Jan 12, 2015 at 5:55 PM, Oleg Mürk <[email protected]> wrote: > >> Hello! >> >> I would like to better understand and confirm consistency guarantees of >> Akka Cluster and Persistence (ideally in terms of CAP and CRDTs) and >> apologise if these questions have been discussed before :) >> >> Akka Cluster: >> Is it possible to configure Akka Cluster with auto-downing so that with >> 100% probability at each moment at most one cluster node thinks that he is >> the leader (or singleton) even in case of cluster partitions? Some >> emails/blog posts imply that requiring cluster partition to be majority >> quorum does the trick? How to do it with auto-scaling clusters? >> >> Akka Persistence: >> Am I correct that if two actors with the same persistence id work >> concurrently on different cluster partitions the sequence numbers of events >> will get mixed up? Does it imply that if Akka Persistence is used with Akka >> Cluster Sharding then Akka Cluster must guarantee uniqueness of >> ShardCoordinator? >> >> Some relevant messages that I managed to find are: >> * Quorum-based split brain resolution >> https://groups.google.com/d/msg/akka-user/UBSF3QQnGaM/r8YUEzr2EtEJ >> https://groups.google.com/d/msg/akka-user/UBSF3QQnGaM/Jsg30N9ORQQJ >> * Exactly *one* persistence actor with the same persistenceId active ? >> https://groups.google.com/d/msg/akka-user/9E65YaKI2BE/SA3px74h0ugJ >> https://groups.google.com/d/msg/akka-user/9E65YaKI2BE/hZon4M_z0AgJ >> >> Thank You! >> Oleg Mürk >> >> -- >> >>>>>>>>>> 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 [email protected]. >> To post to this group, send email to [email protected]. >> Visit this group at http://groups.google.com/group/akka-user. >> For more options, visit https://groups.google.com/d/optout. >> > > > > -- > Akka Team > Typesafe - The software stack for applications that scale > Blog: letitcrash.com > Twitter: @akkateam > > -- > >>>>>>>>>> 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 [email protected]. > To post to this group, send email to [email protected]. > 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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