You can however implement your own akka.cluster.sharding.ShardCoordinator.ShardAllocationStrategy which will allow you to take whatever you want into account, and deal with the consequences thereof ofc ;)
-- Johan On Friday, August 5, 2016 at 12:04:12 AM UTC+2, Justin du coeur wrote: > > It does do reassignment -- but it has to know to do that. Keep in mind > that "down" is the master switch here: until the node is downed, the rest > of the system doesn't *know* that NodeA should be avoided. I haven't dug > into that particular code, but I assume from what you're saying that the > allocation algorithm doesn't take unreachability into account when choosing > where to allocate the shard, just up/down. I suspect that unreachability > is too local and transient to use as the basis for these allocations. > > Keep in mind that you're looking at this from a relatively all-knowing > global perspective, but each node is working from a very localized and > imperfect one. All it knows is "I can't currently reach NodeA". It has no > a priori way of knowing whether NodeA has been taken offline (so it should > be avoided), or there's simply been a transient network glitch between here > and there (so things are *mostly* business as usual). Downing is how you > tell it, "No, really, stop using this node"; until then, most of the code > assumes that the more-common transient situation is the case. It's > *probably* possible to take unreachability into account in the case you're > describing, but it doesn't surprise me if that's not true. > > Also, keep in mind that, IIRC, there are a few cluster singletons involved > here, at least behind the scenes. If NodeA currently owns one of the key > singletons (such as the ShardCoordinator), and it hasn't been downed, I > imagine the rest of the cluster is going to *quickly* lock up, because the > result is that nobody is authorized to make these sorts of allocation > decisions. > > All that said -- keep in mind, I'm just a user of this stuff, and am > talking at the edges of my knowledge. Konrad's the actual expert... > > On Thu, Aug 4, 2016 at 4:59 PM, Eric Swenson <e...@swenson.org> wrote: > >> While I'm in the process of implementing your proposed solution, I did >> want to make sure I understood why I'm seeing the failures I'm seeing when >> a node is taken offline, auto-down is disabled, and no one is handling the >> UnreachableNode message. Let me try to explain what I think is happening >> and perhaps you (or someone else who knows more about this than I) can >> confirm or refute. >> >> In the case of akka-cluster-sharding, a shard might exist on the >> unreachable node. Since the node is not yet marked as "down", the cluster >> simply cannot handle an incoming message for that shard. To create another >> sharded actor on an available cluster node might duplicate the unreachable >> node state. In the case of akka-persistence actors, even though a new >> shard actor could resurrect any journaled state, we cannot be sure that the >> old unreachable node might not at any time, add other events to the >> journal, or come online and try to continue operating on the shard. >> >> Is that the reason why I see the following behavior: NodeA is online. >> NodeB comes online and joins the cluster. A request comes in from >> akka-http and is sent to the shard region. It goes to NodeA which creates >> an actor to handle the sharded object. NodeA is taken offline (unbeknownst >> to the akka-cluster). Another message for the above-mentioned shard comes >> in from akka-http and is sent to the shard region. The shard region can't >> reach NodeA. NodeA isn't marked as down. So the shard region cannot >> create another actor (on an available Node). It can only wait (until >> timeout) for NodeA to become reachable. Since, in my scenario, NodeA will >> never become reachable and NodeB is the only one online, all requests for >> old shards timeout. >> >> If the above logic is true, I have one last issue: In the above >> scenario, if a message comes into the shard region for a shard that WOULD >> HAVE BEEN allocated to NodeA but has never yet been assigned to an actor on >> NodeA, and NodeA is unreachable, why can it simply be assigned to another >> Node? is it because the shard-to-node algorithm is fixed (by default) and >> there is no dynamic ability to "reassign" to an available Node? >> >> Thanks again. -- Eric >> >> On Wednesday, August 3, 2016 at 7:00:42 PM UTC-7, Justin du coeur wrote: >>> >>> The keyword here is "auto". Autodowning is an *incredibly braindead* >>> algorithm for dealing with nodes coming out of service, and if you use it >>> in production you more or less guarantee disaster, because that algorithm >>> can't cope with cluster partition. You *do* need to deal with downing, but >>> you have to get something smarter than that. >>> >>> Frankly, if you're already hooking into AWS, I *suspect* the best >>> approach is to leverage that -- when a node goes offline, you have some >>> code to detect that through the ECS APIs, react to it, and manually down >>> that node. (I'm planning on something along those lines for my system, but >>> haven't actually tried yet.) But whether you do that or something else, >>> you've got to add *something* that does downing. >>> >>> I believe the official party line is "Buy a Lightbend Subscription", >>> through which you can get their Split Brain Resolver, which is a fairly >>> battle-hardened module for dealing with this problem. That's not strictly >>> necessary, but you *do* need to have a reliable solution... >>> >>> On Wed, Aug 3, 2016 at 8:42 PM, Eric Swenson <er...@swenson.org> wrote: >>> >>>> We have an akka-cluster/sharding application deployed an AWS/ECS, where >>>> each instance of the application is a Docker container. An ECS service >>>> launches N instances of the application based on configuration data. It >>>> is >>>> not possible to know, for certain, the IP addresses of the cluster >>>> members. Upon startup, before the AkkaSystem is created, the code >>>> currently polls AWS and determines the IP addresses of all the Docker >>>> hosts >>>> (which potentially could run the akka application). It sets these IP >>>> addresses as the seed nodes before bringing up the akka cluster system. >>>> The >>>> configuration for these has, up until yesterday always included the >>>> akka.cluster.auto-down-unreachable-after configuration setting. And it >>>> has >>>> always worked. Furthermore, it supports two very critical requirements: >>>> >>>> a) an instance of the application can be removed at any time, due to >>>> scaling or rolling updates >>>> b) an instance of the application can be added at any time, due to >>>> scaling or rolling updates >>>> >>>> On the advice of an Akka expert on the Gitter channel, I removed the >>>> auto-down-unreachable-after setting, which, as documented, is dangerous >>>> for >>>> production. As a result the system no longer supports rolling updates. A >>>> rolling update occurs thus: a new version of the application is deployed >>>> (a new ECS task definition is created with a new Docker image). The ECS >>>> service launches a new task (Docker container running on an available >>>> host) >>>> and once that container becomes stable, it kills one of the remaining >>>> instances (cluster members) to bring the number of instances to some >>>> configured value. >>>> >>>> When this happens, akka-cluster becomes very unhappy and becomes >>>> unresponsive. Without the auto-down-unreachable-after setting, it keeps >>>> trying to talk to the old cluster members. which is no longer present. It >>>> appears to NOT recover from this. There is a constant barrage of messages >>>> of the form: >>>> >>>> [DEBUG] [08/04/2016 00:19:27.126] >>>> [ClusterSystem-cassandra-plugin-default-dispatcher-27] >>>> [akka.actor.LocalActorRefProvider(akka://ClusterSystem)] resolve of path >>>> sequence [/system/sharding/ExperimentInstance#-389574371] failed >>>> [DEBUG] [08/04/2016 00:19:27.140] >>>> [ClusterSystem-cassandra-plugin-default-dispatcher-27] >>>> [akka.actor.LocalActorRefProvider(akka://ClusterSystem)] resolve of path >>>> sequence [/system/sharding/ExperimentInstance#-389574371] failed >>>> [DEBUG] [08/04/2016 00:19:27.142] >>>> [ClusterSystem-cassandra-plugin-default-dispatcher-27] >>>> [akka.actor.LocalActorRefProvider(akka://ClusterSystem)] resolve of path >>>> sequence [/system/sharding/ExperimentInstance#-389574371] failed >>>> [DEBUG] [08/04/2016 00:19:27.143] >>>> [ClusterSystem-cassandra-plugin-default-dispatcher-27] >>>> [akka.actor.LocalActorRefProvider(akka://ClusterSystem)] resolve of path >>>> sequence [/system/sharding/ExperimentInstance#-389574371] failed >>>> [DEBUG] [08/04/2016 00:19:27.143] >>>> [ClusterSystem-cassandra-plugin-default-dispatcher-27] >>>> [akka.actor.LocalActorRefProvider(akka://ClusterSystem)] resolve of path >>>> sequence [/system/sharding/ExperimentInstance#-389574371] failed >>>> >>>> and of the form: >>>> >>>> [WARN] [08/04/2016 00:19:16.787] >>>> [ClusterSystem-akka.actor.default-dispatcher-9] [akka.tcp:// >>>> ClusterSystem@10.0.3.103:2552/system/sharding/ExperimentInstance] >>>> Retry request for shard [5] homes from coordinator at [Actor[akka.tcp:// >>>> ClusterSystem@10.0.3.100:2552/system/sharding/ExperimentInstanceCoordinator/singleton/coordinator#1679517511]]. >>>> >>>> [1] buffered messages. >>>> [WARN] [08/04/2016 00:19:18.787] >>>> [ClusterSystem-akka.actor.default-dispatcher-9] [akka.tcp:// >>>> ClusterSystem@10.0.3.103:2552/system/sharding/ExperimentInstance] >>>> Retry request for shard [23] homes from coordinator at [Actor[akka.tcp:// >>>> ClusterSystem@10.0.3.100:2552/system/sharding/ExperimentInstanceCoordinator/singleton/coordinator#1679517511]]. >>>> >>>> [1] buffered messages. >>>> [WARN] [08/04/2016 00:19:18.787] >>>> [ClusterSystem-akka.actor.default-dispatcher-9] [akka.tcp:// >>>> ClusterSystem@10.0.3.103:2552/system/sharding/ExperimentInstance] >>>> Retry request for shard [1] homes from coordinator at [Actor[akka.tcp:// >>>> ClusterSystem@10.0.3.100:2552/system/sharding/ExperimentInstanceCoordinator/singleton/coordinator#1679517511]]. >>>> >>>> [1] buffered messages. >>>> [WARN] [08/04/2016 00:19:18.787] >>>> [ClusterSystem-akka.actor.default-dispatcher-9] [akka.tcp:// >>>> ClusterSystem@10.0.3.103:2552/system/sharding/ExperimentInstance] >>>> Retry request for shard [14] homes from coordinator at [Actor[akka.tcp:// >>>> ClusterSystem@10.0.3.100:2552/system/sharding/ExperimentInstanceCoordinator/singleton/coordinator#1679517511]]. >>>> >>>> [1] buffered messages. >>>> [WARN] [08/04/2016 00:19:18.787] >>>> [ClusterSystem-akka.actor.default-dispatcher-9] [akka.tcp:// >>>> ClusterSystem@10.0.3.103:2552/system/sharding/ExperimentInstance] >>>> Retry request for shard [5] homes from coordinator at [Actor[akka.tcp:// >>>> ClusterSystem@10.0.3.100:2552/system/sharding/ExperimentInstanceCoordinator/singleton/coordinator#1679517511]]. >>>> >>>> [1] buffered messages. >>>> >>>> and then a message like this: >>>> >>>> [WARN] [08/03/2016 23:50:34.690] >>>> [ClusterSystem-akka.remote.default-remote-dispatcher-11] [akka.tcp:// >>>> ClusterSystem@10.0.3.103:2552/system/endpointManager/reliableEndpointWriter-akka.tcp%3A%2F%2FClusterSystem%4010.0.3.100%3A2552-0] >>>> >>>> Association with remote system [akka.tcp:// >>>> ClusterSystem@10.0.3.100:2552] has failed, address is now gated for >>>> [5000] ms. Reason: [Association failed with [akka.tcp:// >>>> ClusterSystem@10.0.3.100:2552]] Caused by: [Connection refused: / >>>> 10.0.3.100:2552] >>>> >>>> The 10.0.3.100 host is the one that was taken out of service. The >>>> 10.0.3.103 host is the remaining instance (to narrow down the issue, we >>>> have one node, to which a second was added and then the first removed). >>>> >>>> But the fact remains that no messages to the sharding region are >>>> handled -- they all timeout like this: >>>> >>>> [ERROR] [08/03/2016 23:50:35.821] >>>> [ClusterSystem-akka.actor.default-dispatcher-6] >>>> [akka.actor.ActorSystemImpl(ClusterSystem)] Error during processing of >>>> request HttpRequest(HttpMethod(POST), >>>> http://eim.dev.genecloud.com/eim/v1/experimentinstance,List(Host: >>>> eim.dev.genecloud.com, X-Real-Ip: 10.0.3.157, X-Forwarded-For: >>>> 10.0.3.157, Connection: upgrade, Accept-Encoding: gzip, deflate, Accept: >>>> */*, User-Agent: python-requests/2.10.0, Accept-Type: application/json, >>>> Authorization: Bearer 9wDoBBnbzD7XHmWt8Qk-OaanGlaHPzul8PQrnzPrwW4, >>>> Timeout-Access: >>>> <function1>),HttpEntity.Strict(application/json,{}),HttpProtocol(HTTP/1.1)) >>>> akka.pattern.AskTimeoutException: Ask timed out on >>>> [Actor[akka://ClusterSystem/system/sharding/ExperimentInstance#- >>>> 2107641834]] after [10000 ms]. Sender[null] sent message of type >>>> "com.genecloud.eim.ExperimentInstance$Commands$NewExperimentInstanceV1". >>>> at >>>> akka.pattern.PromiseActorRef$$anonfun$1.apply$mcV$sp(AskSupport.scala:604) >>>> at akka.actor.Scheduler$$anon$4.run(Scheduler.scala:126) >>>> at >>>> scala.concurrent.Future$InternalCallbackExecutor$.unbatchedExecute(Future.scala:601) >>>> at >>>> scala.concurrent.BatchingExecutor$class.execute(BatchingExecutor.scala:109) >>>> at >>>> scala.concurrent.Future$InternalCallbackExecutor$.execute(Future.scala:599) >>>> at >>>> akka.actor.LightArrayRevolverScheduler$TaskHolder.executeTask(LightArrayRevolverScheduler.scala:331) >>>> at >>>> akka.actor.LightArrayRevolverScheduler$$anon$4.executeBucket$1(LightArrayRevolverScheduler.scala:282) >>>> at >>>> akka.actor.LightArrayRevolverScheduler$$anon$4.nextTick(LightArrayRevolverScheduler.scala:286) >>>> at >>>> akka.actor.LightArrayRevolverScheduler$$anon$4.run(LightArrayRevolverScheduler.scala:238) >>>> at java.lang.Thread.run(Thread.java:745) >>>> >>>> And even though the cluster of one node (10.0.3.103) should be >>>> perfectly able to handle messages from akka-http, it doesn't. They all >>>> timeout. And the barrage of unhappy messages continues forever. It never >>>> recovers and the service cannot handle any akka-http requests that send >>>> messages to the cluster-sharing region. >>>> >>>> I "fixed" everything, by simply adding back >>>> the auto-down-unreachable-after parameter. Now, when a new node comes >>>> into >>>> the cluster, it is recognized and used, and when an old node is removed >>>> from the cluster, after the configured time period, it is removed the >>>> cluster and everything is happy again. >>>> >>>> What is the recommended way to deal with cluster nodes being added and >>>> removed outside the control of the application? >>>> >>>> -- Eric >>>> >>>> -- >>>> >>>>>>>>>> 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. >>>> To post to this group, send email to akka...@googlegroups.com. >>>> Visit this group at https://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. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to akka-user+unsubscr...@googlegroups.com. >> To post to this group, send email to akka-user@googlegroups.com. >> Visit this group at https://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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