Thanks for the clear and simple explanation. However, will the cluster (with auto expand replicas) ever go green if it has been grown from 2 to 3 (triggering replicas to grow to to) and then downsized to two nodes again? In other words, do the auto grow replicas setting work both ways or just upwards?
Thanks again. G. On 11 Jul 2014 12:11, "Glen Smith" <[email protected]> wrote: > Hi Goncalo, > > I think it's important that you understand: multiple copies of a shard > will never be located on the same node. > Not two replicas, and not the primary and one replica. > To witness this, run a server on your local machine, and create an index > with the defaults - 5 shards, one replica. > You will see that your cluster is "yellow", and has 5 unallocated shards. > > How that helps create a better mental picture of shard allocation. > > > On Friday, July 11, 2014 2:00:47 AM UTC-4, Gonçalo Luiz wrote: >> >> Hi Ivan, >> >> Does this mean that if a note comes back and a replication is underway >> we'll end up with a node holding 2 replicas and 1 node holding node ? >> >> Scenario: >> >> Node A - Replica 2 >> Node B - Replica 3 >> Node C - Replica 1 >> >> If node A dies and Node B get's Replica 2, as soon as node A (or a >> replacement) is brought up, is the final configuration likely to be >> >> Node A (or replcament) - No replicas >> Node B .- Replica 3 and 2 >> Node C - Replica 1 >> >> or is there a re-balance that takes place ? >> >> Thanks, >> Gonçalo >> >> Gonçalo Luiz >> >> >> On 10 July 2014 22:11, Ivan Brusic <[email protected]> wrote: >> >>> It's only been around for 3.5 years: https://github.com/ >>> elasticsearch/elasticsearch/issues/623 :) >>> >>> I should clarify part of my previous statement. >>> >>> *"By default, the ongoing recovery is not cancelled when the missing >>> node rejoins the cluster. You can change the gateway settings [2] to >>> control when recovery kicks in."* >>> >>> What I meant to say is that an ongoing recovery is never cancelled once >>> it has commenced, no matter what settings. By default, recovery happens >>> immediately, but can be changed with the gateway settings. >>> >>> -- >>> Ivan >>> >>> >>> On Thu, Jul 10, 2014 at 1:48 PM, [email protected] <[email protected]> >>> wrote: >>> >>>> Indeed, auto_expand_replicas "all" triggers an update cluster >>>> settings action each time a node is added. >>>> >>>> Still blown by the many settings Elasticsearch provides. Feeling small. >>>> Homework: collecting a gist textfile of all ES 1.2 settings. >>>> >>>> Jörg >>>> >>>> >>>> On Thu, Jul 10, 2014 at 9:57 PM, Ivan Brusic <[email protected]> wrote: >>>> >>>>> Sticking to your use case, you might want to use >>>>> the auto_expand_replicas setting to "all" [1]: Never used it, but it >>>>> sounds >>>>> what you are looking for. >>>>> >>>>> By default, the ongoing recovery is not cancelled when the missing >>>>> node rejoins the cluster. You can change the gateway settings [2] to >>>>> control when recovery kicks in. >>>>> >>>>> [1] http://www.elasticsearch.org/guide/en/elasticsearch/ >>>>> reference/current/indices-update-settings.html >>>>> [2] http://www.elasticsearch.org/guide/en/elasticsearch/ >>>>> reference/current/modules-gateway.html >>>>> >>>>> Cheers, >>>>> >>>>> Ivan >>>>> >>>>> >>>>> On Thu, Jul 10, 2014 at 12:39 PM, Gonçalo Luiz <[email protected]> >>>>> wrote: >>>>> >>>>>> I get it know. >>>>>> >>>>>> I agree that setting the number of replicas is connected to the >>>>>> deployment reality in each case and it's derived variables and thus there >>>>>> is no one formula to fit all cases (it would't be a setting in that >>>>>> case). >>>>>> >>>>>> What I was trying to cover was the theoretical / extreme case where >>>>>> any node may fail at any time and what is the best way to go to minimize >>>>>> the chance of losing data. Also, in the case you want to scale down the >>>>>> installation (pottentially down to one node) without having to worry >>>>>> about >>>>>> selecting nodes that hold different replicated shards is an example that >>>>>> can beneffit from such configuration. >>>>>> >>>>>> I'm however not clear yet on what happens when a node goes down >>>>>> (triggering extra replication amongst the survivors) and then comes up >>>>>> again. Is the ongoing replication cancelled and the returning node >>>>>> brought >>>>>> up to date? >>>>>> >>>>>> Thanks for your valuable input. >>>>>> >>>>>> G. >>>>>> On 10 Jul 2014 18:07, "[email protected]" <[email protected]> >>>>>> wrote: >>>>>> >>>>>>> All I say is that it depends on the probability of the event of >>>>>>> three nodes failing simultaneously, not on the total number of nodes >>>>>>> having >>>>>>> a replica. You can even have 5 nodes and the probability of the event >>>>>>> of 4 >>>>>>> nodes failing simultaneously, and so on. >>>>>>> >>>>>>> As an illustration, suppose you have a data center with two >>>>>>> independent electric circuits and the probability of failure corresponds >>>>>>> with power outage, then it is enough to distribute nodes equally over >>>>>>> servers using the two independent power lines in the racks. If one >>>>>>> electric >>>>>>> circuit (plus UPS) fails, half of the nodes go down. With replica level >>>>>>> 1, >>>>>>> ES cluster will keep all the data. There is no need to set replica level >>>>>>> equal to node number. >>>>>>> >>>>>>> Jörg >>>>>>> >>>>>>> >>>>>>> On Thu, Jul 10, 2014 at 8:55 AM, Gonçalo Luiz <[email protected]> >>>>>>> wrote: >>>>>>> >>>>>>>> Hi Joe, >>>>>>>> >>>>>>>> Thanks for your reply. >>>>>>>> On this thougth: >>>>>>>> >>>>>>>> " >>>>>>>> From my view your idea of better fault tolerance does not make much >>>>>>>> sense. The replica number is a statistical entity that is related to >>>>>>>> the >>>>>>>> probability of faults. The higher the replica, the higher the >>>>>>>> probability >>>>>>>> of surviving faults. There is no correlation to the total number of >>>>>>>> nodes >>>>>>>> in a cluster to ensure better fault tolerance. The fault tolerance >>>>>>>> depends >>>>>>>> on the probability of a node failure." >>>>>>>> >>>>>>>> I'm not getting it. If we have 4 nodes with 2 replicas it means >>>>>>>> that 3 of the nodes will have data of a given index (assuming 0 shards >>>>>>>> to >>>>>>>> ease the discussion), ritght? If those three nodes fail simultaneously >>>>>>>> the >>>>>>>> 4th will have no way of grabbing a copy and data will be lost forever. >>>>>>>> However if nr of replicas is 3, the 4th would be able to keep serving >>>>>>>> the >>>>>>>> requesrs and eventually handover a copy to a new node joining the >>>>>>>> cluster. >>>>>>>> How does this not help fault tolerance? I'm I missing something? >>>>>>>> >>>>>>>> Thanks, >>>>>>>> G. >>>>>>>> On 10 Jul 2014 00:21, "[email protected]" <[email protected]> >>>>>>>> wrote: >>>>>>>> >>>>>>>>> 1. You can set replica number at index creation time or by cluster >>>>>>>>> update settings action org.elasticsearch. >>>>>>>>> action.admin.cluster.settings.ClusterUpdateSettingsAction >>>>>>>>> >>>>>>>>> 2. You will get an index with lower replica number :) >>>>>>>>> >>>>>>>>> 3. Yes. Quick code example: >>>>>>>>> >>>>>>>>> ClusterState clusterState = clusterService.state(); >>>>>>>>> // find number of data nodes >>>>>>>>> int numberOfDataNodes = 0; >>>>>>>>> for (DiscoveryNode node : clusterState.getNodes()) { >>>>>>>>> if (node.isDataNode()) { >>>>>>>>> numberOfDataNodes++; >>>>>>>>> } >>>>>>>>> } >>>>>>>>> >>>>>>>>> 4. Yes. Use org.elasticsearch.cluster.ClusterStateListener >>>>>>>>> >>>>>>>>> From my view your idea of better fault tolerance does not make >>>>>>>>> much sense. The replica number is a statistical entity that is >>>>>>>>> related to >>>>>>>>> the probability of faults. The higher the replica, the higher the >>>>>>>>> probability of surviving faults. There is no correlation to the total >>>>>>>>> number of nodes in a cluster to ensure better fault tolerance. The >>>>>>>>> fault >>>>>>>>> tolerance depends on the probability of a node failure. >>>>>>>>> >>>>>>>>> From the viewpoint of balancing load, it makes much sense. When >>>>>>>>> setting replica number to the number of nodes, the cluster can balance >>>>>>>>> search requests to all nodes which is optimal. >>>>>>>>> >>>>>>>>> Jörg >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> On Wed, Jul 9, 2014 at 11:57 PM, <[email protected]> wrote: >>>>>>>>> >>>>>>>>>> Hi all, >>>>>>>>>> >>>>>>>>>> I'm considering using elasticsearch as a repository for a PoC I'm >>>>>>>>>> currently developing. >>>>>>>>>> >>>>>>>>>> This PoC models an application that needs durability but not >>>>>>>>>> isolability, so I'm fine with the eventual consistency of reads >>>>>>>>>> against the >>>>>>>>>> most recent writes. >>>>>>>>>> >>>>>>>>>> As durability is paramount (we can't affort to lose the data >>>>>>>>>> unless 100% of the nodes die) I've been exploring the option of >>>>>>>>>> setting >>>>>>>>>> every shard to have N replicas where N is the number of nodes in the >>>>>>>>>> cluster. >>>>>>>>>> >>>>>>>>>> From what I've read so far it is possible to dynamically set the >>>>>>>>>> number or replicas which triggers a replication throttled replication >>>>>>>>>> process. >>>>>>>>>> >>>>>>>>>> I would like to have some help on the following steps (I'm >>>>>>>>>> running ES in embedded mode in a Java application): >>>>>>>>>> >>>>>>>>>> 1 - How can I set the number or replicas using the native Java >>>>>>>>>> client ? >>>>>>>>>> 2 - What happens if a node dies and the number of replicas is >>>>>>>>>> lowered to the number of surviving ones? >>>>>>>>>> 3 - Is it possible, from a participating node, to access the list >>>>>>>>>> of nodes in the cluster so I can use their count to set the number of >>>>>>>>>> replicas (step 1) ? >>>>>>>>>> 4 - is it possible to hook a callback to the event of a node >>>>>>>>>> joining or leaving the cluster ? >>>>>>>>>> >>>>>>>>>> I envisioning the following mechanism: >>>>>>>>>> >>>>>>>>>> a) - Start with one node, a given number of shards and 1 replica >>>>>>>>>> b)- Each time a node joins I adjust the number or replicas to >>>>>>>>>> match the new node count. In this case, there would be 2 replicas >>>>>>>>>> c) - An arbitrary number of nodes might be added and I'd execute >>>>>>>>>> step b) accordingly >>>>>>>>>> d) - At any time a node might leave the cluster and thus I need >>>>>>>>>> to lower the number or replicas to the new node count (I assume that >>>>>>>>>> the >>>>>>>>>> cluster would go ahead and proceed to compensate the lost replica by >>>>>>>>>> asking >>>>>>>>>> an existing node to hold 2 replicas instead of one; is this stopped >>>>>>>>>> by >>>>>>>>>> lowering the number or replicas?) >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> The ultimate goal is to make sure no data is loss unless 100% of >>>>>>>>>> the nodes die before a new one can acquire a full replica. >>>>>>>>>> >>>>>>>>>> Is this doable? Does this make sense at all ? >>>>>>>>>> >>>>>>>>>> For the time being, I'm not worried about lack of disk space or >>>>>>>>>> bandwidth as I'm still in the very early days of the PoC. >>>>>>>>>> >>>>>>>>>> Thank you very much for all your work and help. >>>>>>>>>> >>>>>>>>>> Gonçalo >>>>>>>>>> >>>>>>>>>> -- >>>>>>>>>> You received this message because you are subscribed to the >>>>>>>>>> Google Groups "elasticsearch" group. >>>>>>>>>> To unsubscribe from this group and stop receiving emails from it, >>>>>>>>>> send an email to [email protected]. >>>>>>>>>> To view this discussion on the web visit >>>>>>>>>> https://groups.google.com/d/msgid/elasticsearch/276418fa- >>>>>>>>>> 812f-4af5-94a0-7362f5ba7931%40googlegroups.com >>>>>>>>>> <https://groups.google.com/d/msgid/elasticsearch/276418fa-812f-4af5-94a0-7362f5ba7931%40googlegroups.com?utm_medium=email&utm_source=footer> >>>>>>>>>> . >>>>>>>>>> For more options, visit https://groups.google.com/d/optout. >>>>>>>>>> >>>>>>>>> >>>>>>>>> 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