Hi Ramith, +1. This minimum number of nodes is what you called a Quorum_ https://en.wikipedia.org/wiki/Quorum_(distributed_computing).
We can support a 2 nodes mode where we do not support partition tolerance. However, IMO that is a bad idea as most users would not understand/ look at the details and run in to trouble. --Srinath On Mon, May 16, 2016 at 1:41 PM, Ramith Jayasinghe <ram...@wso2.com> wrote: > Hi All, > > Global state of the MB cluster becomes inconsistent, when the network > becomes partitioned (split brains) in previous MB version(s). So as a > solution we propose following, > 1) a MB cluster cannot go below a defined number ( a.k.a: minimum cluster > size) > 2) During a network partition if node count (/size) of the particular > partition is less than 'minimum cluster size' then that partition(s) > 2.1) will stop accepting incoming traffic/connections > 2.2) disconnect all active connections ( > publishers/subscribers) > > So idea is to let only a single partition ( which has the cluster size >= > minimum cluster size) keep working while other(s) stop working. > Therefore, choosing the number 'minimum cluster size' is important when > deploying MB. > otherwise user will have multiple network partitions ( where size >= > minimum cluster size) working in parallel creating the problem we are > trying to solve here. > > So here's the way to pick the number: > > | Cluster size | Minimum Node Count | > |-------------------|--------------------| > | 2 | 2 | > | 3 | 2 | > | 4 | 3 | > | 5 | 3 | > | N | (N / 2) + 1 | > > > So this will have a direct effect on minimum HAed deployment for MB which > used to 2. > why? > suppose, users now deploy 2 node MB cluster with this feature enabled. > then during a network partition both nodes will stop working. this may be > fine since it will make MB cluster reliable but in users point of view its > a complete outage (since none of the nodes except traffic). > > Therefore now minimum HAed node count for MB become 3. > When cluster size is 3, it will be able to withstand 1 node being in a > network partition (and other 2 nodes will work). > > thoughts? > > > Jira: https://wso2.org/jira/browse/MB-1664 > > -- > Ramith Jayasinghe > Technical Lead > WSO2 Inc., http://wso2.com > lean.enterprise.middleware > > E: ram...@wso2.com > P: +94 772534930 > -- ============================ Blog: http://srinathsview.blogspot.com twitter:@srinath_perera Site: http://home.apache.org/~hemapani/ Photos: http://www.flickr.com/photos/hemapani/ Phone: 0772360902
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