During a networking partition, cluster states updates (like mapping changes or shard assignments)
are committed if a majority of the masters node received the update correctly. This means that the current master has access to enough nodes in the cluster to continue to operate correctly. When the network partition heals, the isolated nodes catch up with the current state and get the changes they couldn't receive before. However, if a second partition happens while the cluster
is still recovering from the previous one *and* the old master is put in the minority side, it may be that a new master is elected which did not yet catch up. If that happens, cluster state updates can be lost.
This commit fixed 95% of this rare problem by adding the current cluster state version to `PingResponse` and use them when deciding which master to join (and thus casting the node's vote).
Note: this doesn't fully mitigate the problem as a cluster state update which is issued concurrently with a network partition can be lost if the partition prevents the commit message (part of the two phased commit of cluster state updates) from reaching any single node in the majority side *and* the partition does allow for the master to acknowledge the change. We are working on a more comprehensive fix but that requires considerate work and is targeted at 6.0.
We have long worked to capture different partitioning scenarios in our testing infra. This PR adds a new variant, inspired by the Jepsen blogs, which was forgotten far - namely a partition where one node can still see and be seen by all other nodes. It also updates the resiliency page to better reflect all the work that was done in this area.
During master election each node pings in order to discover other nodes and validate the liveness of existing nodes. Based on this information the node either discovers an existing master or, if enough nodes are found (based on `discovery.zen.minimum_master_nodes>>) a new master will be elected.
Currently, the node that is elected as master will currently update it the cluster state to indicate the result of the election. Other nodes will submit a join request to the newly elected master node. Instead of immediately processing the election result, the elected master
node should wait for the incoming joins from other nodes, thus validating the elections result is properly applied. As soon as enough nodes have sent their joins request (based on the `minimum_master_nodes` settings) the cluster state is modified.
Note that if `minimum_master_nodes` is not set, this change has no effect.
Closes#12161
Boaz Leskes
Ali Beyad
Yannick Welsch
Clinton Gormley
Lee Hinman
Jason Tedor
debadair
Jose Diaz-Gonzalez
Andrzej Wisłowski
Alex Chan
Pascal Borreli
Martijn van Groningen