Fault tolerance for Elastic Cloud Enterprise is based around the concept of availability zones.
An availability zone contains resources available to an Elastic Cloud Enterprise installation that are isolated from other availability zones to safeguard against potential failure.
Planning for a fault-tolerant installation with multiple availability zones means avoiding any single point of failure that could bring down Elastic Cloud Enterprise.
The main difference between Elastic Cloud Enterprise installations that include two or three availability zones is that three availability zones enable Elastic Cloud Enterprise to create clusters with a tiebreaker. If you have only two availability zones in total in your installation, no tiebreaker is created.
Tiebreakers are used in distributed clusters to avoid cases of split brain, where a cluster splits into multiple, autonomous parts that continue to handle requests independently of each other, at the risk of affecting cluster consistency and data loss. A split-brain scenario is avoided by making sure that a minimum number of master-eligible nodes must be present in order for any part of the cluster to elect a master node and accept user requests. To prevent multiple parts of a cluster from being eligible, there must be a quorum-based majority of (n/2)+1 nodes, where n is the number of nodes in the cluster. The minimum number of master nodes to reach quorum in a two-node cluster is the same as for a three-node cluster: two nodes must be available.
When you create a cluster with nodes in two availability zones when a third zone is available, Elastic Cloud Enterprise can create a tiebreaker in the third availability zone to help establish quorum in case of loss of an availability zone. The extra tiebreaker node that helps to provide quorum does not have to be a full-fledged and expensive node, as it does not hold data. For example: By tagging allocators hosts in Elastic Cloud Enterprise, can you create a cluster with eight nodes each in zones
ece-1b, for a total of 16 nodes, and one tiebreaker node in zone
ece-1c. This cluster can lose any of the three availability zones whilst maintaining quorum, which means that the cluster can continue to process user requests, provided that there is sufficient capacity available when an availability zone goes down.
By default, each node in an Elasticsearch cluster is a master-eligible node and a data node. In larger clusters, such as production clusters, it’s a good practice to split the roles, so that master nodes are not handling search or indexing work. When you create a cluster, you can specify to use dedicated master-eligible nodes, one per availability zone.