CODEWITHSUNDEEP
elasticsearch
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Reputation: 4528

elasticsearch 7.X cluster with specified master node

I have 3 elastic node , How can I cluster there three nodes with always same master node , I didn't find any good docs about new elastic 7 way of specify discovery and master node:

discovery.seed_hosts: [ ]

cluster.initial_master_nodes: []

for example I have node a, b, c and I want node a to be master what what should be discovery.seed_hosts and cluster.initial_master_nodes for master node and child nodes

UPDATE

with using Daniel answer , and checking ports are open and node have same cluster name , other nodes didn't join cluster, is there any additional config needed?

UPDATE 2

looks like nodes found each other but for some reason can't choose master node with election:

master not discovered or elected yet, an election requires 2 nodes with ids [wOZEfOs9TvqGWIHHcKXtkQ, Cs0xaF-BSBGMGB8a-swznA]

Solution

Deleting folder data of all nodes start a node and then add other nodes with first node (as master) as seed host.

Upvotes: 2

Views: 14497

Answers (1)

Daniel Schneiter
Daniel Schneiter

Reputation: 1996

Elasticsearch allows you to specify the role of a node. A node (an instance of Elasticsearch) can serve as a coordinating node, master node, voting_only node, data node, ingest node or machine learning node.

With respect to master nodes you can only configure which nodes potentially can become the (active) master, but you cannot specify which one of the so-called master-eligible nodes will be the active master node.

The only exception to this is when you only configure one master-eligible node, then obviously only this one can become the active master. But be aware that in order to get true high availability you need to have at least 3 master-eligible nodes (this ensures that your cluster will still be 100% operational even when losing one of the master-eligible nodes).

Therefore Elastic always recommends to configure 3 or 5 nodes in your cluster as master-eligible nodes. You can configure that role via the node.master property in the Elasticsearch.yml-file. Setting it to true (default) allows that node to become master, while false will ensure that this node never ever will become master and also will not participate in the master election.

Over the life-time of your cluster (master-eligible) nodes might get added and removed. Elasticsearch automatically manages your cluster and the master node election process with the ultimate goal to prevent a split brain scenario from happening, meaning you eventually end up having 2 clusters which go by the same name but with independent master nodes. To prevent that from happening when starting up your cluster for the very first time (bootstrapping your cluster) Elastic requires you to configure the cluster.initial_master_nodes property with the names of the nodes that initially will serve as master-eligible nodes. This property only needs to be configured on nodes that are master-eligible and the setting will only be considered for the very first startup of your cluster. As values you put in the names as configured with the node.name property of your master-eligible nodes.

The discovery.seed_hosts property supports the discovery process which is all about enabling a new node to establish communication with an already existing cluster and eventually joining it when the cluster.name matches. You are supposed to configure it with an array of host names (not node names!) on which you expect other instances of Elasticsearch belonging to the same cluster to be running. You don't need to add all 100 host names of the 100 nodes you may have in your cluster. It's sufficient to list host names of the most stable node names there. As master (eligible) nodes are supposed to be very stable nodes, Elastic recommends to put the host of all master-eligible nodes (typically 3) in there. Whenever you start/restart a node, it goes through this discovery process.

Conclusion

With a cluster made up of 3 nodes you would configure all of them as master-eligible nodes and list the 3 node names in the cluster.initial_master_nodes setting. And you would put all the 3 host names also in the discovery.seed_hosts setting to support the discovery process.

Useful information from the Elasticsearch reference:

Upvotes: 11

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