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What Does it Mean?
There is some confusion in the use of coordinating node terminology. Client nodes were removed from Elasticsearch after version 2.4 and became Coordinating Nodes. At the same time a new node type, Ingest Node, also appeared. Many clusters do not use dedicated coordinating or ingest nodes, and leave the ingest and coordination functions to the data nodes.
A coordinating (or client) node is a node which has:
node.master: false node.data: false node.ingest: false
This means that the only functions it can carry out are those related to relaying queries to other data nodes, acting as a load balancer. This configuration is only recommended for large clusters (usually 20 nodes or more).
If you have coordinating nodes, then search requests should be load-balanced across the coordinating nodes rather than sending directly to the data nodes.
Not to be confused with coordinating (or client) nodes, an ingest node is a specific type of node used for pre-processing documents in ingest pipelines. If you intensively use ingest pipelines for processing documents (eg. log parsing or pdf document processing) then it may make sense to use this type of node on medium to large clusters (10 nodes or more).
An ingest node is created by setting:
node.master: false node.data: false node.ingest: true
All indexing requests that require ingest pipelines should be load balanced across the ingest nodes. Search requests must not be sent to ingest nodes where node.data is false.