Node roles
As we know, an ES cluster consists of one or multiple nodes. Nodes typically contain shards, but this isn't always the case.
This is because nodes can be assigned different roles that do different things. The roles are:
- Master-eligible: A node may be elected as the master node. A master node do cluster wide actions such as shard allocation, creating and deleting indices etc. This role doesn't mean the cluster will be the master, just that it is eligible. There is a voting process for this.
- Data: The data role means the node stores data and performs queries related to the data on that node.
- Ingest: The ingest role enables a node to run ingest pipelines (like adding documents to indices). You can think of this as a simplified Logstack pipeline, but for full functionality, Logstash is better. Ingest roles are best for relatively simple operations.
- Machine learning: The ML role role allows the node to run jobs and machine learning API requests that don't affect other tasks.
- Coordination: The coordination role coordinates queries across the cluster. It doesn't run queries, just delegates the task to other nodes. Note that every node is a coordination node, however for a node to be a dedicated cooordination node, it has to have an empty role list []. There is a section on that here, this is typically best for large clusters that need some load balancing.
- Voting only: Not that important, but read more here
To see node roles:
GET /_cat/nodes?v
When to change node roles
- When optimising the cluster to scale requests
- Often something done for large clusters
Typically, changing nodes, shards and replicas is something you'd do before resorting to changing node roles
Never change roles unless you are sure what you're doing!