We want answer the question: How a node is registered in a cluster?, but first we have to have some foundation to get to that. Let’s begin.
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What is ElasticSearch? ElasticSearch allows you to store, search, and analyze huge volumes of data quickly and in near real-time and give back answers in milliseconds. It’s able to achieve fast search responses because instead of searching the text directly, it searches an index.
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What is a node? A node is a running instance of ElasticSearch which belongs to a cluster. Usually a node represents a single server. There are several type of node roles, and a node can have multiple roles at the same time. The most important ones are:
- Master Node: Controls the cluster
- Data Node: Holds data and performs CRUD operations
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What is a cluster? A collection of connected nodes.
Right now we know that our nodes are the responsible of handling our data. And that clusters are a collection of interconnected Nodes. But we are still missing the main question.
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What happens when a node is created? When a node is created it has to be assigned to a cluster. At startup, a node will use unicast to discover an existing cluster with the same cluster name and will try to join that cluster.
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Remember master nodes? Each cluster has a single master node which is chosen automatically by the cluster and can be replaced if it fails.
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Node join request. During master election or when joining one existing formed cluster, a node sends a join request to the master in order to be officially added to the cluster. This is handled in the java class
MasterNodeReadRequest.java. -
Cluster State Once a node has joined a cluster it’s registered in the cluster state, which is the metadata that represents the state of the whole cluster. It includes:
- the set of nodes in the cluster
- all cluster-level settings
- information about the indices in the cluster, including their mappings and settings
- the locations of all the shards in the cluster.
All this information can be found in the ClusterState.java file.