DevOps Classroomnotes 23/Feb/2022

Core Concepts of ElasticSearch

  • Indexes
  • Types
  • Documents
  • Clusters
  • Nodes
  • Shards and replicas
  • Mappings and types
  • Inverted Indexes


  • An index is container that stores and manages documents of single type in ElasticSearch.


  • Typically documents with mostly common sets of fields are grouped under one type.


  • Json Documents are first class citizens of elastic search.
  • A document conists of multiple fields and is basic unit of information stored in elastic search
  • In addition to the fields that are sent by the user in the document, Elastic search maintains internal metafields. The fields are
  • _id: unique identifier of the document within a type
  • _type: This field contains type of the document
  • _index: this field contains the index name of the document

Lets Experiment with Indexes, types and Documents


  • An Elastic search node is a single server of Elastic Search which may be part of large cluster of nodes.
  • Every ElasticSearch node is assigned an unique ID and name when start.


  • A cluster hosts one or more indexes and is responsible for providing operations such as indexing, searching and aggregations.
  • A cluster is formed by one or more nodes.
  • Elastic search node is always part of cluster

Shards and replicas

  • An index contains documents of one type.
  • Shards help in dividing the documents of single index over multiple nodes.
  • The process of dividing the data among shards is called as sharding.
  • This is built in process of elastic search
  • By default, every index is configured to have five shards in ElasticSearch.
  • At the time of creating the index, we can specify the number of shards.
  • Once the index is created the shards cannot be modified.
  • Consider the below example where we have three nodes and number of shards are five
  • Now if one node goes down, as of now as mentioned below we loose some data (till the node is up)
  • Distributed Systems such as Elastic Search are expected to run inspite of failure. This issue is addressed by replica shards or replaces.
  • Each shard in an index can be configured to have zero or more replica shards.
  • Lets understand this by one replica shard
  • Even if one node goes down we still have the data from replica shards serving the clients of elastic search

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