Elastic search

Useful links

Intro about internals (С чего начинается Elasticsearch)

This is a document oriented storage with Lucene as an index.

Each shard is a Lucene index. How to control number of shards in index:

PUT _template/all
{
  "template": "*",
      "settings": {
        "number_of_shards": 1
      }
}

• data node

  • hot (SDD is better)

  • warm (HDD is enough)

  • cold (HDD is enough)

• coordinating node

• master node

  • active master may be only one

  • master manages a topology of the cluster

    • create new index

    • extract shards

    • move shards and join if necessary

  • knows all about cluster state

  • node.master: true

Each ElasticSearch instance is a node. To join nodes in cluster:

• Nodes need to have same version

• cluster.name should be equal

How to control number of replicas

PUT / _settings {
  "index": {
      "number_of_replicas": someVal
  }
}

Deletion of data from node

First deletion happens only in primary shard. And after flush and commit in primary shard => internal request happens for changing replicas.

Cluster health status

• green - all good

• yellow - there are lost shards. Cluster is fully operating, but uses replicas

• red - there are lost shards. Cluster is broken or part of the data is not available

num(data nodes) >= num(replicas)

"Replica" is applicable for shards.

Fault tolerance

Split-brain problem.

КОЛИЧЕСТВОКАНДИДАТОВ = ОБЩЕЕКОЛИЧЕСТВО_НОД/2 + 1