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:
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
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
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