ElasticSearch: Unassigned Shards, how to fix?

Question

I have an ES cluster with 4 nodes:

number_of_replicas: 1
search01 - master: false, data: false
search02 - master: true, data: true
search03 - master: false,          


        

Answer 1

Another possible reason for unassigned shards is that your cluster is running more than one version of the Elasticsearch binary.

shard replication from the more recent version to the previous versions will not work

This can be a root cause for unassigned shards.

Elastic Documentation - Rolling Upgrade Process