ElasticSearch works in the cluster.
Is there a quorum concept? If the replication factor is 2 or 3 and one
replication fails, is the content still available?
Is there a param to read from _all_replications and read from
_one_replication to maintain the consitency?
Will the replication be datacenter awared in the future?
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ElaticSearch clustering module is different than cassandra and other dynamo based systems. You don't have to read from all/quorum replicas in order to maintain consistency. When something is indexed, it is indexed on a primary shard and then replicated to all its replicas synchronously (if you have more than one replica, it is done in parallel using async io).
Note, that elasticsearch provides near real time for indexed documents to be visible when searching/getting them. But, if (when) it provides real time support, then once you indexed, it will be provided immediately and no need for quorum based reads.
When doing a read / search, one of the replicas is chosen and the reads is done from it. Search is never done on more than one replica of the same shard. This gives you better performance and better read/search scalability just by increasing the number of replicas.