CAP Theorem and Beyond

Pick two out of three

Consistency - all nodes see all the data at the same time
Availability - any node can execute read/write operations
Partition tolerance - cluster continues to operate despite node(s) failure

Realistic choices

CP - Consistency/Partition Tolerance: wait for a response from the partitioned node which could result in a timeout error
AP - Availability/Partition Tolerance: get the most recent version of the data, possibly stale
ElasticSearch is a search engine: consistency is at the document level