One of the major new features introduced in MongoDB version 3 was the integration of the WiredTiger storage engine. Prior to December 2014, WiredTiger Inc. was a company that specialized in database storage engine technology. Its impressive list of customers included Amazon Inc. In December 2014, WiredTiger was acquired by MongoDB after partnering with them on multiple projects.
In MongoDB version 3, multidocument transactions were not supported. Furthermore, in the replication process (https://docs.mongodb.com/manual/replication/#replication), changes accepted by the primary server in a replica set were pushed out to the secondary servers in the set, which were largely controlled through oplogs (https://docs.mongodb.com/manual/core/replica-set-oplog/#replica-set-oplog) and programming logic outside of the storage engine. Simply stated, the oplog represents changes made to the database. When a secondary synchronizes with a primary,...
