*MongoDB 3.0*
*WiredTiger storage engine* introduced *MongoDB 3.2*
WiredTiger the default, introduced *read concerns* *MongoDB 3.6*
introduced *logical sessions*, the underlying framework for causal consistency and retryable writes *MongoDB 4.0*
introduced *multi-document transactions* for replica sets *MongoDB 4.2*
will add transaction support for sharded clusters Transactions were the long-term solution to support retryability for multi-document writes