Durability in relational database systems refers to the property that every transaction that has successfully committed will survive in the face of failure. This usually refers to writing the contents of the committed transaction in persistent storage (such as a hard disk or SDD). RDBMSes are always following the durability concept by writing every committed transaction to a transaction log or write-ahead log (WAL). MongoDB, using the WiredTiger storage engine, is committing writes using WAL to its persistent storage-based journal every 60 msec and is, for all practical purposes, durable. As durability is important, every database system prefers relaxing other aspects of ACID first, and durability usually gets relaxed last.