What is MVCC (Multi-version concurrency control)?

A method used to achieve both performant concurrent access and strict isolation level. The idea is to give each transaction a unique versioned view of the data, thus each transaction can operate on their own. Only at the commit phase, the database system will perform the resolution, the conflict might arise at this point and may cause the transaction to abort. However, the assumption is such scenario is less likely to happen and that's why MVCC is referred as optimistic concurrency control. A classic saying for MVCC is 'Reads do not block Writes, writes do not block Reads'.