Optimistic vs Multi Version Concurrency Control - Differences?

Question

I am trying to find out, what the difference between optimistic concurrency control (OCC) and multi version concurrency control (MVCC) is?

So far I know that both is bas

Answer 1

I think they are sometimes used interchangeably, and if the transaction only involves one object then they are essentially the same, but MVCC is an extension of optimistic concurrency (or a version of it) that provides guarantees when more than one object is involved. Say that you have two objects, A and B, which must maintain some invariant between them, e.g. they are two numbers whose sum is constant. Now, a transaction T1 subtracts 10 from A and adds it to B, while, concurrently, another transaction T2 is reading the two numbers. Even if you optimistically update A and B independently (CAS them), T2 could get an inconsistent view of the two numbers (say, if it reads A before it's modified but reads B after it's been modified). MVCC would ensure T2 reads a consistent view of A and B by possibly returning their old values, i.e., it must save the old versions.

To sum up, optimistic locking (or optimistic concurrency control), is a general principle for synchronization w/o locks. MVCC is an optimistic technique which allows isolated transactions which span multiple objects.