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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 based on version checking for updates. In OCC, I read about transactions that acquire no locks for reading access, only for the later update which will fail if in between the version was incremented and version checking fails. In this case the transaction will be rolled back. In MVCC, it is basically the same, or not? Where is the difference?
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.
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