Optimistic vs Multi Version Concurrency Control - Differences?

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?

Solution

To directly reply to the question, multi version concurrency control (MVCC) is a concurrency control method, (typically) belonging in the category of optimistic concurrency control (OCC).

There are 2 main concurrency control approaches:

Pessimistic Concurrency Control: this approach assumes that conflicting operations happen more frequently (that's why it's called pessimistic). Since the conflicts are common, this approach makes use of locks to prevent conflicting operations from executing, assuming that there is no significant overhead from their usage.
Optimistic Concurrency Control: this approach assumes that conflicting operations are rare and they do not happen so frequently. Under this assumptions, the locks would impose significant & not needed overhead to the performance. For this reason, this approach generally avoids locking and attempts to execute the operations, checking (at the commit of each transaction), whether there has been a conflict with another transaction during its operations. If there was any conflict, this approach proceeds with aborting the transactions that had conflicting operations.

One widely known algorithm of pessimistic concurrency control is the 2-phase locking.

Two widely known algorithms of optimistic concurrency control are:

The main difference between these 2 algorithms is the following. The timestamp-based algorithm assigns a single (more correctly one for each kind of operation, read & write) timestamp to each object, denoting the last transaction that accessed it. So, each transaction checks during the operation, if it conflicts with the last transaction that accessed the object. The multi-version approach maintains multiple versions of each object, each one corresponding to a transaction. As a result, the multi-version approach manages to have fewer aborts than the first approach, since a potentially conflicting transaction can write a new version, instead of aborting in some cases. However, this is achieved at the cost of more storage required for all the versions.

Strictly speaking, MVCC is concerned mostly with how data are stored, i.e. the fact that there can be multiple physical versions for each data item. As a result, it is theoretically possible to combine it with pessimistic methods as well(e.g. locking), but its multi-version nature is best combined with optimistic methods.