Concurrency Control in distributed system including pessimistic control (such as read-write lock) and optimistic control. Here we discuss the optimistic approaches:

Basic approach:

1. Check for serializability at commit.
2. If not, abort, and roll back updates it did.

This approach may causes other transaction to read dirty data, which may be aborted as well.

Multi-version approach

1. For each object, we maintain per-transaction version of it, called “tentative version”, and a final version of “committed version”.
2. “tentative version” has timestamp, sometimes we have timestamp for both read and write.
3. On read and write, find the correct version.

Vector Clock Approach

​ 1. For each object, maintain a vector clock that records the timestamp for which have modified it.

2. It detect whether new write is strictly newer than current value. If it is, then do the update, if not, create a **sibling value ** which can be solved by user or pre-defined strategy in application.
3. To prevent vector is too long (too many entries),use size-based pruning. To prevent timestamp is too old and unnecessary to maintain, use time-based pruning.