How do key/value stores manage concurrency?

Question

I have read a number of articles stating key value stores only require two operations:

• set(key, value)
• get(key)

This is fine for a single process, but when you have multiple processes, how does the key value store manage concurrency? I would have thought a version number (E.g., an unsigned integer) used for compare-and-swap style concurrency would be required. E.g., the two operations would be:

• set(key, value, version), where version is the condition - a mismatch causes a concurrency error and a successful match causes an increment.
• get(key) (returning both the value and the version).

Answer 1

There is two kind of design. Some are using locks and other are using MultiVersion Concurrency Control.

MVCC achieve concurrency without locks. It can be summed up as:

• In a single read, the database return the latest version of the data
• In a single write, the databae add a new version for the data
• In the case of read/write concurrent request, the read get the most recent version of the data, that is before the current write.
• In the case of write/write concurrent request, I think that one of the write is abandonned and replayed later.