How to do distributed locking (2016)

This overcomplicates things...

* If you have something like what the article calls a fencing token, you don't need any locks.

* The token doesn't need to be monotonically increasing, just a passive unique value that both the client and storage have.

Let's call it a version token. It could be monotonically increasing, but a generated UUID, which is typically easier, would work too. (Technically, it could even be a hash of all the data in the store, though that's probably not practical.) The logic becomes:

(1) client retrieves the current version token from storage, along with any data it may want to modify. There's no external lock, though the storage needs to retrieve the data and version token atomically, ensuring the token is specifically for the version of the data retrieved.

(2) client sends the version token back along with any changes.

(3) Storage accepts the changes if the current token matches the one passed with the changes and creates a new version token (atomically, but still no external locks).

Now, you can introduce locks for other reasons (hopefully goods ones... they seem to be misused a lot). Just pointing out they are/should be independent of storage integrity in a distributed system.

(I don't even like the term lock, because they are temporary/unguaranteed. Lease or reservation might be a term that better conveys the meaning.)

This is known as 'optimistic locking'. But I wouldn't call it a distributed locking mechanism.

Optimistic locks are absolutely a distributed locking mechanism, in that they are for coordinating activity among distributed nodes - but they do require the storage node to have strong guarantees about serialization and atomicity of writes. That means it isn’t a distributed storage solution, but it is something you can build over the top of a distributed storage solution that has strong read after write guarantees.