Do you define locks as things that guarantee data integrity? Is not offering concurrency at all locking? Is your point that optimistic is locking or that it bears some surface similarities to locking? I accept that surface similarities exist. But that doesn’t really help anyone understand the difference. Nobody is struggling to see that these two solutions are applicable to some of the same problems.
Locks don’t directly guarantee data integrity. They guarantee mutual exclusion around a critical section or mutable access to data they logically contain. Mutual exclusion is a coarse-grained way to get actually serialized execution of concurrent accesses. That gives you the data integrity. Mutual exclusion also gives you all the problems when you send an acquired lock over a network. It also explains why parallelism on the guarded section/data is 1. This can be blocky in a read heavy environment, because each read blocks all the others. Two transactions can get into deadlock by acquiring in a particular order. These are characteristics of all locks.
Optimistic locking (also known as optimistic concurrency control, because it’s not locking) does not provide mutual exclusion, in any way, shape, or form. You cannot acquire any kind of lock with it. There aren’t any. A lock has a pair of operations, one to begin mutual exclusion and one to end it, but OCC doesn’t have either. Because it’s not a lock. It therefore does not suffer from network partitions between the lock and the client, does not shove all readers through a parallelism=1 bottleneck, you do not have to worry about deadlock prevention or avoidance. Those problems simply do not exist for OCC. It is literally lock-free. It almost couldn’t be ANY further from being a lock. You don’t have to accept that retries being mandatory in the API makes it not a lock. You just have to observe that it isn’t a lock.
Some things are locks, some things aren’t. I think we should avoid calling things that aren’t locks locks, because it’s confusing enough as it is. Frankly, I think if people hadn’t been calling OCC “optimistic locking” then this blog post would have been easier to write. Similarly, as I illustrated above, calling it “lock-free” helps people understand OCC’s advantages and challenges in the same terms we use to talk about lock-free algorithms on a multi-core CPU. It gives you the intuition that every client side call should be a loop. It gives you the intuition that overflow ≈ the ABA problem. All of these bits of understanding flow from calling things what they are.
