optimistic locking is a useful technique that seems unfamiliar to many - it's worth looking into
Same goes for `git push --force`, always use `git push --force-with-lease` instead.
e.g. if the version column is an incrementing number, then it relies on no client unilaterally incrementing the value on failure and retrying--not much of a 'lock'.
Nobody would call CAS a lock. If your system only uses CAS, it should rightly be called “lock-free”.
In the spirit of trying to keep this complex subject free of misrepresentation... Compare-and-set (CAS, or alternatively compare-and-swap) is commonly used to implement lock-free algorithms.
A 'spin lock' where a thread uses only CAS in a tight loop and not carrying on until the lock is acquired is indeed a lock and not a kind of non-blocking or lock-free algorithm.
Performance and implementation characteristics differ but the end result is functionally the same as a pessimistic lock, which is why they're both called "locking" mechanisms.
The functional difference is crucial here, the non-lock characteristics are what allows them to offer atomic updates via HTTP. They couldn’t have used pessimistic locking to do this, not only because it is is limited to a single db transaction, but because they can not trust API users or the network to ensure the rows ever get unlocked.
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.
It is a lock in the sense that it allows success only.
Pesimistic lock usually means "maybe wait then success". In complex locking spaghetti it may mean deadlock. It may also mean wait then timeout. It may also mean wait then timeout then I don't know what actually happened, maybe success, maybe not.