But where this kind of concurrency without corresponding parallelism make sense? There's no point in trying to do thousands of concurrent reads if there's no a

But where this kind of concurrency without corresponding parallelism make sense? There's no point in trying to do thousands of concurrent reads if there's no available parallelism on the same scale, especially keeping transaction / isolation level snapshots open, it's just an overloaded system. Then you want to do backpressure with a maybe some queuing.