Hi, is

Semaphore

meant to be at application layer? My case it to enforce only 1 caller of synchronization at a time

If you only want one caller at a time and you know it's not going to change to another number, you want to use

Mutex

instead.

right, it will work, but im confused if there should be such api used in between my business logic which syncing basically is

There's multiple differences. Semaphore can allow a given number of acquirers that is greater than one for example. The docs are good at explaining each of these.

I know what those two do, I only meant them as a coroutine api, vs their thread equivalent

Blocking threads is more wasteful, and that's the only thing the non coroutines-friendly version can do.