Useful or not, from you.
reactive-streams-jvm Subscription.cancel MUST BE thread-safe

Rule 3.5

Subscription.cancel MUST respect the responsivity of its caller by returning in a timely manner, MUST be idempotent and MUST be thread-safe.

mentions thread-safety and also links the keyword to it's definition in this context, which mentions "async".

Can be safely invoked synchronously, or asychronously, without requiring external synchronization to ensure program correctness.

The rule seems to contradict other rules, like:

  • 2.7

A Subscriber MUST ensure that all calls on its Subscription's request and cancel methods are performed serially.

  • 3.1

Subscription.request and Subscription.cancel MUST only be called inside of its Subscriber context.

And lastly the "MUST be thread-safe" doesn't equally apply to 'Subscription.request'. Is this obsolete, or is my understanding wrong?

That's a useful answer
Without any help

The definition of serially should clear things up.

Serial(ly) In the context of a Signal, non-overlapping. In the context of the JVM, calls to methods on an object are serial if and only if there is a happens-before relationship between those calls (implying also that the calls do not overlap). When the calls are performed asynchronously, coordination to establish the happens-before relationship is to be implemented using techniques such as, but not limited to, atomics, monitors, or locks.

In practice, implementations of the Subscription methods end up thread-safe anyway.