NOTE
Hystrix Thread Isolation (Historical)
Thread/semaphore bulkhead isolation, bounded queues, timeout behavior, and modern resilience interpretation.
This is a historical learning note and may contain outdated or incomplete understanding.
Hystrix historically isolated dependency calls with dedicated thread pools or semaphore limits. The core pattern is a bulkhead: one dependency should not consume all request-processing capacity when it slows down.
Thread-pool isolation adds scheduling/queue overhead but can separate blocking capacity. Semaphore isolation is cheaper but does not move work off the caller thread.
Whichever implementation is used, limits must be bounded and coordinated with deadlines/downstream capacity. Replacing one unbounded queue with another does not create resilience.