Implement a Thread-Safe Per-Key Ordered Executor

Implement an in-process component constructed with `maxConcurrent` and an existing thread-pool executor. It exposes `submit(key, task)` and must preserve task order within each key while allowing independent keys to execute concurrently. ### Constraints & Assumptions - Multiple threads may call `submit` concurrently. Define the linearization point that establishes order for concurrent submissions sharing a key. - Tasks with one key must execute in submission order and must never overlap. - Tasks with different keys may run concurrently, with at most `maxConcurrent` tasks running globally. - `submit` returns after enqueueing/scheduling; it does not wait for the task to finish. - Do not implement the underlying thread pool. For the core algorithm, assume it accepts scheduled work while this component is open and that tasks eventually return or throw. - A long task for one key must not hold a lock or occupy extra waiting workers that unnecessarily block other keys when capacity is available. ### Clarifying Questions to Ask - Should queue capacity be bounded, and how should overload be reported? - What should task exceptions do to later tasks for the same key? - Is shutdown or executor rejection part of the required interface? ```hint Schedule ready keys, not blocked tasks Only the next eligible task for a key should consume a global execution slot. Later tasks for that key can remain in its own queue. ``` ### What a Strong Answer Covers - A per-key FIFO queue, an eligibility state, and a global concurrency count. - Thread-safe submission and completion transitions with a clear ordering definition. - Dispatch without scanning all other key queues or executing user work under the scheduler lock. - Slot release and continued progress after task exceptions. - Cleanup of idle keys, fairness considerations, and explicit lifecycle assumptions. ### Follow-up Questions - How would simultaneous submissions for the same key be ordered? - How would you add bounded queues and shutdown without losing accepted tasks?