Uber | Goldman Sachs | Rubrik LLD Interview Question : Design Scheduled Executor

The interview question required designing a custom ScheduledThreadPoolExecutor or ScheduledExecutorService from scratch. This is a classic concurrency problem commonly seen in interviews for senior engineering roles.

Key Concepts:

• Thread pool with worker threads
• DelayQueue to hold tasks ordered by next execution time
• Delayed interface for time‑based ordering
• Scheduling strategies:
• schedule after initial delay
• schedule at fixed‑rate
• schedule at fixed‑delay
• Thread safety and rescheduling logic

Solution Overview:

1. Use a DelayQueue to store ScheduledTask objects (implements Runnable + Delayed).
2. Worker threads loop, calling take() – this blocks until a task’s delay expires.
3. Once executed, if the task is periodic, update its next trigger time and put it back into the queue.
4. For fixed‑rate, next trigger = previous trigger + period;
5. for fixed‑delay, next trigger = current time + delay.
6. Shutdown interrupts workers and clears the queue.