Design a Multi-Threaded-Task-Scheduler

Requirements

1. User should be able to schedule ad-hoc events
2. User should be able to schedule a recurring event with fixed rate
3. User should be able to schedule a recurring event with some delay rate

Extended Requirements

1. start time, end time for job scheduler

Non functional requirements

1. Async
2. Concurrent
3. Multithreading

Proposed Solutions

I will be using Java as a preferred language

Entities

1. ScheduledTask - What I want to actually run
2. TaskCreatorService -> factory class to create ScheduledTask.
3. SchedulerExecutor - this is where i'll trigger all APIs
4. SchedulerService - Can I delegate the actual business logic here
5. TaskType - {RUN_ONCE, RECUR, RECUR_WITH_WAIT}
6. Queue - PriorityQueue -> push the job and poll from it

Design patterns

1. Singleton - SchedulerService

Data structure and algorithms

1. PriorityQueue

Multithreading

1. Executors
2. Concurrency
3. Synchronization
4. Locks
5. Conditions (ThreadPoolExecutor)

Questions

1. Deadlock ?
2. How to track error i. while scheduling not much to fail ii. While execution, yes it can -- logging and monitoring