Design a simple task management system that supports: ( 1) Add a new task with a unique ID and description, ( 2) Mark a task as completed by ID, ( 3) Get all incomplete tasks, ( 4) Get all completed tasks, and ( 5) Get a task by ID. Specify the data model and core APIs/classes, choose in-memory vs. persistent storage, and describe how you'd ensure thread safety and consistency. Discuss ID generation strategies (auto-increment, UUID, or timestamp plus attributes hash) and their trade-offs (ordering, collisions, sharding). Outline performance considerations and a brief testing strategy.