Design a Simple Task Management Service

Requirements

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.
5. Get a task by ID.

What to Deliver

• Data model (entities, fields, and indexes).
• Core APIs/classes (language-agnostic or your preferred language).
• Choose in-memory vs. persistent storage and justify.
• How you ensure thread safety and consistency.
• ID generation strategies (auto-increment, UUID, timestamp-based/Snowflake-like): trade-offs for ordering, collisions, and sharding.
• Performance considerations (time/space complexity, pagination, indexing).
• Brief testing strategy (functional, concurrency, and performance).

Assume this is a single service used by multiple threads/clients. You may make minimal assumptions explicit to keep the design coherent.