Design Collaborative To-Do List System

Problem Statement

Design a collaborative to-do list system. Users can create lists, add tasks, share lists with collaborators, update tasks, and receive near real-time updates when collaborators make changes. Common operations include:

• Creating a new to-do list
• Adding a task to a list
• Updating a task (e.g., marking it as completed)
• Deleting a task
• Sharing a list with other users
• Receiving updates when other users modify a shared list

Constraints and Considerations

1. Scalability: The system should handle a large number of users and lists.
2. Real-time updates: Users should see changes made by collaborators as soon as possible.
3. Consistency: Ensure that all users see the same list state.
4. Security: Only authorized users should be able to access and modify lists.
5. Offline support: Users should be able to view and modify their lists offline, with changes synced when they reconnect.

Examples

1. Creating a list: A user creates a new to-do list titled "Groceries."
2. Adding tasks: The user adds tasks to the "Groceries" list: "Milk," "Eggs," "Bread."
3. Updating tasks: The user marks "Milk" as completed.
4. Sharing a list: The user shares the "Groceries" list with a collaborator.
5. Receiving updates: The collaborator adds "Butter" to the list, and the original user sees this change in real-time.

Hints

1. Database design: Consider how you would store lists, tasks, and user information.
2. API design: Design RESTful APIs for creating, reading, updating, and deleting lists and tasks.
3. Real-time updates: Use WebSockets or long polling to push updates to clients.
4. Caching: Implement caching to reduce database load and improve performance.
5. Conflict resolution: Design a strategy for handling conflicts when multiple users modify the same task simultaneously.

Solution (High-Level)

1. Database: Use a relational database with tables for Users, Lists, Tasks, and List_Users (for sharing). Each task has a unique ID, list ID, and user ID for updates.
2. APIs: Create RESTful APIs for CRUD operations on lists and tasks. Use authentication (e.g., OAuth) to secure endpoints.
3. Real-time updates: Implement WebSocket connections between clients and servers. When a task is updated, broadcast the change to all connected clients.
4. Caching: Use a caching layer (e.g., Redis) to store frequently accessed data, reducing database reads.
5. Conflict resolution: Use optimistic concurrency control by including a version number with each task. If a task's version doesn't match the server's, reject the update and prompt the user to refresh.

NOT_FOUND

After conducting a thorough search across various platforms including Reddit, 1point3acres, PracHub, Glassdoor, Blind, GitHub, and interview prep sites, no additional substantive text or complete problem statement beyond the provided excerpt was found. The information available is limited to the excerpt given, and no further details or examples are available online.