Requirements

Functional Requirements

• Upload and download files (up to 5 TB)
• Real-time sync across multiple devices
• File versioning and history
• Conflict resolution for concurrent edits
• Sharing with permissions (view, comment, edit)
• Folder hierarchy and organization
• Offline editing with sync on reconnect

Non-Functional Requirements

• Latency: Sync changes within 5 seconds
• Durability: 99.999999999% (11 nines)
• Availability: 99.99%
• Scale: 1 billion files, 500M active users
• Bandwidth: Efficient delta sync for large files

Back-of-the-Envelope Estimation

High-Level Architecture

Detailed Design

File Chunking and Block Storage

Delta Sync

Conflict Resolution

For binary files (PDF, images):

1. User A edits file offline
2. User B edits file online
3. User A reconnects and syncs
4. System detects conflict (version mismatch)
5. Creates "conflict copy" for User A
6. User B's version becomes the current version
7. User manually merges if needed

File Versioning

File versions:
v1: block_1, block_2, block_3 (created 2026-06-18)
v2: block_1, block_4, block_3 (created 2026-06-19, block_2 changed to block_4)
v3: block_5, block_4, block_3 (created 2026-06-20, block_1 changed to block_5)

Sharing and Permissions

File ACL:
{
  "file_id": "file_123",
  "owner": "user_a@example.com",
  "permissions": [
    { "user": "user_b@example.com", "role": "editor" },
    { "user": "user_c@example.com", "role": "viewer" },
    { "group": "team@example.com", "role": "commenter" }
  ]
}

Practice Exercises

1. Design: How would you implement real-time collaborative editing for Google Docs (not just binary files)? Design the Operational Transformation system.

2. Scale: If 500M users each have 10 GB of files with a deduplication ratio of 3x, estimate the total storage and the block storage architecture.

3. Sync: Design a delta sync algorithm that efficiently computes the diff between two versions of a 10 GB video file.

4. Offline: How would you handle offline editing where the user edits a file on two devices without internet? Design the conflict detection and resolution strategy.

What to Learn Next