System Design FAQ: Top Questions

A File Storage System allows users to upload, retrieve, and synchronize files across devices while ensuring durability, security, and efficient version management. Examples include Dropbox, Google Drive, and Box.

📋 Functional Requirements

• Upload/download files
• File versioning
• Sharing and permissions
• Cross-device sync

📦 Non-Functional Requirements

• High durability (99.999999999%)
• Efficient metadata and large file handling
• Scalability and fault tolerance

🏗️ Architecture Overview

• Frontend: File UI + Sync client
• API Gateway: Auth, routing, metadata access
• Metadata Service: Stores file paths, versions, ownership
• Storage Layer: S3 or object store for file blobs
• Database: PostgreSQL or Cassandra for metadata

📁 File Metadata Table

CREATE TABLE files (
    id UUID PRIMARY KEY,
    user_id UUID,
    filename TEXT,
    version INT,
    path TEXT,
    size BIGINT,
    blob_key TEXT,
    created_at TIMESTAMP DEFAULT now()
);
        

📤 Upload Flow

1. Client calculates file hash (e.g., SHA-256)
2. Checks server for existing file (deduplication)
3. Uploads to object storage (S3/MinIO)
4. Stores metadata in DB

🔐 Signed Upload URLs (AWS S3)

import boto3

s3 = boto3.client('s3')
url = s3.generate_presigned_url(
    'put_object',
    Params={'Bucket': 'myapp-bucket', 'Key': 'user123/file.txt'},
    ExpiresIn=3600
)
print(url)
        

🧬 Chunked Upload Strategy

• Split large files into parts (e.g., 5 MB)
• Upload each chunk in parallel
• Use checksum to verify and reassemble

🔄 Sync Design

• Client watches file system for changes (e.g., inotify)
• Syncs diffs to backend
• Version conflict resolution at server

☁️ Example S3 Bucket Policy for Read/Write

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": "*",
      "Action": [
        "s3:GetObject",
        "s3:PutObject"
      ],
      "Resource": "arn:aws:s3:::myapp-bucket/*"
    }
  ]
}
        

🧪 Observability

• Track API usage and upload success/failure
• Use Prometheus to monitor file queue lag and sync latency
• Set up alerts for upload spikes and storage thresholds

📌 Final Insight

A file storage system needs to separate file content (blobs) from metadata, support versioning, and efficiently scale reads/writes. Using signed URLs, chunking, and object storage systems enables safe, durable file management across millions of users.