System Design Interview Query: Planning a Design for Dropbox
In a bid to create a Dropbox-like file hosting service, we'll outline the key components and design considerations for a robust, scalable, and user-friendly solution.
Core Tables and Data Structure
To model users, devices, file/folder objects, file chunks, and access permissions efficiently, we recommend the following core tables:
- Users: Store user information such as user ID, name, email, password hash, and timestamps for account activity.
- Devices: Each user may have multiple devices; this table tracks device IDs linked to users.
- Objects: Represent files and folders with fields for object ID, device ID owner, object type (file/folder), parent folder ID (to represent hierarchy), object name, and timestamps.
- Chunks: Files are often split into chunks; this table stores chunk IDs, the associated file object ID, chunk storage URLs (e.g., in cloud storage like Amazon S3), and timestamps.
- AccessControlList (ACL): To manage sharing, this table maps users to objects with whom they have access, tracking share creation and update times.
This structure follows normalization practices to minimize data redundancy and maintain clear relationships.
Additional Design Considerations
File Chunking
File chunking improves upload/download efficiency, allowing partial updates and streaming.
Hierarchical File Structure
The hierarchical file structure is maintained by in Objects, referencing folders.
Cloud Storage Integration
File chunks are stored in external systems such as Amazon S3, while metadata is kept in the database.
Access Control
Access control is handled separately via ACL to flexibly manage sharing of files and folders among users.
System Components
Messaging Service Queue
The messaging service queue is responsible for the asynchronous communication between the clients and the synchronization service, handling lots of reading and writing requests, storing messages in a highly available and reliable queue, and providing high performance and scalability.
Synchronization Service
The synchronization service receives the request from the request queue of the messaging services and updates the metadata database with the latest changes. It also broadcasts the latest update to the other clients through the response queue so that the other client's indexer can fetch back the chunks from the cloud storage and recreate the files with the latest update.
Upload Service
The Upload Service receives file upload requests from clients, generates Presigned URLs for S3, coordinates the upload process, ensures data integrity and completeness, and updates the Metadata Database with file details.
Download Chunk API, Upload Chunk API, and Get Objects API
The Download Chunk API would be used to download the chunk of a file, the Upload Chunk API would be used to upload the chunk of a file, and the Get Objects API would be used by clients to query the Meta Service for new files/folders when they come online.
Indexer
The Indexer is responsible for updating the internal database when it receives the notification from the watcher (for any action performed in folders/files).
Edge Wrapper
An edge wrapper is an abstraction layer that sits between the application and the sharded databases, providing a unified interface for the application to interact with the database system. The edge wrapper integrates ORM functionality to provide a convenient interface for the application to interact with sharded databases.
Chunker
The Chunker breaks the files into multiple small pieces called chunks and uploads them to the cloud storage with a unique id or hash of these chunks.
Watcher
The Watcher is responsible for monitoring the sync folder for all activities performed by the user such as creating, updating, or deleting files/folders.
Scalability and Performance
The system is designed to handle increasing traffic as the number of users grows, ensuring scalability and high performance. Sharding helps distribute the load, improve query performance, and enhance scalability. Each shard is a separate database instance that can be distributed across different servers or locations.
Maintenance, Backup, and Recovery
Maintenance, backup, and recovery operations become more intricate with multiple shards. However, by following best practices and implementing robust data management strategies, these challenges can be effectively addressed.
In conclusion, this design provides a clear foundation for building a scalable, maintainable, and user-friendly file hosting infrastructure similar to Dropbox. The solution incorporates essential features, including file chunking, hierarchical file structure, cloud storage integration, and access control, to deliver an efficient and reliable service for users.
- To maintain high performance and scalability in handling reading and writing requests, a messaging service queue is implemented as a system component, ensuring asynchronous communication between clients and the synchronization service.
- The system design incorporates a trie data structure for the ACL, allowing for efficient management of sharing among users and permissions for files and folders.
- In addition to database management, the technology used for the synchronization service should support hashing to ensure data integrity and completeness during file uploads and updates, similar to Dropbox's system.
