May 8, 2022 6:00 PM PDT
This document summarizes a mock system design interview focused on a notification system. The interview assessed the candidate's ability to design a scalable and reliable notification service capable of handling a high volume of notifications across various devices.
Interview Details
- Topic: Notification System
- Level: L5 (Senior)
- Duration: 45 minutes
- Drawing Tool Used: Whimsical
Requirements
Functional Requirements
- Push Notification Types: SMS messages and emails
- Supported Devices: Mobile and PC
Non-Functional Requirements
- User Load: 10 million users per day
- Notification Size: 200 bytes per notification
- Delay: Soft requirement
- Scalability and Durability: Must ensure notifications are sent reliably
System Design Discussion
External APIs
- Discussed the integration of external APIs for sending notifications.
Optimization for 10M Users
- Suggested adding caching to the frontend service.
- Proposed the use of a queue system for message processing.
Data Loss Prevention
- Recommended adding partitions and logging to the database.
- Suggested incorporating additional disks into the design.
Handling Large Scale Data
- Introduced the concept of a rate limiter to manage traffic.
Notification Volume Calculation
- Total notification count:
- (10M \text{ notifications/day} \times 365 \text{ days} \times 10 \text{ years} \times 200B = 7.3 \text{ TB})
- Queries Per Second (QPS):
- (10M / 24 / 3600 \approx 11.57 \text{ QPS} )
- Max QPS: (20 \times \text{average} \approx 300 \text{ QPS})
User Preferences and Acknowledgments
- Discussed how to check user preferences for notifications.
- Suggested adding acknowledgment (ACK) messages to confirm that users have read notifications.
- For SMS, acknowledgment can be received through the carrier when the message is opened.
Tracking Illegal Emails
- Proposed logging email IDs in the database and adding an authentication service.
Third-Party Authorization
- Discussed the benefits of using third-party authorization for validating user identity.
Further Improvements
- Suggested improving consistency for users on different devices.
Feedback
Interviewer Feedback
- The interviewee performed reasonably well but struggled with clarity and depth in certain areas.
- Noted that the design lacked coverage for stability and reliability.
- Suggested focusing on high-level components and scaling strategies.
- Encouraged the interviewee to clarify requirements and assumptions early in the discussion.
Interviewee Feedback
- Felt the interview went okay but acknowledged difficulties in requirement gathering.
- Recognized the need for deeper engagement with the interviewer.
- Noted that the publishing service could become a bottleneck.
- Suggested that assumptions should be declared when requirements are unclear.
Technical Insights
- Discussed the rationale for adding message queues and the potential for duplicates.
- Emphasized the importance of idempotency in message processing.
- Highlighted the need for retry mechanisms, especially for high-priority messages.
- Mentioned the role of APNs in handling notifications when users turn off their devices.
This summary encapsulates the key points discussed during the interview, focusing on the technical aspects of designing a robust notification system.