When I first faced system design interviews at a FAANG company, I realized how ambiguous and overwhelming the process could be—for both candidates and interviewers. Inspired by that struggle, I took on a side project to build a Yahoo system design interview platform: a web application where candidates could practice design interviews in a realistic, interactive setting.
In this post, I’ll walk you through the 7 key lessons I learned from this journey—lessons packed with actionable insights you can apply whether you’re building interview tools, educational platforms, or scalable web systems.
1. Define the User Journey (Candidates & Interviewers)
Early on, I made the classic mistake of building features before really understanding who the users were and what problems they faced.
- Candidate needs: realistic system design questions, ability to sketch architecture, receive feedback.
- Interviewer needs: tools to present prompts, observe candidate workflows, annotate answers.
- Scenario: In a mock interview mode, the candidate should be able to “code” a design and share it live.
(Pro tip) — Craft user personas and map out interaction flows before writing a single line of code. This helps prioritize features that really matter.
Takeaway: User-centric design is not just UI/UX; it’s about deeper empathy that drives feature development and architecture decisions.
2. Balancing Complexity vs. Usability in the UI
The core technical challenge was how to represent system design diagrams interactively:
- I initially tried building a drag-and-drop UI from scratch using pure canvas APIs—Big mistake. It took forever and was buggy.
- Instead, I switched to leveraging a mature open-source library (JointJS or Draw.io), saving weeks of development time.
- For real-time collaboration, I integrated WebRTC and WebSocket for low latency updates.
Tradeoff: Building my own UI gave ultimate control but sacrificed maintainability. Using libraries improved agility but limited certain customizations.
Takeaway: When building developer tools, lean on battle-tested libraries for core functionalities to speed up development and reduce technical debt.
3. Architecture: Scalable Backend Design
To support potentially thousands of simultaneous mock interviews, scalability was crucial:
- Used serverless architecture (AWS Lambda) for functions like user authentication and activity logging.
- Employed WebSocket clusters for real-time data exchange. Managing state consistency across clusters became tricky — I implemented a state synchronization protocol using Redis pub/sub.
- Backend APIs decoupled via GraphQL to allow flexible data querying by the frontend.
Lesson: Scalability comes with managing complexity in state synchronization—a classic tradeoff between performance and consistency (see Educative’s System Design complete guide.
Takeaway: Design your backend communication with eventual consistency in mind for real-time collaborative apps.
4. Integrating Version Control for Diagram History
One of the features I’m most proud of: a version control system that saved candidates’ architecture diagrams after every interaction, allowing them to backtrack or “undo” changes.
- I implemented a diff-based model that stored incremental changes rather than full copies.
- This concept was inspired by Git’s efficient snapshot management.
- Allowed candidates to review the history during post-interview feedback sessions.
(Solution) — This pattern inspired by Git can be generalized for complex UI state management in interactive tools.
Takeaway: Implementing version control for designs boosts both usability and teaching impact. You create a narrative of design decisions rather than just a snapshot.
5. Handling Network Failures with Offline Support
I vividly remember encountering a chaotic interview session where the candidate’s WiFi dropped midway. Without offline protection, all work was lost.
I knew offline-first design was critical:
- Employed IndexedDB for local caching of user changes.
- Created a sync mechanism that reconciled local edits with the server when the connection returned.
- UI showed status indicators like “You’re offline, changes will sync soon.”
Engineering tradeoff: Offline support adds complexity and potential merge conflicts but improves user trust drastically.
Takeaway: Don’t neglect poor network scenarios in your design interview platform. Your users’ experience depends heavily on graceful degradation.
6. Security and Privacy Considerations
System design interviews often involve sensitive intellectual property or personal data. This necessitated:
- End-to-end encryption of communication channels.
- Role-based access control preventing unauthorized access to interview sessions.
- GDPR-compliant user data management.
- Regular audits and penetration testing of the platform.
(Pro tip) — Neglecting security risks your platform’s reputation and legal standing. Invest early in secure design principles.
Takeaway: Treat interview platforms like any sensitive enterprise application with strict authentication, authorization, and data privacy policies.
7. Gathering Continuous Feedback for Improvement
Perhaps the most valuable lesson was how I embedded analytics and feedback mechanisms directly into the platform:
- Interviewers could rate candidate answers and leave timestamped comments tied to diagram elements.
- Candidates received tailored learning paths based on recurring weaknesses detected.
- Heatmaps showed commonly discussed system components during interviews.
This created a virtuous cycle of data-driven improvements.
Use case: Interview coaches can customize training for individuals vs generic advice.
Takeaway: Measure EVERYTHING. Analytics can reveal your blind spots and how users truly interact with your product.
Final Thoughts: You’re Closer Than You Think
Building a Yahoo-style system design interview platform was a rollercoaster—debugging real-time sync issues at 3 AM, iterating on UI prototypes, and learning devops on the fly.
If you’re inspired to build tools that democratize tech education, remember:
- Start with user empathy.
- Balance engineering tradeoffs consciously.
- Embrace open source and cloud services.
- Protect user data meticulously.
- Iterate relentlessly based on real user feedback.
And above all, enjoy the process. The best engineer-teachers are those who learn alongside their users.
Resources to Level Up Your System Design Skills:
Thanks for reading! If you’re designing your own interview tools or facing interview challenges — drop a comment or reach out. You’re closer than you think to cracking the code.
Happy designing!
— A fellow dev who’s been in the trenches
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