Interview with a Meta engineer.

An Interview with a Meta engineer

Watch someone solve the design live comments problem in an interview with a Meta engineer and see the feedback their interviewer left them. Explore this problem and others in our library of interview replays.

Design Live Comments Feature: System Design Interview with a Meta Engineer - YouTube

Interview Summary

Problem type
Design Live Comments

Interview question
Design a live comments feature. Live commenting is a feature that allows clients to publish real-time comments on live videos or pictures without the need to refresh or reload.

Interview Feedback

Feedback about Utilitarian Lemur (the interviewee)
Advance this person to the next round?
No
How were their technical skills?
4/4
How was their problem solving ability?
3/4
What about their communication ability?
3/4

Overall weak no hire as M1.

✅ What went well:

  1. Good functional requirement
  2. Good non-functional requirement and why they’re there
  3. Good assumption read vs write ratio and calling out about hot spots (celebrities)
  4. Good API design, lacking response
  5. Decent data schema. Includes the “live” problem
  6. Good data flow
  7. Good component responsibilities
  8. TC drove the discussion well
  9. Core puzzle: talk about using push (websocket/long polling). Didn’t talk about the alternatives
  10. Deep dive: talk about CAP theorem and how it applies in this app. And decided to go with high availability. But doesn’t tell us what is the implication on the system
  11. Deep dive: talk about fail tolerance on storage

❌ What can be improved:

  1. Spent 20 mins in functional + non-functional + numbers. Benchmark is 10 mins
  2. Doesn’t really explain what non-functional mean. What’s the implications
  3. Doesn’t really explain what the numbers mean. What’s the implications
  4. Lack of tradeoffs discussion
  5. Lack 2-3 more deep dives (DB choices, DB sizes, connection with choices numbers)
  6. API lacks responses

Action items:

  1. Memorize all non-functional requirement possibilities, then just shoot them (yes, no)

    1. Reliability: 99.99% system availability
    2. Scalability: can handle up and down traffic
    3. Security: only one public endpoint. Code is executed safely
    4. Durability: store data for 10 years
    5. Latency: p95 200ms
    6. High Availability vs Strong Consistency

  2. Explain by use cases (read, end to end) vs by sections (API, then data schema, then high level design)

  3. Don’t clarify, just shoot ahead. Let the interviewer disrupt you on checkpoints

  4. Not all numbers are important. QPS is always important. Storage number is usually important.

  5. Think about Core Puzzle, and at least make the tradeoff about Core Puzzle

Level implications:

  1. E4 you have to do things right
  2. E5 you have to do trade-offs (SQL vs NoSQL, core puzzle, push vs pull, REST vs GraphQL, sync vs async)
  3. E6 you have to go deep (offline support, multi-language support, battery optimization)
  4. E7 you have to impress (something that most people don’t know)

Tips:

  1. Easiest way to sound smart (and have an opportunity for deep-dive): user → LB → API gateway → service (don’t forget to mention API gateway)
    1. API Gateway allows you to put these following deep dives: authentication, security, rate limiting, throttling, transformations, analytics and monitoring
  2. Keep functional + non-functional + quantitative analysis down to 10 mins, no more than this.
  3. On quantitative analysis, you can use quick numbers:
    1. 1MB → 1e6
    2. 1M DAU → 1e6
    3. 1 day → 86400 sec → close to 100,000 sec → 1e5
    4. Usage on computing QPS → 1M DAU → 1e6 / 1e5 → 10 request/sec.
    5. Usage on computing storage -> 1M DAU * 1 MB * 5 years -> 1e6 * 1e6 * 5 * 400 -> 1e6 * 1e6 * 2e3 -> 2 * e15
  4. You can talk high level design + API design + data schema + data flow at the same time. You can save 5-10 mins by doing this.
  5. Persist all the discussion into writing/drawing. You don't know when your interviewer will actually get a chance to write your feedback.
  6. Master your drawing tool. Practice with excalidraw (cuz Facebook uses excalidraw)
  7. Stop at each milestone and ask questions: "Am I going in the right direction, or do you want me to go deep into somewhere else?"
  8. Your biggest enemy is not your tech skill. It's usually time. Remember that you only have 35 mins.

Feedback about Digital Cactus (the interviewer)
Would you want to work with this person?
Yes
How excited would you be to work with them?
2/4
How good were the questions?
3/4
How helpful was your interviewer in guiding you to the solution(s)?
3/4

Interview Transcript

... (transcript continues with detailed interaction between the interviewer and interviewee about the design of the live comments feature, including their approaches to functional requirements, non-functional requirements, and system design. The discussion includes deep dives into technical aspects such as data storage, API design, message queue handling, etc.)

Summary of Key Points: