L5
L5 — Senior Engineer (end-to-end design with trade-offs)
At L5, you own an end-to-end feature. The interviewer wants to see multiple components fit together, explicit trade-offs, and awareness of scale beyond your one service.
Day-to-day responsibilities
- Design a service and its integrations, not just a component
- Reason about multiple databases, caches, and message paths
- Evaluate build vs. buy for infrastructure choices
- Own an on-call rotation and incident response
Interview signals
- Explicit trade-off statements ('we picked X because…, at cost of Y')
- At least two scale tiers considered (today and 10× today)
- A concrete NFR discussion (latency budget, error budget, availability target)
- At least one failure scenario per major component
Problem focus
- Instagram, Twitter, Uber, Slack, Ticket Booking, Payment System
- Multi-service designs, read/write split, background jobs
Delivery advice
- State an assumption every time you make a choice — otherwise it looks accidental
- Compare at least two options for every non-trivial decision
- Reserve 5 minutes at the end for NFRs and failure discussion