Load Balancer - Interview Tips

Interview Approach

Initial Questions (5 min)

Traffic: How many requests per second?
Protocol: HTTP, TCP, or both?
Backends: How many backend servers?
Session: Stateful or stateless?
Geographic: Single region or global?

Design Progression

Step 1: High-Level (5 min)

Clients → Load Balancers → Backend Servers

Step 2: Algorithm (5 min)

Round-robin for simple cases
Least connections for long-lived
Weighted for heterogeneous backends
IP hash for session persistence

Step 3: Health Checking (5 min)

Active probes every 5s
Mark unhealthy after 3 failures
Remove from pool
Re-add when healthy

Step 4: High Availability (5 min)

Multiple LB instances
DNS/Anycast routing
Automatic failover
No single point of failure

Step 5: Scaling (5 min)

Add LB instances
Add backend servers
Auto-scaling integration
Connection draining

Step 6: Advanced Features (5 min)

SSL/TLS termination
Rate limiting
Request routing
Metrics and monitoring

Key Topics

Must Cover

✅ Load balancing algorithms
✅ Health checking
✅ High availability
✅ Session persistence
✅ Failover

Should Cover

✅ SSL/TLS termination
✅ Connection draining
✅ Auto-scaling
✅ Monitoring

Talking Points

Round-Robin

"Round-robin distributes requests evenly across backends. It's simple and works well for stateless applications with homogeneous servers."

Health Checking

"We send periodic probes to backends. After 3 consecutive failures, we mark the server unhealthy and remove it from the pool. When it recovers, we automatically re-add it."

Session Persistence

"For stateful applications, we use sticky sessions. We can use cookie-based (insert cookie with server ID) or IP-based (hash client IP) persistence."

High Availability

"We deploy multiple load balancer instances behind DNS or Anycast. If one fails, traffic automatically routes to healthy instances. No single point of failure."

Time Management

0-5 min: Requirements
5-15 min: Architecture
15-25 min: Deep dive
25-35 min: Scaling/HA
35-45 min: Follow-ups

This guide helps you approach load balancer design interviews with confidence.