✅ Part 1 — Types of AWS Load Balancers (Interview Must-Know)
✅ Q1 — What are the types of AWS Load Balancers?
Four types:
ALB — Application Load Balancer (Layer 7) HTTP/HTTPS routing, path/host rules
NLB — Network Load Balancer (Layer 4) TCP/UDP, ultra high performance
GWLB — Gateway Load Balancer Traffic inspection appliances
CLB — Classic LB (legacy) Avoid for new systems
If you can’t explain ALB vs NLB clearly — interview red flag.
✅ Part 2 — ALB Deep Dive (Most Used)
✅ Q2 — ALB powers — what makes it strong?
ALB is L7 intelligent router. Supports host-based and path-based routing, header rules, redirects, fixed responses, weighted routing. Supports WebSockets and HTTP/2. Integrates with WAF and Cognito auth. Can route to multiple target groups — perfect for microservices.
✅ Q3 — Real production ALB design pattern
Single ALB per environment with multiple listeners and path rules:
/api/*→ service A/payments/*→ service B/auth/*→ service C
Reduces cost and centralizes TLS + WAF. Very common in microservices + EKS.
✅ Q4 — ALB tradeoffs
Higher latency than NLB. Only supports HTTP/HTTPS/gRPC. Not suitable for raw TCP protocols. Can become expensive with many rules + high request rate. Header-based routing increases processing overhead.
✅ Part 3 — NLB Deep Dive (Performance Choice)
✅ Q5 — NLB powers — when is it superior?
NLB is Layer 4 and extremely fast. Handles millions of connections with very low latency. Supports TCP, UDP, TLS passthrough. Preserves source IP by default. Supports static IP and Elastic IP — important for allowlist integrations.
✅ Q6 — Real production NLB use cases
- gRPC services
- database proxying
- high-throughput APIs
- gaming or streaming protocols
- partner allowlist integrations
- TLS passthrough to app
✅ Q7 — NLB tradeoffs
No path routing. No header routing. No WAF directly. No auth layer. Routing is only port/protocol based. You must handle L7 logic in app or proxy behind it.
✅ Part 4 — GWLB (Advanced / Niche but Asked in Senior Interviews)
✅ Q8 — What is Gateway Load Balancer used for?
GWLB is used to route traffic through security appliances like firewalls and IDS/IPS. It sits inline and distributes traffic to inspection tools. Used in security-heavy architectures.
✅ Part 5 — Scaling Behavior (Interview Favorite)
✅ Q9 — Do AWS load balancers scale automatically?
Yes — fully managed and auto-scaling. But scaling is not instant. Sudden extreme spikes can cause initial 5xx or connection drops. Pre-warming is rarely needed now but still exists for extreme cases.
✅ Q10 — What is connection draining (deregistration delay)?
When a target is removed, LB waits before closing connections so in-flight requests finish. Critical for zero-downtime deploys. Must align with app shutdown time.
✅ Part 6 — Health Checks & Failure Behavior
✅ Q11 — Health checks — how do they really affect traffic?
Targets failing health checks are removed from rotation. Bad health check design = false positives = traffic loss. Health endpoint should check critical dependencies — not just “process alive”.
✅ Q12 — Common health check mistake
Checking /health that always returns 200 even if DB is down. That makes LB think service is healthy when it’s not.
✅ Part 7 — Security & TLS Design
✅ Q13 — Where should TLS terminate — LB or app?
Usually at ALB for simplicity and cert management. Terminate at app only if end-to-end encryption required. NLB supports TLS passthrough when needed.
✅ Q14 — ALB + WAF — why common?
ALB integrates directly with WAF for L7 filtering. Blocks attacks before app. Common fintech edge pattern: CloudFront → WAF → ALB → services.
✅ Part 8 — Kubernetes / EKS Patterns
✅ Q15 — ALB vs NLB in EKS — when to use which?
ALB → Ingress for HTTP routing across many services. NLB → Service type LoadBalancer for TCP/gRPC or static IP needs.
Many clusters use both.
✅ Part 9 — Cost Tradeoffs
✅ Q16 — What drives LB cost?
- hours running
- LCU usage (new connections, active connections, bytes, rules)
- NLB data processed
ALB rule count increases LCU cost.
✅ Q17 — Cost optimization pattern
One shared ALB per env instead of many ALBs. Path routing saves cost.
✅ Part 10 — Limits & Quotas
✅ Q18 — Important limits interviewers expect awareness of
- listener rules limit
- target group limits
- targets per group
- request size limits
- idle timeout limits
Design must consider rule explosion.
✅ Part 11 — Failure & Debugging
✅ Q19 — LB returns 502 — common causes?
- backend timeout
- wrong port
- TLS mismatch
- health check mismatch
- target crashed
- security group block
✅ Part 12 — When NOT to Use Which
✅ Q20 — When not to use ALB vs NLB
Don’t use ALB for raw TCP. Don’t use NLB when you need path routing or WAF. Don’t put internal east-west traffic through ALB — use cluster networking/service mesh.