Understanding Load Balancers Through Real-World Scenarios: A Technical Guide

Requirement:

/products → Product Service
/orders   → Order Service
/users    → User Service

This requires path-based routing.

I would use a Layer 7 load balancer, such as AWS Application Load Balancer.

Client
  ↓
ALB
 ├── /products → Product Service
 ├── /orders   → Order Service
 └── /users    → User Service

Interview answer: Use ALB because it can inspect HTTP paths and route requests accordingly.

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4. Different Domains Need Different Backend Services. What Would You Do?

Suppose:

shop.example.com  → Shopping Service
admin.example.com → Admin Service
api.example.com   → API Service

This requires host-based routing.

I would use an Application Load Balancer.

The ALB reads the hostname and forwards the request to the correct target group.

Key concept: Host-based routing is useful when multiple applications share one load balancer.

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5. Users Lose Their Shopping Cart When Requests Go to Another Server. Why?

Imagine:

Request 1 → Server 1
Request 2 → Server 2

If Server 1 stores the user's shopping cart only in local memory, Server 2 will not know about it.

One quick solution is sticky sessions, where the same user is sent back to the same server.

User A → Server 1
User A → Server 1
User A → Server 1

However, a better long-term architecture is usually to store session state in shared storage such as Redis or a database.

Interview answer: Sticky sessions can help, but stateless application servers with shared session storage are generally easier to scale.

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6. One Availability Zone Goes Down. What Happens?

Suppose your servers are distributed like this:

Availability Zone A
 ├── Server 1
 └── Server 2

Availability Zone B
 ├── Server 3
 └── Server 4

If Zone A fails, the load balancer should continue sending traffic to healthy servers in Zone B.

That is why production applications should normally deploy servers across multiple Availability Zones.

Key point: Do not keep all backend servers in one failure zone.

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7. Your Application Needs Millions of TCP Connections. ALB or NLB?

For a high-performance TCP workload, I would normally consider a Network Load Balancer.

Examples include:

• Gaming systems
• IoT connections
• Real-time network services
• High-volume TCP applications

Clients
   ↓
  NLB
 ├── Server 1
 ├── Server 2
 └── Server 3

Interview answer: NLB is better suited to high-performance Layer 4 workloads such as TCP and UDP.

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8. Your Application Uses WebSockets. Can a Load Balancer Handle It?

Yes, but I would first confirm that the chosen load balancer supports the application's protocol and connection behavior.

WebSockets create long-lived connections.

Client
   ↕
Load Balancer
   ↕
Application Server

I would also check:

• Idle timeout settings
• Connection limits
• Backend scaling
• Session handling
• Health checks

Key point: Long-lived connections require different capacity planning than short HTTP requests.

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9. You Want to Deploy a New Version Without Downtime. How Can a Load Balancer Help?

Suppose the current application is Version 1 and you want to deploy Version 2.

You can gradually shift traffic:

          ┌── Version 1
Users → LB
          └── Version 2

For example:

90% → Version 1
10% → Version 2

If Version 2 works correctly, gradually increase its traffic.

This supports deployment strategies such as:

• Blue-green deployment
• Canary deployment
• Weighted traffic shifting

Interview answer: Use the load balancer to gradually route traffic to the new version and quickly roll back if problems appear.

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10. One Backend Server Keeps Failing Health Checks. What Would You Check?

I would investigate:

• Is the health-check URL correct?
• Is the correct port being checked?
• Is the application actually running?
• Are security rules blocking the load balancer?
• Is the health-check timeout too short?
• Is the server overloaded?
• Is the application returning the expected status code?

Example:

Load Balancer
   ↓
/health
   ↓
200 OK → Healthy
500    → Unhealthy

Key point: Do not immediately assume the load balancer is the problem. The backend or health-check configuration may be incorrect.

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11. The Load Balancer Is Working, but the Website Is Still Slow. Why?

A load balancer cannot fix every performance problem.

The bottleneck might be:

• Database queries
• Cache misses
• External APIs
• Slow application code
• Storage
• Network latency

For example:

Users
  ↓
Load Balancer
  ↓
Servers
  ↓
Slow Database

Adding more application servers will not solve a database bottleneck.

Interview answer: Use monitoring and tracing to identify the real bottleneck before scaling infrastructure.

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12. Your Company Wants All Traffic Inspected by Firewalls Before Reaching Applications. What Would You Use?

This is a good use case for Gateway Load Balancer.

Internet
   ↓
GWLB
   ↓
Firewall Fleet
   ↓
Application

GWLB can distribute traffic across multiple virtual security appliances.

Interview answer: Use GWLB when traffic needs to pass through firewalls, intrusion detection systems, or other network appliances.

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13. Can a Load Balancer Itself Become a Single Point of Failure?

Yes, if you build your own architecture around only one load-balancer instance.

A production system should use a highly available load-balancing architecture.

Managed cloud load balancers are commonly designed to provide this redundancy for you.

Interview point: Removing the server single point of failure is not enough—you also need to consider the availability of the load-balancing layer.

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Final Interview Tip

For scenario-based questions, do not immediately say:

“I will add a load balancer.”

First identify the actual problem.

A strong answer usually follows this order:

1. Measure the problem
↓
2. Identify the bottleneck
↓
3. Choose the correct load-balancing strategy
↓
4. Add health checks and monitoring
↓
5. Test failure and scaling behavior

The interviewer is usually testing your decision-making, not just whether you know the definition of a load balancer.

Next Episode: When Not to Use a Load Balancer