When applications grow, handling user traffic efficiently becomes a must. Load balancing AWS services help distribute requests across multiple compute resources, improving availability, performance, and fault tolerance. For cloud engineers or developers preparing for interviews, having a strong understanding of ALB, NLB, CLB, and routing logic can give you a competitive edge. These services form the backbone of application reliability in AWS environments and are often major topics in job discussions.
In this blog, we will walk through the most commonly asked AWS load balancing interview questions and answers. The explanations are simple and clear, helping you confidently discuss traffic distribution, routing logic, and how different load balancers work in real-world scenarios. If you are preparing for a cloud role, this guide will help sharpen your knowledge quickly.
AWS Load Balancing Interview Questions and Answers
Question 1. What is AWS load balancing?
Answer: Load balancing AWS refers to distributing incoming traffic across multiple targets such as EC2 instances, containers, IP addresses, or Lambda functions. It helps improve scalability, prevents performance bottlenecks, and ensures the application remains available even if some targets fail.
Question 2. What are the different types of AWS Elastic Load Balancers?
Answer: AWS offers three main types:
- Application Load Balancer (ALB) – Operates at the application layer (Layer 7)
- Network Load Balancer (NLB) – Works at the transport layer (Layer 4)
- Classic Load Balancer (CLB) – Older generation load balancer supporting both Layer 4 and Layer 7
Each load balancer type suits different application needs.
Question 3. When is a Network Load Balancer a better option?
Answer: NLB is ideal for extreme performance use cases. It provides:
- Very low latency
- Millions of requests per second
- Static IP support
- TCP, UDP, and TLS traffic handling
NLB is often used for gaming, streaming, IoT, or financial platforms where speed matters.
Question 4. What is the purpose of the Classic Load Balancer?
Answer: CLB is the earlier version of AWS load balancers and supports basic Layer 4 and 7 traffic. It is useful for legacy systems but is no longer recommended for new applications. Most new workloads are migrated to ALB or NLB for better features and efficiency.
Question 5. How does routing logic work in ALB?
Answer: ALB routing logic evaluates incoming HTTP requests and uses rules to decide where traffic should go. These rules may include:
- URL paths (/login → Auth Service)
- Host headers (api.myapp.com → API Service)
- HTTP headers, methods, or query strings
This makes ALB highly flexible for distributed and microservice architectures.
Question 6. What type of routing logic does NLB support?
Answer: NLB does not support content-based routing. Instead, it forwards traffic based on network-level information such as TCP/UDP ports and IPs. It is purely designed for fast and reliable transport of high-volume traffic.
Question 7. What is a Target Group?
Answer: A target group represents the set of targets (EC2 instances, ECS tasks, Lambda functions) where the load balancer routes traffic. It also defines health checks and how traffic should be distributed among targets.
Question 8. What are health checks in load balancing AWS?
Answer: Health checks evaluate whether targets are available and able to handle traffic. If a target fails the check, the load balancer automatically stops sending traffic to it until recovery. ALB uses HTTP-based checks, while NLB uses TCP-based checks.
Question 9. How does session stickiness work?
Answer: Load balancers can maintain a client’s session by routing repeated requests to the same target. In ALB, session stickiness uses cookies. In CLB, stickiness may use cookies or load balancer–generated tokens. This is helpful for stateful applications.
Question 10. Can ALB distribute traffic to Lambda functions?
Answer: Yes, ALB supports Lambda as a target type, enabling serverless applications to scale seamlessly with standard HTTP workloads.
Question 11. What is cross-zone load balancing?
Answer: Cross-zone load balancing distributes traffic evenly across all registered targets in all Availability Zones. It helps avoid situations where AZ imbalance affects performance. ALB provides it by default, while NLB allows enabling/disabling based on cost preference.
Question 12. How does AWS manage SSL or TLS certificates with load balancers?
Answer: AWS Certificate Manager (ACM) integrates with ALB and NLB to manage certificates for HTTPS/TLS encryption. This simplifies secure inbound traffic without installing certificates manually on instances.
Question 13. Can NLB preserve the client IP?
Answer: Yes, NLB supports client IP pass-through, making it visible to the backend service. ALB can provide client IP using the X-Forwarded-For header.
Question 14. What are listener rules in ALB?
Answer: Listeners inspect incoming traffic on predefined ports. Listener rules define how to evaluate requests and forward them to the correct target group. They are key in routing logic for microservices.
Question 15. What monitoring tools are available for load balancers?
Answer: AWS CloudWatch tracks performance metrics like latency, request count, and healthy host count. AWS CloudTrail logs configuration and API actions for auditing. Together they help troubleshoot issues faster.
Conclusion
Mastering AWS load balancing is a big step toward becoming a strong cloud engineer. Understanding ALB, NLB, CLB, routing logic, and traffic distribution gives you the ability to design scalable and resilient applications that can handle real-world demands. By reviewing these interview questions and answers, you’ll be more prepared to explain how different load balancing AWS services work and when to use the right one.
Continue practicing real architecture scenarios and pair this knowledge with hands-on experience using EC2, ECS, and Lambda deployments. The more familiar you become with AWS traffic routing behavior, the more confident you will be in your interviews.
