What is the difference between an Application Load Balancer and a Network Load Balancer?
Interviewer Expectations for this Question
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Core Competency
Assesses fundamental understanding of AWS conventions, runtime behavior, and memory/performance considerations.
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Evaluation Criteria
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Comprehensive Model Answer
Verified Solution
Both are Elastic Load Balancers, but they operate at different OSI layers.
- Application Load Balancer (layer 7) understands HTTP and HTTPS, supports host and path-based routing, sticky sessions, and WebSockets, and integrates with WAF. It terminates TLS and adds X-Forwarded-For headers.
- Network Load Balancer (layer 4) handles TCP, UDP, and TLS, offers extremely high throughput and ultra-low latency, and preserves the client source IP. It can have a static IP per Availability Zone.
Choose ALB for web applications and microservices that route by URL. Choose NLB for non-HTTP protocols, gaming, IoT, or when you need static IPs or millions of requests per second.
aws elbv2 create-listener --load-balancer-arn arn:aws:elasticloadbalancing:... \
--protocol HTTPS --port 443 --certificates CertificateArn=arn:aws:acm:...
Both support health checks and cross-zone load balancing.
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