ALB vs NLB decision in AWS - Trade-offs & Expert Analysis

Overview - ALB vs NLB decision

What is it?

ALB (Application Load Balancer) and NLB (Network Load Balancer) are two types of load balancers offered by AWS. They help distribute incoming network traffic across multiple servers to keep applications running smoothly. ALB works at the application level, handling web traffic with advanced routing, while NLB works at the network level, focusing on high performance and low latency. Choosing between them depends on the needs of your application.

Why it matters

Without the right load balancer, applications can slow down, crash, or become unreachable when many users connect at once. ALB and NLB solve this by spreading traffic efficiently, improving speed and reliability. Picking the wrong one can cause wasted resources, poor user experience, or security risks. Understanding their differences helps build strong, scalable systems that keep users happy.

Where it fits

Before learning this, you should understand basic networking concepts like IP addresses and ports, and what a load balancer does. After this, you can explore deeper topics like auto scaling, security groups, and multi-region architectures to build resilient cloud applications.

Mental Model

Core Idea

ALB directs traffic based on web application rules, while NLB routes traffic quickly at the network level for high performance.

Think of it like...

Think of ALB as a smart receptionist who reads your request and sends you to the right department, while NLB is like a fast highway toll booth that quickly lets cars through without checking details.

┌───────────────┐       ┌───────────────┐
│   Client      │──────▶│ Load Balancer │
└───────────────┘       └───────────────┘
                           │          
          ┌────────────────┴───────────────┐
          │                                │
   ┌───────────────┐                ┌───────────────┐
   │ ALB (App Layer)│                │ NLB (Network) │
   └───────────────┘                └───────────────┘
          │                                │
  Routes by URL, Host, HTTP headers   Routes by IP, Port
  Handles HTTP/HTTPS traffic           Handles TCP/UDP traffic

Build-Up - 7 Steps

1

Foundation

What is a Load Balancer?

Concept: Introduces the basic idea of a load balancer and why it is used.

A load balancer is like a traffic manager for internet requests. When many users want to use a website or app, the load balancer spreads their requests across several servers. This keeps the app fast and available, even if some servers are busy or fail.

Result

You understand that load balancers help keep websites and apps running smoothly by sharing work among servers.

Knowing what a load balancer does is key to understanding why different types exist and how they improve reliability.

2

Foundation

Basic AWS Load Balancer Types

Concept: Introduces the main AWS load balancers: ALB and NLB.

AWS offers different load balancers. ALB works with web traffic and understands details like URLs and headers. NLB works at a lower level, handling raw network connections quickly. Each serves different needs.

Result

You can name ALB and NLB and know they serve different layers of network traffic.

Recognizing that load balancers operate at different layers helps you choose the right one for your app's needs.

3

Intermediate

How ALB Routes Traffic

🤔Before reading on: do you think ALB can route traffic based on the website address or just the server IP? Commit to your answer.

Concept: Explains ALB's ability to route traffic based on web request details.

ALB works at the application layer, so it can look inside web requests. It can send users to different servers based on the URL path (like /images or /login), the hostname (like app.example.com), or HTTP headers. This lets you run many services behind one ALB.

Result

You understand ALB can direct users to different servers depending on what they ask for.

Knowing ALB's smart routing lets you design flexible web apps that share one entry point but serve many purposes.

4

Intermediate

How NLB Handles Traffic

🤔Before reading on: do you think NLB inspects the content of network packets or just forwards them quickly? Commit to your answer.

Concept: Describes NLB's focus on speed and handling raw network connections.

NLB works at the network layer. It forwards traffic based on IP address and port without looking inside the data. This makes it very fast and able to handle millions of connections with low delay. It supports TCP, UDP, and TLS traffic.

Result

You know NLB is best for fast, simple routing of network connections without inspecting content.

Understanding NLB's speed and simplicity helps you pick it for performance-critical or non-web apps.

5

Intermediate

Comparing Features and Use Cases

🤔Before reading on: which load balancer do you think supports WebSocket connections? Commit to your answer.

Concept: Compares ALB and NLB features and when to use each.

ALB supports HTTP/HTTPS, WebSockets, and advanced routing rules. NLB supports TCP, UDP, and TLS with very high throughput and static IPs. Use ALB for web apps needing smart routing and NLB for high-performance or non-HTTP traffic.

Result

You can match load balancer types to application needs based on features.

Knowing feature differences prevents choosing a load balancer that can't handle your app's traffic type.

6

Advanced

Security and Integration Differences

🤔Before reading on: do you think ALB or NLB integrates better with AWS WAF (Web Application Firewall)? Commit to your answer.

Concept: Explores security features and AWS service integrations for ALB and NLB.

ALB integrates with AWS WAF to protect against web attacks and supports SSL termination. NLB can pass encrypted traffic through or terminate TLS but does not integrate with WAF. ALB supports user authentication features, while NLB is simpler.

Result

You understand which load balancer fits security needs and AWS integrations.

Knowing security integration helps you design safer applications and meet compliance requirements.

7

Expert

Choosing Load Balancers in Complex Architectures

🤔Before reading on: can you think of a scenario where using both ALB and NLB together is beneficial? Commit to your answer.

Concept: Discusses advanced patterns combining ALB and NLB for optimized performance and flexibility.

In complex systems, you might use NLB to handle raw TCP traffic for microservices and ALB for web frontends. NLB can front ALB to provide static IPs or handle sudden traffic spikes. Understanding how to chain them improves scalability and fault tolerance.

Result

You can design multi-layered load balancing architectures for demanding applications.

Knowing how to combine ALB and NLB unlocks powerful, flexible cloud infrastructure designs.

Under the Hood

ALB operates at Layer 7 (application layer) of the OSI model, inspecting HTTP/HTTPS requests to make routing decisions based on content like URLs and headers. It terminates SSL connections and can modify requests before forwarding. NLB operates at Layer 4 (transport layer), forwarding TCP/UDP packets without inspecting payloads, enabling ultra-low latency and high throughput. Both use AWS-managed infrastructure to distribute traffic across healthy targets in multiple availability zones.

Why designed this way?

AWS designed ALB to support modern web applications needing flexible routing and security features, while NLB was built for performance-critical applications requiring static IPs and handling millions of connections. This separation allows AWS to optimize each load balancer for its specific use case rather than one tool trying to do everything.

┌───────────────┐
│   Client      │
└──────┬────────┘
       │
┌──────▼────────┐
│  Load Balancer│
│ ┌───────────┐ │
│ │ ALB (L7)  │ │
│ │ - Inspect │ │
│ │   HTTP    │ │
│ │ - SSL Term│ │
│ └───────────┘ │
│               │
│ ┌───────────┐ │
│ │ NLB (L4)  │ │
│ │ - Fast    │ │
│ │ - TCP/UDP │ │
│ └───────────┘ │
└──────┬────────┘
       │
┌──────▼────────┐
│   Targets     │
└───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Do you think ALB can handle UDP traffic? Commit to yes or no before reading on.

Common Belief: ALB can handle all types of network traffic including UDP.

Reality: ALB only supports HTTP and HTTPS protocols, not UDP. NLB is needed for UDP traffic.

Why it matters: Using ALB for UDP traffic will cause failures and unreachable services, leading to downtime.

Quick: Do you think NLB can inspect HTTP headers to route traffic? Commit to yes or no before reading on.

Common Belief: NLB can inspect HTTP headers and route traffic based on them.

Reality: NLB does not inspect HTTP headers; it routes traffic based only on IP and port.

Why it matters: Expecting NLB to do application-level routing can cause misrouted traffic and broken app behavior.

Quick: Do you think ALB always has higher latency than NLB? Commit to yes or no before reading on.

Common Belief: ALB is always slower than NLB because it inspects traffic deeply.

Reality: While ALB adds some latency due to inspection, in many cases it is optimized and latency difference is minimal for typical web apps.

Why it matters: Assuming ALB is too slow might lead to unnecessary use of NLB, losing valuable routing features.

Quick: Do you think NLB provides static IP addresses by default? Commit to yes or no before reading on.

Common Belief: NLB does not provide static IP addresses.

Reality: NLB can have static IP addresses, which is useful for firewall rules and whitelisting.

Why it matters: Not knowing this can cause architects to overcomplicate network design or miss security benefits.

Expert Zone

1

ALB supports HTTP/2 and gRPC protocols, enabling modern web and microservice communication patterns.

2

NLB preserves the source IP address of clients, which is important for logging and security policies.

3

ALB can perform content-based routing with path and host-based rules, but complex rules can impact performance.

When NOT to use

Avoid using ALB for non-HTTP protocols like TCP or UDP; use NLB instead. Conversely, avoid NLB if you need advanced HTTP routing, SSL offloading, or integration with AWS WAF. For extremely simple use cases with low traffic, a Classic Load Balancer or direct server access might suffice.

Production Patterns

In production, teams often use ALB for web frontends with multiple microservices behind it, leveraging path-based routing. NLB is used for database proxies, gaming servers, or IoT devices needing fast, stable connections. Some architectures combine NLB in front of ALB to provide static IPs and handle TLS termination at scale.

Connections

OSI Model Layers

ALB operates at Layer 7, NLB at Layer 4, showing practical use of OSI layers in cloud networking.

Understanding OSI layers clarifies why ALB can inspect web content and NLB cannot, helping choose the right tool.

Traffic Routing in Road Networks

Both load balancers route traffic but at different levels: ALB like city streets directing cars by destination, NLB like highways moving cars fast without stops.

Seeing load balancers as traffic systems helps grasp their roles in managing flow and speed.

Human Immune System

ALB acts like an immune system recognizing specific threats (web attacks) and filtering them, while NLB is like the circulatory system moving blood quickly without filtering.

This analogy highlights why ALB integrates with security tools and NLB focuses on speed.

Common Pitfalls

#1 Using ALB for UDP traffic.

Wrong approach:

Create an ALB and try to route UDP-based game server traffic through it.

Correct approach:

Use an NLB configured to handle UDP traffic for the game servers.

Root cause:

Misunderstanding that ALB only supports HTTP/HTTPS protocols leads to traffic failures.

#2 Expecting NLB to route based on URL paths.

Wrong approach:

Configure NLB with rules to send traffic to different servers based on URL paths.

Correct approach:

Use ALB for path-based routing and NLB for simple IP/port forwarding.

Root cause:

Confusing network layer routing with application layer routing causes misconfiguration.

#3 Not enabling health checks properly on load balancers.

Wrong approach:

Deploy ALB or NLB without configuring health checks for backend servers.

Correct approach:

Configure health checks so the load balancer only sends traffic to healthy targets.

Root cause:

Overlooking health checks leads to traffic being sent to unhealthy or down servers, causing downtime.

Key Takeaways

ALB and NLB are AWS load balancers designed for different layers: ALB for application-level web traffic, NLB for fast network-level traffic.

Choosing the right load balancer depends on your application's protocol, performance needs, and routing complexity.

ALB supports advanced routing, SSL termination, and security integrations, making it ideal for modern web apps.

NLB offers ultra-low latency, static IPs, and supports TCP/UDP, suited for high-performance or non-web applications.

Understanding these differences helps build scalable, reliable, and secure cloud architectures.