Amazon API Gateway vs Elastic Load Balancer: Which one to choose?
As a Solutions Architect working with AWS services, I've seen many teams struggle with choosing between Amazon API Gateway and Elastic Load Balancer (ELB) for their applications. Both serve as entry points to your infrastructure but are designed for different purposes. In this post, I'll share practical insights on when to use each service, how they can work together, and how to make the right choice for your specific needs.
The Core Differences at a Glance
Before diving into use cases, let's quickly understand what each service is optimized for:
API Gateway is a fully managed service specifically designed for API management, offering features like request validation, authentication, throttling, and API version management.
Elastic Load Balancer (ELB) primarily distributes incoming traffic across multiple targets (like EC2 instances, containers, or IP addresses) to ensure high availability and fault tolerance. The 2 most common types of ELB are Application Load Balancer (ALB), which operates at the application layer (Layer 7) and supports HTTP and HTTPS, and Network Load Balancer (NLB), which operates at the transport layer (Layer 4) and supports TCP, UDP, and TLS.
When to Choose API Gateway
1. Building RESTful or WebSocket APIs
API Gateway shines when you're building APIs that need to be consumed by mobile apps, web applications, or other services. For example a fintech startup can use API Gateway to create a payment processing API that handles authentication, rate limiting, and request validation before the requests even reach their backend services.
# Example API Gateway REST API endpoint structure
/api/v1/payments
GET - retrieve payments
POST - create new payment
/api/v1/payments/{id}
GET - get specific payment
PUT - update payment
DELETE - cancel payment
2. Serverless Architectures
If you're building with Lambda functions, API Gateway is practically a default choice. It provides the HTTP/WebSocket endpoint that triggers your functions and manages the request/response cycle.
3. When You Need API-Specific Features
Choose API Gateway when you need:
Request/response transformations
API keys and usage plans
Schema validation
OAuth/Custom authorizers
CORS support
API documentation via Swagger/OpenAPI
4. Fine-Grained Access Control
When implementing microservices where different endpoints need different security policies, API Gateway's resource policies and integration with IAM, Cognito, and Lambda authorizers provide much more flexibility than what ELB offers.
When to Choose Elastic Load Balancer
1. High-Volume Web Applications
For a high-traffic e-commerce platform I worked with, we chose Application Load Balancer (ALB) to distribute traffic across dozens of EC2 instances. ELB was the better choice because it handled millions of requests per hour without the per-request pricing model of API Gateway.
2. Traditional Multi-Tier Applications
If you have traditional web applications running on EC2 instances or containers:
Client → Application Load Balancer → EC2 instances/ECS containers → Database layer
3. TCP/UDP Traffic or Pure WebSocket Applications
Network Load Balancer (NLB) is ideal when you need to handle non-HTTP traffic or when extreme performance and low latency are crucial.
4. When You Need SSL Termination for Web Applications
Application Load Balancer can handle SSL termination for your web applications, reducing the compute burden on your servers.
Using Both Together: A Layered Approach
In many real-world architectures, I've implemented both services together to get the best of both worlds:
Internet → API Gateway (for API management) → ALB (for load distribution) → EC2/ECS/EKS
This approach makes sense when:
You need API management features but have non-Lambda backends: For example, a media streaming company can use API Gateway for authentication and rate limiting but have their video processing running on EC2 instances behind an ALB.
You have a mix of serverless and container/VM workloads: API Gateway can route different API paths to different backends - some to Lambda and others to your ALB-fronted services.
# Example architecture
/api/users/* → Lambda functions
/api/content/* → ALB → Container services
You want to gradually migrate from EC2 to serverless: Starting with an ALB and gradually moving some endpoints to API Gateway + Lambda can be a practical migration strategy.
Practical Decision Framework
When deciding between the two, ask yourself these questions:
Are you primarily building APIs? If yes, start with API Gateway.
Is your application traditional web-based or a mix of HTTP and non-HTTP protocols? If yes, start with ELB.
Is cost a major concern with high traffic volume? If yes, ELB might be more economical as API Gateway charges per request.
Do you need fine-grained API controls? If yes, API Gateway provides more API-specific features.
Is your primary concern distributing load across a fleet of servers? If yes, ELB is purpose-built for this.
Performance and Cost Considerations
API Gateway
Latency: Adds a small overhead (typically 10-50ms)
Cost: Pay per request (around $3.50 per million requests) + data transfer
Scaling: Handles millions of requests without configuration
Elastic Load Balancer
Latency: Very low added latency (single-digit ms)
Cost: Hourly charge + LCU (Load Balancer Capacity Units) + data transfer
Scaling: Scales automatically but may require pre-warming for sudden traffic spikes
Conclusion
In my years working with AWS architectures, I've found that the choice between API Gateway and ELB isn't always either/or. Often, the best architectures leverage both services for what they do best.
API Gateway excels at API management, security, and serverless integrations, while ELB provides cost-effective, high-performance load distribution for your compute resources.
Understanding these differences has helped me design more efficient, scalable, and cost-effective cloud architectures for clients across various industries. I hope these practical insights help you make the right choice for your specific use cases.