ALB vs API Gateway: Choosing the Right Front Door for HTTP APIs#

When you’re building applications on AWS, you’ll quickly encounter a fundamental decision: which service should handle incoming HTTP traffic? Two obvious candidates emerge—Application Load Balancer (ALB) and API Gateway. Both sit at the edge of your infrastructure, both accept HTTP requests, and both can route traffic to your backend services. Yet they’re built for fundamentally different purposes, and choosing incorrectly can leave you struggling with missing features or paying for capabilities you don’t need.

This article cuts through the confusion by examining when to use each service, what makes them different beyond their surface similarities, and how to make the right choice for your specific workload. Whether you’re building microservices, mobile backends, or hybrid architectures, understanding these differences will shape how you design your infrastructure.

Understanding the Core Purpose of Each Service#

Before diving into features, it helps to understand the philosophical difference between these two services.

The Application Load Balancer is fundamentally a network load balancer optimized for Layer 7 (application layer) HTTP and HTTPS traffic. It evolved from the older Elastic Load Balancer and was purpose-built to distribute traffic across multiple targets—EC2 instances, ECS containers, Lambda functions, or IP addresses. Think of ALB as a sophisticated traffic cop that looks at HTTP headers, paths, and hostnames to decide where traffic should go. It’s excellent at distributing load and ensuring high availability across your backend infrastructure.

API Gateway, by contrast, is an AWS-managed service purpose-built for creating, publishing, and managing APIs. It’s not primarily about load balancing—it’s about API management. This distinction matters enormously. API Gateway sits between clients and your backend logic with a focus on API-specific concerns: request validation, throttling, authentication, transformation, and monetization through usage plans. If you’re building an API that external developers will consume, or if you need sophisticated API management features, API Gateway is in its element.

Layer 7 Routing and Path-Based Distribution#

Both services operate at Layer 7 and can make intelligent routing decisions based on HTTP characteristics, but they approach this differently.

ALB excels at simple, efficient routing rules. You can route based on hostnames—directing api.example.com to one target group and images.example.com to another. You can route based on URL paths—/admin/* traffic goes to one set of instances while /public/* goes elsewhere. You can even route based on HTTP headers, query parameters, or HTTP methods. These routing rules are lightweight and fast because they’re designed specifically for distributing load.

API Gateway takes a different approach. You define resources and methods within your API, then attach integrations to each endpoint. A GET request to /users/{id} could integrate with a Lambda function, while POST to /users could trigger an entirely different function. The routing is explicit and API-centric rather than pattern-based.

For simple, performance-critical load distribution scenarios, ALB’s approach is leaner. For API-specific routing with complex integration patterns, API Gateway’s resource-method model aligns better with how you think about your API.

Request Validation and Transformation#

Here’s where a significant capability gap emerges. If you care about validating requests before they reach your backend, API Gateway offers built-in validation powered by JSON Schema. You can specify that a request body must conform to a particular schema, and API Gateway will reject malformed requests before they consume your backend resources. This is valuable not just for protecting your backend but also for providing immediate, clear feedback to API clients.

API Gateway also includes request and response transformation capabilities. You can modify headers, map request parameters to integration parameters, or transform the response structure before it reaches the client. Imagine a backend service that returns data in one format, but your API contract promises a different shape. API Gateway can transform it without requiring custom code in your backend.

ALB doesn’t provide these capabilities. If you need request validation or transformation, that logic must live in your backend service. In many cases, this isn’t a limitation—it’s actually the right design. But when you’re managing many different API endpoints or want to enforce consistency across your API surface, having these capabilities at the gateway level is genuinely convenient.

Authentication and Authorization#

Both services can authenticate requests, but the mechanisms and sophistication differ.

API Gateway provides multiple built-in authorizer options. You can use AWS Identity and Access Management (IAM) authentication, making it natural for AWS service-to-service communication. You can implement request-based authorization using Lambda authorizers, where your custom code examines the request and decides whether to allow it. You can integrate with Amazon Cognito for user pool authentication. For APIs requiring API keys and usage plans, API Gateway handles all the infrastructure.

ALB handles authentication differently. It can integrate with Amazon Cognito or OpenID Connect providers to authenticate users, which is powerful for web applications. However, it doesn’t have native support for API key-based authentication or the same breadth of authorizer options. For service-to-service communication, you’d typically rely on security groups, network ACLs, or application-level authentication logic in your backend.

If you’re building a public API where different consumers might have different rate limits and quota allocations, API Gateway’s usage plans feature is invaluable. ALB simply wasn’t designed for this scenario.

Throttling and Rate Limiting#

API Gateway includes throttling as a first-class feature. You can set request rates at multiple levels: across your entire API, per API stage, or using usage plans to give different clients different rate limits. This is essential when you’re offering an API to external developers or need to protect your backend from traffic spikes.

ALB doesn’t have throttling built in. If you need to rate-limit traffic at the ALB level, you’d have to implement that logic in your backend or use a Web Application Firewall (WAF) rule. This isn’t necessarily bad—sometimes you want rate limiting to happen at the application level so you can make sophisticated decisions. But if you need it at the edge, you’ll need to engineer it separately.

Latency and Performance Characteristics#

For latency-sensitive workloads, this comparison gets concrete.

ALB typically introduces 5-30 milliseconds of latency, depending on how complex your routing rules are and where your targets are located. It’s designed to be a high-performance traffic distribution layer that gets out of the way.

API Gateway typically adds 30-100 milliseconds of latency, depending on the complexity of your integrations and authorizers. This is still acceptable for most use cases—your API endpoint might have dozens of milliseconds of backend latency anyway. But for ultra-low-latency applications where every millisecond matters, this difference is noticeable.

The difference exists because API Gateway performs more sophisticated processing: it validates requests against your schema, executes authorizers, applies throttling, potentially transforms the request, and logs detailed metrics. These features cost clock cycles.

Scaling and Capacity Management#

ALB uses a metric called Load Balancer Capacity Units (LCUs) to measure consumption. An LCU represents a combination of new connections, active connections, processed bytes, and rule evaluations per hour. You’re billed for LCUs plus an hourly charge for each ALB. This model works well when you have steady, predictable traffic patterns.

API Gateway charges per million requests plus data transfer costs. No hourly minimum. This model aligns well with variable traffic patterns, especially for APIs that have periods of inactivity. If your API receives sporadic traffic, the pay-per-request model is more economical.

For a sustained, high-volume workload—say, 1 million requests per hour consistently—you should calculate the costs for each service. ALB’s hourly charge plus LCU costs might be cheaper. For variable workloads with periods of low traffic, API Gateway’s pay-per-request model usually wins.

Integration Targets and Flexibility#

ALB can route to EC2 instances, ECS containers, Lambda functions, or IP addresses (useful for on-premises backends or other cloud providers). It’s truly flexible in where traffic can go.

API Gateway primarily integrates with AWS services: Lambda, EC2, ECS, HTTP endpoints, and various managed services like DynamoDB or SNS through service integrations. While you can integrate with HTTP endpoints—essentially any backend reachable via HTTP—the native integrations are AWS-focused.

If you’re running backend services in your own data center or another cloud provider, ALB is the more natural choice. If you’re all-in on AWS, both services offer excellent flexibility.

A Concrete Comparison Table#

To organize these differences clearly:

Real-World Scenarios: When to Choose Each#

Choose ALB when: You’re distributing traffic across a fleet of EC2 instances or ECS services running a traditional application. You need low latency and your traffic is predictable and steady. You’re routing to multiple backend services based on path or hostname. You don’t need API-specific features like request validation or throttling. Your backend logic handles authentication and authorization.

A concrete example: You’re running a web application with three microservices—user service, product service, and order service—each deployed on ECS. An ALB can route /users/* to the user service, /products/* to the product service, and /orders/* to the order service. The ALB distributes traffic across multiple containers within each service for high availability.

Choose API Gateway when: You’re building an API for external consumption, whether public or restricted to specific partners. You need to impose rate limits and quotas on different consumers. You want built-in request validation to reject malformed requests early. You’re integrating primarily with Lambda functions. You need to transform request or response formats. You benefit from the pay-per-request pricing model because your traffic is variable.

A concrete example: You’re building a SaaS product where customers integrate via REST API. Different subscription tiers should have different rate limits. You want to validate that incoming requests conform to your OpenAPI specification. You run most logic in Lambda. API Gateway is purpose-built for this scenario.

The hybrid approach: Many sophisticated architectures use both. API Gateway sits in front of public-facing APIs with all the API management features, while ALB distributes traffic among backend services. Or ALB fronts your web application while a separate API Gateway instance manages your API endpoints. They’re complementary, not competitors, in many architectures.

Making Your Decision#

When you’re faced with choosing between these services, ask yourself a series of questions:

First, what’s the nature of your traffic? Is it external-facing API traffic or internal application traffic?

Second, do you need API management features like request validation, throttling, or API keys? If yes, API Gateway is strongly indicated.

Third, what are your latency requirements? If you’re building real-time applications where every millisecond matters, ALB’s lower latency overhead is significant.

Fourth, what’s your traffic pattern? Steady and predictable favors ALB’s pricing. Variable and bursty favors API Gateway’s pay-per-request model.

Fifth, where does your backend live? Exclusively on AWS generally makes API Gateway more convenient. Running on-premises or multi-cloud makes ALB more flexible.

Sixth, how many different backend services need routing? Simple binary choices (route to service A or B) favor ALB. Complex API surfaces with dozens of endpoints favor API Gateway.

The correct answer is rarely “it doesn’t matter.” These services are specialized for different problems, and matching the service to the problem is how you build elegant, cost-effective infrastructure.

Conclusion#

ALB and API Gateway both expose HTTP endpoints to the internet, which is why they seem similar at first glance. But they emerge from different design philosophies. ALB is a load balancer that distributes traffic efficiently across backend targets. API Gateway is an API management platform that sits in front of your backend with features purpose-built for API providers.

The choice between them hinges on whether you’re solving a load distribution problem or an API management problem. In practice, many organizations find they need both, deployed for their respective strengths. Understanding the distinct capabilities of each—from request validation and throttling to latency and pricing characteristics—lets you architect confident, efficient solutions that scale with your needs.