API Gateway vs. Load Balancer — What’s the Difference?
When a client sends a request to an application, the request may pass through several layers before reaching the actual application instance.
Let’s understand the typical flow step by step.
1. The client sends a request
For example, imagine a shopping application:
amazon.com/products
The client generally knows only the public URL. It does not need to know which service or server will handle the request.
2. DNS resolves the domain name
A DNS service such as AWS Route 53 helps resolve a domain name to the appropriate endpoint, such as an IP address or another domain name.
For example:
amazon.com → appropriate public endpoint
Important: Route 53 is a DNS service. It is not responsible for routing an individual API request between microservices.
3. The request reaches the API Gateway
The API Gateway acts as an entry point for APIs.
Its responsibilities can include:
- Routing requests to the appropriate service
- Authentication and authorization
- Rate limiting/throttling
- Request validation
- Logging and monitoring
- Request/response transformation
- Sometimes caching
- Sometimes aggregating responses from multiple backend services
For example:
GET /products → Product Service
POST /users → User Service
POST /purchase → Purchase Service
POST /payment → Payment Service
4. What does a shopping application look like?
A shopping application might have different services such as:
- User Service
- Authentication/Login Service
- Product Service
- Purchase/Order Service
- Payment Service
- Purchase History Service
Each service may have multiple instances running for scalability and availability.
For example:
Product Service
→ Instance 1
→ Instance 2
→ Instance 3
This is where the Load Balancer becomes important.
5. The Load Balancer distributes traffic
Once the API Gateway determines which backend service should handle the request, the request may go through a Load Balancer.
The Load Balancer distributes requests among the available healthy instances of that service.
For example:
Product Service
→ Load Balancer
→ Instance 1
→ Instance 2
→ Instance 3
If Instance 1 becomes unhealthy, the Load Balancer can stop sending new requests to that instance and continue sending traffic to healthy instances.
6. How does the Load Balancer know which instance is healthy?
A Load Balancer typically performs health checks on the instances.
The Load Balancer can then route traffic only to the healthy instances.
Also, the traffic does not necessarily have to be distributed equally. The distribution depends on the load-balancing algorithm and configuration.
7. What is rate limiting?
API Gateway can apply rate limiting or throttling.
Rate limiting means restricting the number of requests a client can make within a particular time period.
For example:
100 requests per minute
If a client exceeds the configured limit, additional requests may be throttled or rejected.
8. What about authentication?
The API Gateway can also participate in authentication and authorization.
For example, it can validate an access token before allowing the request to reach the backend service.
If the request is not authorized, the API Gateway can reject it before it reaches the application.
However, authentication and authorization can also be handled by dedicated identity services and/or by the application itself. It is not always the API Gateway's responsibility alone.
9. API Gateway can also provide caching
Depending on the technology and architecture, caching can be used to improve response time and reduce the number of requests reaching backend services.
For example, in an AWS architecture, Amazon CloudFront can cache content at edge locations.
However, CloudFront is not the same thing as API Gateway. They are separate services that can work together.
10. What happens when the response comes back?
The backend service processes the request and sends the response back through the infrastructure.
In some architectures, an API Gateway can also aggregate responses from multiple backend services.
For example, a single client request might require information from:
Product Service + Inventory Service + Review Service
The gateway or an aggregation layer can combine those responses and return a single response to the client.
This pattern is sometimes implemented using a Backend-for-Frontend (BFF) or another dedicated aggregation layer.
11. Logging and monitoring
API Gateway and other components can generate logs and metrics for monitoring and troubleshooting.
In AWS, Amazon CloudWatch can be used to collect and monitor logs and metrics from various services.
12. What happens if the API Gateway fails?
The API Gateway can become a single point of failure if it is deployed as a single instance.
So, if the API Gateway fails, the system may become unavailable.
To avoid this, multiple API Gateway instances can be deployed with a load balancer in front of them.
13. So, what is the actual difference?
The easiest way to remember it is:
API Gateway → "Where should this API request go, and is this request allowed?"
Load Balancer → "Which healthy instance should handle this request?"
The exact architecture can vary, and sometimes the load balancer may appear before the API Gateway or be integrated into a managed gateway service.
14. Is a Load Balancer part of an API Gateway?
Not necessarily.
This is an important distinction.
An API Gateway and a Load Balancer are different architectural components with different primary responsibilities.
Some managed API gateway products may internally use load-balancing mechanisms, but from an architectural perspective, they are different, API Gateway and Load Balancer can work together.
15. What about a monolithic application?
In a traditional monolithic application, an API Gateway may not be necessary.
The application may look more like:
Client
↓
Load Balancer
↓
Application Instance 1 / 2 / 3
The application itself contains the business logic.
In a microservices architecture, an API Gateway can provide a common entry point and handle cross-cutting concerns such as routing, authentication, rate limiting, and monitoring.
The business logic should generally remain in the services, rather than being placed in the API Gateway.
The simple way to remember
API Gateway = Routing + Protection + API Management
Load Balancer = Traffic Distribution + Health-Based Routing
They solve different problems, but they can work together to build a scalable and highly available application.
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