Grokking the System Design Interview
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Load Balancer vs. API Gateway

Load Balancer and API Gateway are two crucial components in modern web architectures, often used to manage incoming traffic and requests to web applications. While they have some overlapping functionalities, their primary purposes and use cases are distinct.

Load Balancer

  • Purpose: A Load Balancer is primarily used to distribute network or application traffic across multiple servers. This distribution helps to optimize resource use, maximize throughput, reduce response time, and ensure reliability.
  • How It Works: It accepts incoming requests and then routes them to one of several backend servers based on factors like the number of current connections, server response times, or server health.
  • Types: There are different types of load balancers, such as hardware-based or software-based, and they can operate at various layers of the OSI model (Layer 4 - transport level or Layer 7 - application level).

Example of Load Balancer:

Imagine an e-commerce website experiencing high volumes of traffic. A load balancer sits in front of the website’s servers and evenly distributes incoming user requests to prevent any single server from becoming overloaded. This setup increases the website's capacity and reliability, ensuring all users have a smooth experience.

API Gateway

  • Purpose: An API Gateway is an API management tool that sits between a client and a collection of backend services. It acts as a reverse proxy to route requests, simplify the API, and aggregate the results from various services.
  • Functionality: The API Gateway can handle a variety of tasks, including request routing, API composition, rate limiting, authentication, and authorization.
  • Usage: Commonly used in microservices architectures to provide a unified interface to a set of microservices, making it easier for clients to consume the services.

Example of API Gateway:

Consider a mobile banking application that needs to interact with different services like account details, transaction history, and currency exchange rates. An API Gateway sits between the app and these services. When the app requests user account information, the Gateway routes this request to the appropriate service, handles authentication, aggregates data from different services if needed, and returns a consolidated response to the app.

Key Differences:

  • Focus: Load balancers are focused on distributing traffic to prevent overloading servers and ensure high availability and redundancy. API Gateways are more about providing a central point for managing, securing, and routing API calls.
  • Functionality: While both can route requests, the API Gateway offers more functionalities like API transformation, composition, and security.

Is it Possible to Use a Load Balancer and an API Gateway Together?

Yes, you can use a Load Balancer and an API Gateway together in a system architecture, and they often complement each other in managing traffic and providing efficient service delivery. The typical arrangement is to place the Load Balancer in front of the API Gateway, but the actual setup can vary based on specific requirements and architecture decisions. Here’s how they can work together:

Load Balancer Before API Gateway

  • Most Common Setup: The Load Balancer is placed in front of the API Gateway. This is the typical configuration in many architectures.
  • Functionality: The Load Balancer distributes incoming traffic across multiple instances of the API Gateway, ensuring that no single gateway instance becomes a bottleneck.
  • Benefits:
    • High Availability: This setup enhances the availability and reliability of the API Gateway.
    • Scalability: Facilitates horizontal scaling of API Gateway instances.
  • Example: In a cloud-based microservices architecture, external traffic first hits the Load Balancer, which then routes requests to one of the several API Gateway instances. The chosen API Gateway instance then processes the request, communicates with the appropriate microservices, and returns the response.

Load Balancer After API Gateway

  • Alternative Configuration: In some cases, the API Gateway can be placed in front of the Load Balancer, especially when the Load Balancer is used to distribute traffic to various microservices or backend services.
  • Functionality: The API Gateway first processes and routes the request to an internal Load Balancer, which then distributes the request to the appropriate service instances.
  • Use Case: Useful when different services behind the API Gateway require their own load balancing logic.

Combination of Both

  • Hybrid Approach: Some architectures might have Load Balancers at both ends – before and after the API Gateway.
  • Reasoning: External traffic is first balanced across API Gateway instances for initial processing (authentication, rate limiting, etc.), and then further balanced among microservices or backend services.


In a complex web architecture:

  • A Load Balancer would be used to distribute incoming traffic across multiple servers or services, enhancing performance and reliability.
  • An API Gateway would be the entry point for clients to interact with your backend APIs or microservices, providing a unified interface, handling various cross-cutting concerns, and reducing the complexity for the client applications.

In many real-world architectures, both of these components work together, where the Load Balancer effectively manages traffic across multiple instances of API Gateways or directly to services, depending on the setup.

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