API Gateway Overview

API Gateway Overview

An API Gateway is a central entry point for all client requests to your backend services. It acts as a reverse proxy, routing requests to the appropriate microservice while handling cross-cutting concerns like security, rate limiting, and monitoring. This abstracts the complexity of a microservice architecture from the client.

In .NET, a popular open-source option for implementing an API Gateway is Ocelot. Microsoft also offers YARP (Yet Another Reverse Proxy), a project that provides high-performance reverse proxying capabilities.

Example: API Gateway with Ocelot in .NET Core

This example demonstrates how to set up a simple API gateway using Ocelot to route requests to two different microservices.

1. Create a Blank Solution
Start by creating a new, empty solution in Visual Studio.

2. Create the Microservices
Create two new ASP.NET Core Web API projects named UserService and ProductService.

UserService (Controllers/UserController.cs)

using Microsoft.AspNetCore.Mvc;

namespace UserService.Controllers;

[ApiController]
[Route("api/[controller]")]
public class UserController : ControllerBase
{
    [HttpGet]
    public IActionResult Get()
    {
        return Ok("Hello from the User Service!");
    }
}

Use code with caution.

ProductService (Controllers/ProductController.cs)

using Microsoft.AspNetCore.Mvc;

namespace ProductService.Controllers;

[ApiController]
[Route("api/[controller]")]
public class ProductController : ControllerBase
{
    [HttpGet]
    public IActionResult Get()
    {
        return Ok("Hello from the Product Service!");
    }
}

Use code with caution.

3. Create the API Gateway Project
Create a new, empty ASP.NET Core Web project named ApiGateway.

4. Install Ocelot NuGet Package
In the ApiGateway project, install the Ocelot package via the NuGet Package Manager:

Install-Package Ocelot

5. Configure the Ocelot Gateway
Create a new file named ocelot.json in the root of the ApiGateway project. This is the central configuration file that tells Ocelot how to route requests. Remember to configure this file to be copied to the output directory.

ocelot.json

{
  "Routes": [
    // Route for the UserService
    {
      "UpstreamPathTemplate": "/gateway/users",
      "UpstreamHttpMethod": [ "GET" ],
      "DownstreamPathTemplate": "/api/user",
      "DownstreamScheme": "http",
      "DownstreamHostAndPorts": [
        {
          "Host": "localhost",
          "Port": 5001 // Adjust to your UserService port
        }
      ]
    },
    // Route for the ProductService
    {
      "UpstreamPathTemplate": "/gateway/products",
      "UpstreamHttpMethod": [ "GET" ],
      "DownstreamPathTemplate": "/api/product",
      "DownstreamScheme": "http",
      "DownstreamHostAndPorts": [
        {
          "Host": "localhost",
          "Port": 5002 // Adjust to your ProductService port
        }
      ]
    }
  ],
  "GlobalConfiguration": {
    "RateLimitOptions": {
      "DisableRateLimitHeaders": false,
      "QuotaExceededMessage": "Too many requests. Please try again later.",
      "HttpStatusCode": 429
    }
  }
}

Use code with caution.

6. Update Program.cs in the ApiGateway
Modify the Program.cs file to configure Ocelot.

Program.cs

using Ocelot.DependencyInjection;
using Ocelot.Middleware;

var builder = WebApplication.CreateBuilder(args);

// Add Ocelot configuration
builder.Configuration.AddJsonFile("ocelot.json", optional: false, reloadOnChange: true);
builder.Services.AddOcelot(builder.Configuration);

var app = builder.Build();

app.UseRouting();
app.UseAuthorization();

// Use Ocelot Middleware
await app.UseOcelot();

app.Run();

Use code with caution.

7. Run the Application
Start all three projects (UserService, ProductService, and ApiGateway). Make sure the ports in launchSettings.json for each microservice match the ocelot.json configuration.

Access the user service through the gateway:
http://localhost:<ApiGatewayPort>/gateway/users

Access the product service through the gateway:
http://localhost:<ApiGatewayPort>/gateway/products

✅ Advantages of API Gateway

• 🔐 Centralized authentication and authorization
• 📊 Rate limiting and Throttling
• 📦 Request aggregation (combine multiple service responses)
• 🧭 Routing and load balancing
• 🧼 Simplifies client-side logic
• 🧪 Logging, monitoring, and analytics

Centralized management and configuration: Manage cross-cutting concerns like security, monitoring, and rate limiting from a single point, rather than repeating the logic in every microservice.

Decouples clients from microservices: Clients only need to know the API gateway's single endpoint. This allows you to refactor or change your backend microservices without breaking client applications.

Optimal API for each client: Implement the "Backends for Frontends" pattern by creating different API gateways tailored to specific client needs, such as a mobile app or a web application.

Reduced chatty communication: Aggregate multiple downstream microservice requests into a single client request, reducing the number of round trips and improving performance, especially for mobile clients.

Improved security: Gateways act as a strong perimeter defense, handling authentication, authorization, and protocol translation so that internal services are not exposed publicly.

Load balancing and throttling: Automatically balance loads across service instances and protect backend services from being overwhelmed by excessive requests.

🧭 When to Use

• You have multiple microservices and want a unified entry point
• You need centralized security and rate limiting
• You want to hide internal service structure from clients
• You need to aggregate data from multiple services

Microservice architecture: For systems with multiple independent services, an API gateway is essential for managing the complexity of communication.

Publicly exposing services: When you need to provide a controlled, secure interface for external clients to access your backend services.

Backends for frontends: When supporting multiple client types (e.g., web, mobile) that require different data or behaviors.

Centralized security and monitoring: When you need a single place to enforce security policies, monitor traffic, and collect logs.

Complex client interaction: When a single user action requires data from multiple microservices, and you want to reduce the client's burden of making multiple network calls.

🚫 When Not to Use

• For simple monolithic applications
• When latency is critical and you want direct service access
• If you don’t need centralized control or routing
• When your services are already exposed securely and independently

Small, monolithic applications: For simple applications with minimal complexity, a gateway can introduce unnecessary overhead and an additional point of failure.

Internal service-to-service communication: It is generally not recommended to route internal microservice communication through the gateway. For inter-service communication, a service mesh may be a more appropriate solution.

High-performance, low-latency applications: While gateways are typically fast, they do add an extra network hop. In performance-critical applications where every millisecond counts, the extra hop might be unacceptable.

Limited resources or team expertise: An API gateway adds another component to manage, deploy, and monitor. For small teams or projects with limited resources, the added operational complexity may outweigh the benefits.

🛠️ Best Practices

• 🔐 Use JWT or OAuth2 for authentication
• 📊 Monitor performance and latency of gateway
• 🧪 Validate and sanitize incoming requests
• 🧬 Version your APIs to avoid breaking changes
• 🔄 Implement caching for frequently accessed endpoints
• 🧹 Avoid overloading the gateway with business logic

Keep it lightweight: The API gateway should focus on routing, security, and composition. Avoid putting complex business logic into the gateway, as this can turn it into a distributed monolith.

Centralize cross-cutting concerns: Use the gateway to handle authentication, authorization, SSL termination, and rate limiting. This offloads these tasks from individual microservices.

Implement caching: Cache frequently accessed data at the gateway level to reduce load on backend services and improve response times.

Use circuit breakers: Configure a circuit breaker pattern so that if a backend service fails, the gateway can automatically handle the failure gracefully (e.g., return a default response) instead of causing a cascade of failures.

Ensure high availability: To prevent the gateway from becoming a single point of failure, deploy it in a highly available, load-balanced configuration.

Decouple and manage configuration: Externalize the routing configuration (like ocelot.json) from the gateway's code. This allows you to add or change routes without redeploying the gateway.

Monitor and log everything: Implement centralized logging and monitoring to gain a clear view of traffic patterns, API health, and performance. This is critical for debugging and observability.

⚠️ Precautions

• 🐢 Gateway can become a bottleneck if not scaled properly
• 🔄 Ensure failover and retry mechanisms
• 🧱 Avoid tight coupling between gateway and services
• 🧯 Monitor for cascading failures