Implementing API Gateway Design Pattern

In modern microservice architectures, managing communication between clients and multiple backend services can quickly become complex. The API Gateway design pattern addresses this by introducing a single entry point for all client requests. This gateway routes, aggregates, transforms, and secures calls to underlying microservices.

My previous post, explored the API Gateway pattern, its benefits, how it works and many more. Here in this post, we’ll explore how to implement it effectively in a .NET application.

Implementing API Gateway Design Pattern

Getting Started

Implementing an API Gateway pattern in a .NET application involves creating a single entry point (the gateway) that routes requests to multiple backend microservices. This pattern helps with centralizing cross-cutting concerns like authentication, logging, throttling, and load balancing.

Implementing an API Gateway in .NET

Here’s a step-by-step guide to implementing it in .NET:

1. Create a new webapi project for the API Gateway
2. Choose an API Gateway package
   • Ocelot– A lightweight, popular open-source API Gateway built specifically for .NET.
   • YARP (Yet Another Reverse Proxy) – A Microsoft-developed, highly customizable reverse proxy library.
   • AspNetCore.ApiGateway - A newer .NET library targeting .NET 6/7/8 for building your own API Gateway with features. nuget.org
   • Build a Custom API Gateway-You can use ASP.NET Core middleware and routing to manually implement the pattern.
3. Install one of the package from Nuget package Manager
4. Configure Package in Program.cs, below example of Ocelot configuration

   using Ocelot.DependencyInjection;
   using Ocelot.Middleware;
   var builder = WebApplication.CreateBuilder(args);
   builder.Configuration.AddJsonFile("ocelot.json", optional: false, reloadOnChange: true);
   builder.Services.AddOcelot(builder.Configuration);
   var app = builder.Build();
   await app.UseOcelot();
   app.Run();
   

5. Define Routes in appsettings.json
   
   Here’s an example configuration for two backend services — UserService and OrderService:

   {
     "Routes": [
       {
         "DownstreamPathTemplate": "/api/users/{everything}",
         "DownstreamScheme": "https",
         "DownstreamHostAndPorts": [
         { "Host": "localhost", "Port": 7001 }
       ],
       "UpstreamPathTemplate": "/users/{everything}",
       "UpstreamHttpMethod": [ "Get", "Post", "Put", "Delete" ]
       },
       {
         "DownstreamPathTemplate": "/api/orders/{everything}",
         "DownstreamScheme": "https",
         "DownstreamHostAndPorts": [
         { "Host": "localhost", "Port": 7002 }
       ],
       "UpstreamPathTemplate": "/orders/{everything}",
       "UpstreamHttpMethod": [ "Get", "Post" ]
     }
   ],
   "GlobalConfiguration": {
     "BaseUrl": "https://localhost:5000"
   }
   }
   

   Now, when a client sends a request to:
   GET https://localhost:5000/users/123 Ocelot automatically routes it to:
   https://localhost:7001/api/users/123
6. Add Cross-Cutting Features
   • Authentication: Integrate with JWT tokens or IdentityServer.
   • Rate limiting: Use Ocelot’s built-in throttling middleware.
   • Caching: Add response caching for high-traffic endpoints.
   • Logging & Tracing: Use Serilog, OpenTelemetry, or Application Insights.
7. Deploying the Gateway

Best Practices

• Keep gateway logic lightweight — don’t add heavy business logic.
• Use API versioning to manage backward compatibility.
• Apply circuit breakers (e.g., Polly) to handle downstream failures.
• Monitor gateway metrics (latency, failure rates, request volume).

Summary

The API Gateway design pattern is essential for managing complex .NET microservice systems efficiently. By using tools like Ocelot or YARP, developers can implement robust gateways that handle routing, security, and observability — all while providing a simplified interface for clients.

Thanks