Building an API Gateway with Ocelot in .NET

Table of Contents

• Introduction to API Gateway and Ocelot
• Why Use Ocelot as an API Gateway in .NET?
• Setting Up Ocelot in .NET Core
• Configuring Ocelot for API Gateway
• Routing, Security, and Load Balancing with Ocelot
• Code Example: Building an API Gateway with Ocelot
• Common Issues and Troubleshooting
• Conclusion

Introduction to API Gateway and Ocelot

In a microservices architecture, an API Gateway acts as a single entry point for all client requests to the various services in the system. It simplifies client interactions, centralizes security, and facilitates routing and load balancing. Ocelot is an open-source API Gateway built on .NET Core that provides a simple way to manage and configure APIs for microservices.

Why Use Ocelot as an API Gateway in .NET?

Ocelot is a lightweight, fast, and scalable API Gateway solution for .NET applications. Key benefits include:

• Easy to Configure: Ocelot is easy to set up in .NET Core applications with minimal configuration required.
• Flexible Routing: It supports dynamic routing rules, allowing flexible redirection of requests to different services.
• Load Balancing: Ocelot provides simple round-robin load balancing and supports other strategies for scaling your application.
• Security Features: Integrates with OAuth, JWT, and other authentication schemes to secure your API endpoints.
• Cross-platform: Ocelot runs on all platforms supported by .NET Core, making it highly portable.

Setting Up Ocelot in .NET Core

Before you can start using Ocelot, you need to set up a .NET Core project and install the necessary NuGet packages.

1. Create a .NET Core Web API project: Open the terminal and run the following command:

   dotnet new webapi -n ApiGatewayExample

2. Install Ocelot NuGet Package: In the terminal, run:

   dotnet add package Ocelot

3. Configure Ocelot in Startup.cs: Add Ocelot's services and middleware to your Startup.cs file:

public void ConfigureServices(IServiceCollection services)
{
    services.AddOcelot();
}

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseOcelot().Wait();
}
                

Configuring Ocelot for API Gateway

Ocelot requires configuration through a JSON file (typically ocelot.json) to define routing, load balancing, and security. Below is an example configuration:

{
  "ReRoutes": [
    {
      "DownstreamPathTemplate": "/api/values",
      "DownstreamScheme": "http",
      "DownstreamHostAndPorts": [
        {
          "Host": "localhost",
          "Port": 5001
        }
      ],
      "UpstreamPathTemplate": "/gateway/values",
      "UpstreamHttpMethod": [ "Get" ]
    }
  ],
  "GlobalConfiguration": {
    "BaseUrl": "http://localhost:5000"
  }
}
            

This configuration sets up routing from the /gateway/values path to the downstream service running at localhost:5001/api/values.

Routing, Security, and Load Balancing with Ocelot

Ocelot allows you to configure dynamic routing, apply security measures, and manage load balancing easily.

Routing

Routing is set up using the DownstreamPathTemplate and UpstreamPathTemplate properties in the ocelot.json file. You can specify routing for different HTTP methods and endpoints.

Security

Ocelot supports various security mechanisms like JWT authentication, API key validation, and OAuth integration. For example, to secure an endpoint with JWT, add the following to your configuration:

{
  "ReRoutes": [
    {
      "UpstreamPathTemplate": "/secure/values",
      "UpstreamHttpMethod": [ "Get" ],
      "AuthenticationOptions": {
        "AuthenticationProviderKey": "Bearer",
        "AllowedScopes": [ "api1" ]
      }
    }
  ]
}
            

Load Balancing

Ocelot supports load balancing strategies like round-robin for distributing requests between multiple downstream services. This is defined in the DownstreamHostAndPorts section.

Code Example: Building an API Gateway with Ocelot

Step-by-Step Example

// Startup.cs
public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddOcelot();
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        app.UseOcelot().Wait();
    }
}

// ocelot.json
{
  "ReRoutes": [
    {
      "DownstreamPathTemplate": "/api/values",
      "DownstreamScheme": "http",
      "DownstreamHostAndPorts": [
        {
          "Host": "localhost",
          "Port": 5001
        }
      ],
      "UpstreamPathTemplate": "/gateway/values",
      "UpstreamHttpMethod": [ "Get" ]
    }
  ],
  "GlobalConfiguration": {
    "BaseUrl": "http://localhost:5000"
  }
}
            

This is a basic example of using Ocelot to route requests to a downstream service running at localhost:5001.

Common Issues and Troubleshooting

While setting up Ocelot, you may encounter issues such as:

• Invalid Configuration: Ensure the paths in the configuration file are correct and match the downstream service endpoints.
• Authentication Failures: Double-check your security configuration and token validation.
• Performance Issues: Ocelot is lightweight, but consider optimizing your network and load balancing settings if facing high traffic.

Conclusion

Building an API Gateway with Ocelot in .NET provides a flexible, secure, and scalable way to manage API requests for microservices architectures. With its easy configuration, support for routing, load balancing, and security, Ocelot is a powerful tool for modern cloud-based applications.

© 2025 Sandeep Mhaske | Ayodhyya.com