Implementing API Gateways with Ocelot for .Net Core 3.1


When developing Microservice or serverless based architecture models , we might want to use API Gateway to provide API to external users. This solution will allow us to independently evolve our internal architecture since services/functions won’t be exposed anymore.

Ocelot is an API Gateway based on the .NET Core framework and a rich set of features including:

· Request Aggregation

· WebSockets support

· Rate Limiting

· Load Balancing

· Configuration / Administration REST API

· QoS using Consul and Polly

· Distributed Tracing

· Authentication/Authorization

Ocelot requires to provide configuration file, that has a list of ReRoutes (configuration used to map upstream request) and Global Configuration (other configuration like QoS, Rate limiting, etc.).

Prerequisites :

. Docker Desktop (For windows)

.Net core framework ( we are using VS 2019 )

.Working Docker Image of your Microservices in an image repository somewhere. — I’ll be using an image on local machine.

Step 1: Create a containerize image of microservices:

i have already created docker image & container on my machine as below :

I will be using employeeservice & detailservice as microservices running on containers at port 8000 & 8001 as below :

Step 2: Create an APIGateway project :

Create an APIGateway ASP.NET Core Empty project and install Ocelot package at first using below command :

Install-Package Ocelot

After installing this package, you can find some dependence on it.

Step 3: Add a ocelot.json file :

Add an ocelot.json file in solution and delete the contents and add below codes :

This file is the configuration of the API Gateway. There are two sections to the configuration- an array of ReRoutes and a GlobalConfiguration.

The ReRoutes are the objects that tell Ocelot how to treat an upstream request. The Global configuration is a bit hacky and allows overrides of ReRoute specific settings.

Take this part to explain the ReRoutes section.

“DownstreamPathTemplate”: “/api/Employee”,
“DownstreamScheme”: “http”,
“DownstreamHostAndPorts”: [
“Host”: “localhost”,
“Port”: 8000
“UpstreamPathTemplate”: “/api/Employee”,
“UpstreamHttpMethod”: [ “Get” ]

The items start with Downstream which means that our request will be forwarded to http://localhost:8000/api/Employee.

The items start with Upstream which means that we should use HTTP GET method with /Employee to visit this service.

Step 4: Edit the Program.cs class so that we can use Ocelot in this project :

Step 5: Run your API Gateway:

As you can see, our API Gateway is running on http://localhost:5000

Step 6: Access containerize image service from your API Gateway:

We open the browser to visit our services as below :

When visiting http://localhost:5000/api/Employee, we will get the result from http://localhost:8000/api/Employee .

When visiting http://localhost:5000/Detail, we will get the result from http://localhost:8001/api/Detail .

Hence our job is done !!!


In this article, We have looked at how to implement APIGateway while implementing Microservice Architecture.

By the way, this is a very easy demo; there are some important things I did not mention in this article, such as Service discovery, Authentication, and Quality of Service. I will cover remaining feature in my upcoming article.

Has this article been useful to you? please share extensively, we also welcome feedback on content you would like us to cover .

Software Consultant interested in Microservices / Serverless computing, Middleware / SOA, Event Driven Architecture & Machine Learning.