Building Simple API Gateways with Ocelot and ASP.net Core

In the previous article, we've discovered what is an API gateway and the cool things that it brings. This article will demonstrate how to build a simple API gateway that routes incoming HTTP requests to the appropriate downstream services.

Articles in the Series

This article belongs to a series of articles that explains the importance of API gateways and how to build them using ASP.net Core. If you're interested to learn more about API gateways, it might be a good idea to spend some time reading the articles listed below.

Part 1: API Gateway in a Nutshell.
Part 2: Building Simple API Gateways with Ocelot.
Part 3: API Response Aggregation using Ocelot
Part 4: API Defense using Rate Limiting and Ocelot.
Part 5: Containerizing API Gateways
Part 6: Containerizing API Gateways using Alpine Base Image

Introducing Ocelot

Download POC from Github

Ocelot is an open source framework used for building .NET core API gateways, the project is aimed at people using .NET / .NET Core to build applications designed with microservices or SOA architectures. Ocelot provides an easy way to write a mapping file (ocelot.json) that could be used to route incoming HTTP requests to the appropriate downstream services.

See: GITHUB | Official Website

Battle Plan

In this guide, we'll build an API gateway for an e-commerce website. The e-commerce API will contain three sub-domains that we'll call "Authentication", "Catalog" and "Ledger" contexts. The gateway we'll build in this article will behave like a reverse proxy that routes incoming HTTP requests to the following downstream services:

Authentication Service - Bounded context in-charge of managing all authentication related things like end-users and access to system.
Catalog Service - Bounded context in-charge of managing all offered products.
Ledger Service - Bounded context in-charge of managing all monetary-product transactions in the system.

Setup

If you want to skip the setup phase, you can download a bunch of bash scripts that I wrote to automate the generation of the four ASP.net core projects. Once you've downloaded the scripts, place them in a folder to group the output of the automation process and execute the script named 100_build-api-gateway.sh.

If you are a newbie in ASP.net Core or just keen to learn how the setup works, you can follow the steps below:

Generate 4 ASP.net Core Projects
Generate Solution and Attach Projects
Install Ocelot Nuget Package
Generate POCOs and Controllers
You can navigate to a sample project from GITHUB and copy the following models and controllers to your solutions:
- User Model
  Path: ./Authentication/Models/User.cs
  Github Link
- User Controller
  Path: ./Authentication/Controllers/UserController.cs
  Github Link
- Product Model
  Path: ./Catalog/Models/Product.cs
  Github Link
- Project Controller
  Path: ./Catalog/Controllers/ProductController.cs
  Github Link
- Transaction Model
  Path: ./Ledger/Models/Transaction.cs
  Github Link
- Transaction Contorller
  Path: ./Ledger/Controllers/TransactionController.cs
  Github Link
Configure Service Ports via launchSettings.json
In order to route HTTP requests properly, you have to ensure that each service will be able to run on the following local ports:
- Authentication: http://localhost:52792/
- Catalog: http://localhost:52791
- Ledger: http://localhost:52790
- Gateway: http://localhost:52793
Start and test individual bounded context services
Test the solutions by running each of them and issuing a GET request against the following URLS:
- Authentication: http://localhost:52792/api/user
- Catalog: http://localhost:52791/api/product/
- Ledger: http://localhost:52790/api/transaction
NOTE: You can configure a solution to run multiple services at the same time by following this awesome guide:

Configuring Ocelot

Now lets focus on the most important detail which is configuring Ocelot. The first thing that we have to do is configure the ocelot.json file and add the code below. Note that the BaseUrl is required by Ocelot for it to be aware of the URL it is running in order to perform a header find & replace for certain administrative configurations.

Navigate to the Program.cs file of the Gateway project and add the following configurations on the web host builder method. The code below will setup all the middlewares required by ocelot to function.

Setting up your Routes

Now that everything we need in the gateway side is configured, its time to setup routes using the ocelot.json file. Update your ocelot.json file by updating the re-routes section. The code above setups up the mappings dictionary of (see below) your application gateway to the downstream services.

Gateway to Authentication
Gateway to Catalog
Gateway to Ledger

Testing the routes out

To test the routes, start all projects and navigate to each link listed below.

User Endpoint http://localhost:52793/api/user
Products Endpoint http://localhost:52793/api/product
Transaction Endpoint http://localhost:52793/api/transaction

Conclusion

In this article, we've learned how to perform basic re-routing of HTTP requests from a gateway to downstream services using Ocelot. Please do note that the article only explained basic re-routing and the power of API gateways starts to shine with request aggregation which is the subject of the next article.

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