For the last 2 days I have been struggling with a breaking change I had in my ASP.NET Core web api that caused the consuming application to fail. I had a header parameter that was optional but became required after changing the nullability of my project to enabled.
Although I found the issue and was able to fix it quite fast, I was not happy with my current process and was wondering how I could prevent this from happening again.
This brought me to a final solution where I introduced some extra tests that compared the OpenAPI metadata between different implementations. Let me show you how I did it…
Generate OpenAPI documents at build time
To compare 2 OpenAPI metadata documents we first need to get them. For the already released version of the API, I can download the document through the OpenAPI endpoint (/openapi/v1.json by default). But what about the new version of the API that is still in development?
I solve this by generating the OpenAPI document at build time. This makes it easy to use and compare the OpenAPI documents without the need of hosting our OpenAPI.
Enabling build time generation is easy. We only need to add the following NuGet package to our project:
dotnet add package Microsoft.Extensions.ApiDescription.Server
Now if we build our application, it will:
- Automatically generate the Open API document(s) associated with the app during build.
- Populate the Open API documents in the app's output directory as
{ProjectName}.json.
You can further customize this if you want. For more info on how to do this, have a look at the documentation:
Compare OpenAPI documents
Now that we have 2 OpenAPI documents available, we can compare them. Therefore I use the Open API Comparator. This tool is available both as a command line tool and as a nuget package.
To install it run the following command:
dotnet add package Criteo.OpenApi.Comparator
We can now integrate this in a unit test as you can see below:
If API changes are found, you can see the details on what has changed:
