Now that I know my YAML file works, because the generated curl command succeeds, it’s time to look at why the interactive version doesn’t.

The way this is supposed to go is that you click the little red dot on the right in your spiffy new doc section, enter your credentials, and then fill in the blanks to test right there from the documentation. Magic! But why would this fail when the similarly auto-generated curl command succeeds? It’s because the server restricts access to the resource the originating webserver is requesting. This is good for safety, but sometimes it’s what you need to do. It’s called cross-origin resource sharing (CORS) and it was mentioned in the Swagger docs. But until I ran into a problem, I wasn’t sure what I was supposed to do with this information.

Swagger UI sends an OPTIONS request to the server as the opening of a handshake that results in the resource being accessible when the subsequent actual request shows up. (That is a terrible description, hopefully someone who actually understands this can do better.) I saw those OPTIONS requests, but then nothing happened, there was no POST, and my new message in Zulip was never created.

I still can’t fully explain it, but basically some extra headers are required from the server to verify that the requesting domain should be allowed to request the desired resource. The generated sample back-end already took care of that and it wasn’t a problem. But for a real API like Zulip, in an environment not expecting what Swagger needs, it means changing something with Django. I know nothing about Django.

I did some research and found a Django app (I had no idea there was such a thing) that handled this: django-cors-headers. I installed and configured it and suddenly everything worked. The bad part is that means a 3rd party package must be installed on any Zulip server that wants to work with Swagger UI documentation.

I looked into what django-cors-headers does, but there is a lot more than just creating the necessary header. Much of it is the tests, administrative features and other things that make it a useful piece of software. If I continued to investigate I certainly could figure out the actual minimal necessary change I’d need on the server side. But why do that when there’s already a nice package available? Except not everyone has the luxury of just installing something. That is a question I will need to find an answer for before deploying this to production.

But now I have reached the end of my proof-of-concept experiment, and I have nice interactive documentation to demonstrate how my API works. There’s much work yet to cover all the endpoints and all the features, but this shows that it can be done.

The basic steps are these:

  • Download the swagger-ui repo from github and drop the dist directory it contains in an appropriate location on your webserver
  • Build a YAML file that describes your API, according to the OpenAPI/Swagger spec
  • Update index.html to point to your file and make any other desired changes (like I did for language)
  • Configure your webserver, if needed, to handle CORS correctly

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