This is Day 19 of Butter Days, from my apartment in New York, NY.

COVID-19 is happening right now, which is the main reason I’m not working from a cafe as usual. The Federation of American Scientists made this site which has a lot of great information about the virus.

As far as this project goes, last week I had an unexpected breakthrough, but there are still some implementation issues to work through before I can fully generate this client without any manual fixes.

AWS Service Definitions

This spec has never actually been used to generate a real client for the AWS API before, so it was put together with the best guesses based on the documentation and the fields in the AWS service definition. They did a great job, considering that.

This service definition was mysteriously updated by a user called “awstools” that I can’t click on to learn more (no github account), with commits that look like:

commit 3cb57c7462874bf62fb44275752ba91a244ca7be
Author: awstools <>
Date:   Fri Nov 8 19:26:46 2019 +0000

    Updates SDK to v2.568.0

I don’t see pull requests either, so I wonder if Amazon has a special user that has write permission to master on all the SDKs. It actually looks like these specs get updated quite frequently:

Fri Mar 13 18:05:25 2020 +0000  Updates SDK documentation
Thu Mar 12 19:53:38 2020 +0000  Updates SDK to v2.639.0
Wed Mar 11 18:27:13 2020 +0000  Updates SDK to v2.638.0
Tue Mar 10 18:22:18 2020 +0000  Updates SDK to v2.637.0
Mon Mar 9 18:19:44 2020 +0000   Updates SDK to v2.636.0
Fri Mar 6 19:21:26 2020 +0000   Updates SDK to v2.635.0
Thu Mar 5 19:29:41 2020 +0000   Updates SDK to v2.634.0
Wed Mar 4 19:20:21 2020 +0000   Updates SDK to v2.633.0
Tue Mar 3 19:21:17 2020 +0000   Updates SDK to v2.632.0
Mon Mar 2 19:30:59 2020 +0000   Updates SDK to v2.631.0
Sat Feb 29 00:48:37 2020 +0000  Updates SDK to v2.630.0

That’s good to know. When this is working it’ll probably be worth setting up some kind of hook to automatically regenerate the OpenAPI specs when these update.

OpenAPI Spec Errors

Because the OpenAPI specs aren’t maintained by Amazon and haven’t actually been used before, there are still some errors (unsurprisingly).

I filed this issue because there was a “members” field in the AWS spec that the author of the converter reasonably assumed was equivalent to the “properties” field in OpenAPI, but it actually meant that every object should be literally surrounded by a “member” sub-object (but apparently this is not true for all endpoints).

I also filed this issue because the spec was generating both GET and POST requests for all the endpoints, but the original AWS service definition only specified POST. Well, apparently both work, so I’ll have to actually figure out the bugs I had when trying to get the GET request functions to compile in the generated Rust code.

Once these errors are resolved, I’ll probably set up some regression tests so that these specs will stay correct (and functional). This will work well in combination with automatically regenerating the specs when AWS makes an update.

Generating a Client That Supports XML

I don’t think the Rust client generator currently supports XML, and trying to support XML and correct the spec at the same time seems like something I want to avoid if I can, just so I can isolate whether my bugs are in the generator or in the spec itself.

I’m looking through the OpenAPI Generator project to try to understand which generators support generating XML-based clients. In looking at the rust generator here, I stumbled upon a field called XMLStructureDefinitions. This seems promising. In the source I see this comment that says “this option indicates whether XML structures can be defined by spec document and honored by the caller”. Well that sounds like exactly what I want.

Through some more searching, I found this directory with docs on all the generators including whether they support that feature. Let’s find out which generators actually support XML:

$ git grep "|XMLStructureDefinitions|✓|"

Yikes. Those aren’t very many options. Most things in that list are documentation generators and “spring” is a server generator. That doesn’t help me. It’s pretty shocking that not a single client supports this feature. Sure enough, when I search for “XML” in the issues, most of the results are for the spring server.

I think it’s time to file an issue. I might be the first one to implement XML support in a client, and Rust might be the first client to have XML support.

It’s not going to be as easy as just switching the parser from a JSON parser to an XML parser, despite the existense of this great library which so far has worked as a drop in replacement for the JSON parser that’s used in the generated code. The OpenAPI spec also has some XML only features that were actually used to fix my “members” sub-object issue, so I’ll have to implement those as well.

I’m going to file an issue first, just to have some kind of sanity check and make sure my understanding is correct.

All right issue filed. I’m going to leave that there for now to see if anyone has corrections for me. It is very strange that no client supports XML, so I’m hoping that I’m just misunderstanding something, but we’ll see.

Next Time

Next time will definitely be based on the response to that issue. I’ll either end up trying to make the rust generator support XML or end up using whatever they recommend instead.

Either way, my current goal is still to get this spec working. I would like to start moving on to some more interesting things, like actually exporting some things and thinking about what the offline data model would be for AWS state (which I suspect I can do by dumping everything in the “schemas” section of the spec), but I think that will be much more interesting when I’m actually working with real data that I’ve exported from my account.