Monday, November 23, 2009

Classy hData

I've been working on a team that is looking at ways in which we can simplify the exchange of information in Health IT. This effort is called hData. We just released a new version of our packaging and network transport spec, and I would like to talk a bit about how we arrived at this version.

I think it is really important for IT specifications to have a reference implementation available. If you build a spec without code, it's really hard to see where you have gone wrong. To make sure we are on the right track, I built a small web application that implements the spec. I was able to quickly uncover some bugs in our work. Bugs I'm sure we would have missed by just reading the document.

Technology Choices

Since my preferred language of choice is Ruby, it would be natural to think I would want to tackle this project in Rails. However, in hData we make some good use of the HTTP Verbs, and I'm not so sure that they would line up seamlessly with Rails conventions. I decided to go with a much simpler choice. Sinatra is a small web framework that seems perfect for this job. It makes the HTTP Verbs central to your code, so it should be fairly obvious on how we go from the spec to implementation.

There are a few other tools that I used on this adventure. DataMapper was just right for the ORM needs of the project. I could have used ActiveRecord to persist data, but DataMapper has a really nice auto-migration feature, which will save me from writing all of the database creation code. I also used Bundler to manage my application's dependencies.

Getting Started

The best way to get started here is by taking a test driven approach to the spec. For that I will be using Shoulda and Rack Test. With my TDD tools in place, I can take part of the spec that looks like this:

3.1.2 POST Parameters: type, typeId, requirement

For this operation, the value of type MUST equal "extension". The typeId MUST be a URI string that represents a type of section document. The requirement parameter MUST be either "optional" or "mandatory". If any parameters are incorrect or not existent, the server MUST return a status code of 400.

If the system supports the extension identified by the typeId URI string, this operation will modify the extensions node in the root document and add this extension with the requirement level identified by the requirement parameter. The server MUST return a 201 status code.

If the system does not support the extension, it MUST not accept the extension if the requirement parameter is "mandatory" and return a status code of 409. If the requirement is "optional" the server MAY accept the operation, update the root document and send a status code of 201.

Status Code: 201, 400, 409

and turn it into the a Shoulda context block. In the spec above, we're talking about what should happen when you POST to the root of an hData Record. The functionality being described here is how an extension can be added to the record, or how you can register a different type of thing for a record. For example, you could use this feature to add an medications extension to a record, if one did not exist there already. In our test code, we're going to try to register an allergies extension:

As you can see from the code, the combination of Shoulda and Rack Test make it really easy to express the requirements set forth in the specification. The first test tries to POST and incomplete request and should receive an error. The second sends a properly formed request and should get an appropriate response. The last test tries to POST a duplicate extension.

With the tests in place, we can move on to implementation.

I have created a DataMapper Resource to capture all of the information we want to store about an extension. I will also use the validation framework of DataMapper to make sure that all of the requirements for an extension are met. I end up with the resulting code:

With my model in place, I can implement the code to handle the web request:

The code above is pretty typical for Sinatra. The post block handles POST's to the root URL. There I call a method to check and make sure that the type parameter is set. If it isn't I halt the processing and let the user know that the request is malformed with a 400 code. If the type is set to extension, then we drop into the handle_extension method. Inside of the method, I build an Extension object and check it using the DataMapper validation framework.

There is a little bit of funkiness at the end of the handle_extension method where I need to check the type of error. This is due to the fact that I need to return different status codes depending on the error. Unfortunately, with the DataMapper validations, I didn't see any way to return anything with the errors other than a text message, so this seemed like the best way of doing things.

The handle_section at the end of the post block handles another part of the spec. Don't worry, I didn't write it until I had the tests done first.

Lather, Rinse, Repeat

Implementing the rest of the hData Packaging and Transport spec followed the same process. Take the spec and write a matching unit test. Implement the spec and refine the code until the test passed.

In doing this, I found a couple of bugs in our spec. We hadn't provided parameter names for POSTing section documents. Our description of how to add metadata to documents was ambiguous at best. The nice part was that I was able to discover these things before even digging into the implementation.

What still needs to be done

While the Sinatra app that I wrote is a pretty good implementation of the hData Packaging and Transport spec, it still has some gaps. It doesn't support POSTing metadata on documents, it only creates and serves it's own. It also doesn't support nested sections, but that shouldn't be too hard to add.

Wrap Up

You can find the code at eedrummer/classy-hdata on github. Even if you aren't interested in hData, this application should serve as an example Sinatra/DataMapper application. If you dig into the code and the hData spec, I think you'll see that hData is really easy to implement, especially in a classy framework like Sinatra.