While it’s true that there are still a lot of places where software isn’t leveraged and many places where software needs to evolve, software is nearly everywhere!. The type of software we write today needs to interact with other software via some sort of network. Any web developer out there is used to that, (s)he writes software that runs on a server somewhere on the internet and users via a local software (browser) consume their web applications using an internet connection.
Many are used to doing certain things a certain way and it’s really hard to unlearn old habits. The more concerning part of this fact is that it makes it hard for someone to step back and evaluate honestly if a technical decision is due to comfort or if it’s truly the right choice to achieve a given goal. Ruby on Rails revolutionized the way we wrote web applications almost 10 years ago and since then, many web frameworks have adopted the Rails philosophy. As a matter of fact, I personally think that when it comes to writing front end applications, Rails is still one if not the best web frameworks out there. But in its current state, I’m far from convinced that it’s a great tool to write web APIs mainly because it was not designed for that and because it comes with a lot of baggage.
Focusing on the API
Let’s take a step back and consider what is critical when developing a web API:
- Documentation for end users to know how to consume your services.
- Consistent and reliable output so you don’t break client applications relying on your services.
Note that I didn’t mention performance because I consider performance almost always being important. I also didn’t mention the whole REST/RPC/Hypstermedia debate since I consider it being an implementation detail and a totally orthogonal discussion. But for the record, I personally don’t like solutions forcing you into a specific way of providing your data (I’m looking at you webmachine).
Documentation isn’t as important when you consume your own APIs because you can look at your source code. However it gets much more tricky when you start working with other teams. They might not know the language/framework you use. They might not have time to go dig into your source code to figure out what your meta-magical piece of clever code does.
Lately more and more applications provide their raw data to the outside world via web APIs. The developers who will consume your resources don’t have access to your source code, probably don’t use the same programming language and don’t have much time to guess how your API work. Also your test suite won’t help communicate how your API works so we need to find a different approach. Also note that in this scenario, TDD/BDD won’t help us communicate better and tests can’t be used as documentation since your tests aren’t exposed.
Documentation is your #1 way to communicate with your human audience. Communication is key and even if as engineers we love to focus on code, if we can’t communicate about it, end users will have a hard time using our code and might just not even do it. The key is to communicate what your API does, why someone might want to use it and how to use it.
Consistent and reliable output
This point seems obvious but what we realize in reality is that for many, API’s consistency and reliability doesn’t include documentation. You find a lot of APIs out there poorly documented or out of sync with the actual implementation.
Documentation is a contract between you: the developer and the other developers consuming your APIs. As a developer, when you write any type of tests, they become some sort of quality contract. If someone changes your code and break your tests, they break the implicit contract. However, when talking about consuming data via an API, things get a bit more complicated. We need a way to ensure that the end user expectations are matching our implementation/documentation. Unit testing simply can’t guarantee that. Unit testing will guarantee that the logic of your units of code is intact but it can’t easily guarantee that the API output matches the documentation, however this is something that has to be done.
That’s why I personally don’t think it’s a good idea to mix HTML rendering code and web API code. Consider using different controllers or different files depending on the way your code is structured.
I also strongly believe that the implementation details shouldn’t define the way you design your web APIs. Don’t just slap a CRUD API on top of your model and call it done. In most cases, you will pay a high price later on if you take this approach because whenever you will need to change your web API or your model, you will be stuck. This is because your interface is now used by a lot of people and you can’t easily change it. There are many ways to avoid API/model coupling, I don’t advocate one particularly, but whatever you do, be sure you can make your models and APIs can evolve separately.
I’ve been designing and developing web APIs for many years and I’ve been struggling with everything I mentioned until now. I don’t claim that I’ve found the solution but I’d like to think that I found one own way of addressing what are for me some of the most important parts of API design.
I believe that there is value in being explicit in the way we describe web APIs. Sometimes people get confused between being verbose and being explicit. These are two different concepts. Being explicit can sometimes seem verbose, but we need to evaluate the value that can be extracted from the provided information. If there isn’t any clear value and too many words are used, then we aren’t explicit, we are verbose.
When designing a web API, I don’t write it for myself, I do it for someone else. It’s crucial to consider who you are writing for and to expose what’s important for them. A “small” problem is that we don’t show our code to our end users, so we need to find a way to explicitly provide the important information and to have this information provided to our end users.
For that I use a Domain Specific Language (DSL) which is a fancy way to say that I have some specific code allowing me to explicitly define my web services. These services exist as objects that can then be used to process requests but also to validate inputs, and even more importantly to generate documentation.
The DSL allows me to define the following:
- details about consuming the service (uri, HTTP verb, authentication details, other service details…)
- incoming params (which ones are allowed, the type, are they required, optional?, what are they for)
- service output
The input details is really important to validate incoming requests and reject them before even dispatching them. This is done for data sanity and for security reason. It also defines a strong interface that is easier to develop against and to maintain.
The output details might sound quite surprising and redundant. After all, isn’t that a duplication of effort since we already have this information in the “view”? Well, if we consider the important points I highlighted in the first part of this article, we need a way to enforce a “contract” between the end users and our implementation. To do that, we can’t simply rely on our code since we can’t trust it. The output is important to document the expected output but also to validate that our services match our documentation and therefore our “contract”.
Here is an example of a Ruby DSL use to describe a hello world service:
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The DSL style isn’t really important, what’s important is that it allows us to address some of the concerns we discussed earlier such as clearly defined interface that can be communicated as documentation but also acts as code. The focus is really on the API and everything fits in one page. Because everything is an object and can be inspected, proper and up to date documentation can be generated. And the implementation can be tested against the documentation since everything is maintained as code.
While I like Ruby for many reasons, I don’t think that it’s the only good language to implement great web services. It actually has its pros and cons and so have all the web frameworks out there. That’s why I wrote my DSL as a standalone library that could virtually run on top of any web engine since it only outputs a representation of services.
While I hope to one day create an interesting interop solution across programming language (by exporting the objects in a shared data structure for instance), I started by focusing on the various Ruby web frameworks.
The best starting point for me was to use Sinatra. I almost started just using rack, but Sinatra was providing me with a bit more feature for very little headache and little code to grasp. I wrote wd-sinatra which is a Ruby gem providing the WeaselDiesel DSL on top of a Sinatra app. It comes with a generator and all the needed hooks to design, implement, test and generate documentation for modern web APIs.
Using a simple rake command (Ruby’s version of make) one can generate documentation or test the APIs against the implementations. The “mini framework” is still very free form and should let you do whatever you want. I don’t even set a default ORM for you to use since this choice is highly personal. I do however leave you places to set these things.
Sinatra and my “freedom framework” might seem too free form for you. So I’m currently working on getting the DSL to run on top of Rails, and by goal is to actually get it to run with a normal Rails app.
Reconsidering Rails’ MVC
The Rails code base is very deeply marked with its own concept of MVC and having controllers and actions. The challenge I have is that I like my service to be self contained. I want my services to be simple and easy to grasp. I like having 1 service per file. Models, libraries and other optional presenters/decorators live separately but I like to have my service implementation code with the rest of my service description. I honestly don’t like telling developers that they need to go check a route file and that my simple service requires that you open 12 files to understand what’s going on. Simpler is often better and that’s why in wd-sinatra the DSL and the implementation live together which is quite harder to do with Rails.
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Learning from experience
While the DSL fits most of my needs, we all have different use cases. While it’s critical for a library to have a clearly defined objective, it’s also important to have many people help improve it. Part of the learning/vetting excercise is to test an approach against different needs to define when and why it works well in some cases and why sometinmes it doesn’t. This allows us to define a sweet spot that should match the clearly defined objective. However to be able to do that, a design needs to be tested by many people. So far my approach seems to work very well when an API needs to live outside an application and that 3rd parties need to consume the resources. It also seems to work well with edge cases that many API designers seem to encounter sooner or later.
At the end of the day, we have to remember that API stands for Application Programming Interface and that these interfaces have to be programmed so humans can write to comsume them. One of Ruby’s main design points has always been to try to address human needs more than computer’s. As API designers/implementers, it seems important to adopt the same approach and consider who will use our interfaces. I think the discussion should focus more on what to value when defining web APIs, instead of arguing about how to implement APIs. Standardization is a great concept but a really hard to implement. And even with standards, we have to find a way to communicate with the API users to express what standards we follow and where to find the various entry points. Think about your API, how well does it communicate with your future API users, is it good enough for them to get excited? Is it good enough for them to create something amazing with it? If not, why not?
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