A pragmatic review of OAS 3

Disclaimer

Before I go any further I want to address the elephant in the room. Obviously I consider myself a hypermedia evangelist and I’m aware it is easy to make ivory tower arguments from this perspective. I am also an application architect which requires frank pragmatism where today’s OK solution is generally much preferred to next year’s better one.  In most of my previous posts I’ve focused my discussions on the distance between where we are as an industry, where I think we should go, and why it’s important.

Getting started

As part of my process of preparing for my upcoming talks at APIStrat on API Documentation and Hypermedia Clients, I’ve been reviewing the specification in depth for highlights and talking points.

On one of my first forays into the new world of twitter, I rather tongue-in-cheekily(https://twitter.com/hibaymj/status/865054487119089665) pointed out as a hypermedia evangelist my issue with the specification.  Going back, I probably would express the thought differently, but the crux of the issue is OAS does not support late binding.

I’ll get back to this point later, because first I want to talk about the highlights of the specification to acknowledge and applaud the hard work put into such a large undertaking.  Looking back on the state of the art of APIs only 10 years ago, it’s easy to see the vast improvements our current standards and tooling provide.

At this point I’m going to assume most have googled for the changes to the format in OAS 3.  My aim with this post is not to focus on changes, but evaluate OAS as it exists in the current version.

The Great Stuff

Servers Object

This is a very powerful element for the API designer which allows design time orchestration constraints to be placed on the operation of the services. This can greatly enhance the utility of OAS for use in many scenarios, including but not limited to: API Gateways, Microservices orchestration, and enabling implicit support for CQRS designs on separate infrastructure without intermediary.

Components

My previous experience with OAS 1.2 lead to a lot of redundancy, which the components structure of the current version very elegantly eliminates.  The elegance stems from the design choice of composition over definition allowing for reuse without redundancy.  It simplifies the definition of the bodies, headers, request, and response components as reuse becomes a matter composition.  The examples section is a developer experience approval multiplier, which is welcome and should be strongly encouraged.

Linking

As a hypermedia evangelist, my approval of this section should be not come as a surprise.  It mirrors in concept many of the beneficial aspects of an external profile definition like ALPS and is a welcome addition to the spec.

Callbacks

The standardization of the discovery or submission of webhook endpoints within the application contract itself is a very good step in supporting increased interoperability, internally and between organizations.

Runtime Expressions

With the inclusion of this well-defined runtime expression format, OAS removes a large amount of ambiguity for consumers and tool developers. This allows the API designer to add a lot of value enhancing the ease of use for consumers and integrators.

A Mixed Bag

These items are included simply because a tools utility isn’t determined when it is created.  The optional nature of the definition or use cases of the response object and the discriminator open them up the potential of unnecessary ambiguity and misuse.

Responses Object

All of the benefits I mentioned in the components section also apply to the responses object. My concern centers around the enumeration of the different expected responses.  The authors deserve credit in immediately pointing out this shouldn’t be relied on as the full range of possible responses.  My experience has shown that designers, tool developers, and end consumers are prone to missing the fine print or assumption, subsequently over relying on these types of features.

Discriminator

For the purpose it serves I think the discriminator as defined is a very elegant solution which helps to differentiate OAS from standard CRUD.  It allows for the use of hierarchical and non-hierarchical polymorphism alike, for more concise and reusable designs.  However, it still fundamentally ties the API to design time defined data formats.

Room for Improvement

The Extension Mechanism

With obvious resemblance to the now long deprecated format of custom HTTP headers, this section should follow the specs own well designed components format.  This upgrade could use the composition rules defined within the spec to allow much better support from tooling developers, and more consistent interoperability.

It’s All Static

While the authors have done an excellent job removing a lot of static portions out of the spec, it is still fundamentally static at its core.  Fortunately the static nature of the format is largely limited to a small section of the document thus allowing designers and developers much more room to innovate after design time.

Intertwined Protocol and Application Design

In computer science it is always immensely difficult to know precisely where to create boundaries for improved separation of concerns.  The OAS specification was not created from an ivory tower bubble.  It was created to solve real problems in real time.  Unfortunately, it still bears scars from this period by mixing protocol design concerns with application design concerns.  Each application design component is also able to declare protocol properties in a mix which wouldn’t allow for protocol portability.  If protocol concerns like HTTP headers and response codes were abstracted to external definitions or formats, then the reuse of OAS could bridge nearly all relevant protocols.  However, there would be one thing left to prevent the specification portability – the path.

Path Is The Base Abstraction

Getting back to the point raised in my cheeky tweet.  By using the URL path as the primary abstraction the specification creates the possibility of many future; operational, developmental, and maintenance issues.  Recently even the quickly growing GraphQL community has joined voices with hypermedia proponents to point out how this subtle design flaw can develop into severe issues.

Bringing It All Together

The purpose of this post isn’t pointing out all the flaws in OAS but to give a pragmatic review of the state of the specification.  If you want to see a more in depth analysis take a look at Swagger isn’t user friendly.

In the end, if you’re going to opt for an alternative to hypermedia then OAS is about as close as you can get at this point.  The ecosystem fits extremely well in the wide berth between a single user service and massive scale where every byte counts.  If your service design hasn’t been updated in the last 10 years or is nonstandard, it’s very likely OAS 3 would be a massive improvement and represents a today’s best ‘good enough’ solution.

Some of these necessary improvements are easy to handle, others will require more finesse to mitigate if they are addressed at all.  One thing is clear if your project is still using custom API designs, or spend too much time managing older service designs, and you don’t have time to contribute to a hypermedia alternative then OAS is worth your serious consideration.