About API astonishments

Nowadays we developers tend to stand on the shoulders of giants: We put powerful building-blocks from different libraries together to build something worth man-years in hours. Or we fill-in the missing pieces in a framework infrastructure to create a complete application in just a few days.

While it is great to have such tools in the form of application programmer interfaces (API) at your disposal it is hard to build high quality APIs. There are many examples for widely used APIs, good and bad. What does “bad API” mean? It depends on your view point:

Bad API for the API user

For the application programmer a bad API means things like:

  • Simple tasks/use cases are complicated
  • Complex tasks are impossible or require patching
  • Easy to misuse producing bugs

A very simple real life example of such an API is a C++ camera API I had to use in a project. Our users were able to change the area of interest (AOI) of the picture to produce images consisting of only a part of full resolution images. Our application did crash or not work as expected without obvious reasons. It took many hours of debugging to spot the subtle API misuse that could be verified be reading the documentation:

The value of camera.Width.GetMax() changed instead of being constant! The reason is that AOI was meant and not the sensor resolution width. The full resolution width we actually wanted is obtained by calling camera.WidthMax.GetValue(). This kind of naming makes the properties almost undistinguishable and communicates nothing of the implications. Terms like AOI or sensor width or full resolution just do not appear in this part of the API.

Small things like the example above may really hurt productivity and user experience of an API.

Bad API for the API programmer

API programmers can easily produce APIs that are bad for themselves because they take away too much freedom away resulting in:

  • Frequent breaking changes
  • API rewrites
  • Unimplementable features
  • Confusing, not fitting interfaces

Design your interfaces small and focused. Use types in the interface that leave as much freedom as possible without hurting usability (see Iterable vs. Collection vs. List vs. ArrayList for example). Try to build composable and extendable types because adding types or methods is less of a problem than changing them.


Developers should put extra care in interfaces they want to publish for others to use. Once the API is out there breaking it means angry users. Be aware that good API design is hard and necessary for a painless evolution of an API. Consider reading books like “Practical API Design” or “Build APIs You Won’t Hate” if you want to target a wider audience.

Designing an API? Good luck!

If you’ve developed software to some extent, you’ve probably used dozens if not hundreds of APIs, so called Application Programming Interfaces. In short, APIs are the visible part of a library or framework that you include into your project. In reality, the last sentence is a complete lie. Every programmer at some point got bitten by some obscure behavioural change in a library that wasn’t announced in the interface (or at least the change log of the project). There’s a lot more to developing and maintaining a proper API than keeping the interface signatures stable.

A book about API design

practicalapidesignA good book to start exploring the deeper meanings of API development is “Practical API Design” by Jaroslav Tulach, founder of the NetBeans project. Its subtitle is “Confessions of a Java Framework Architect” and it holds up to the content. There are quite some confessions to make if you develop relevant APIs for several years. In the book, a game is outlined to effectively teach API design. It’s called the API Design Fest and sounds like a lot of fun.

The API Design Fest

An API Design Fest consists of three phases:

  • In the first phase, all teams are assigned the same task. They have to develop a rather simple library with an usable API, but are informed that it will “evolve” in a way not quite clear in the future. The resulting code of this phase is kept and tagged for later inspection.
  • The second phase begins with the revelation of the additional use case for the library. Now the task is to include the new requirement into the existing API without breaking previous functionality. The resulting code is kept and tagged, too.
  • The third phase is the crucial one: The teams are provided with the results of all other teams and have to write a test that works with the implementation of the first phase, but breaks if linked to the implementation of the second phase, thus pointing out an API breach.

The team that manages to deliver an unbreakable implementation wins. Alternatively, points are assigned for every breach a team can come up with.

The event

This sounds like too much fun to pass it without trying it out. So, a few weeks ago, we held an API Design Fest at the Softwareschneiderei. The game mechanics require a prepared moderator that cannot participate and at least two teams to have something to break in the third phase. We tried to cram the whole event into one day of 8 hours, which proved to be quite exhausting.

In a short introduction to the fundamental principles of API design that can withstand requirement evolution, we summarized five rules to avoid the most common mistakes:

  •  No elegance: Most developers are obsessed with the concept of elegance. In API design, there is no such thing as beauty in the sense of elegance, only beauty in the sense of evolvability.
  •  API includes everything that an user depends on: Your API isn’t defined by you, it’s defined by your users. Everything they rely on is a fixed fact, if you like it or not. Be careful about leaky abstractions.
  •  Never expose more than you need to: Design your API for specific use cases. Support those use cases, but don’t bother to support anything else. Every additional item the user can get a hold on is essentially accidental complexity and will sabotage your evolution attempts.
  •  Make exposed classes final and their constructor private: That’s right. Lock your users out of your class hierarchies and implementations. They should only use the types you explicitly grant them.
  •  Extendable types cannot be enhanced: The danger of inheritance in API design is that you suddenly have to support the whole class contract instead of “only” the interface/protocol contract. Read about Liskov’s Substitution Principle if you need a hint why this is a major hindrance.

The introduction closed with the motto of the day: “Good judgement comes from experience. Experience comes from bad judgement.” The API Design Fest day was dedicated to bad judgement. Then, the first phase started.

The first phase

No team had problems to grasp the assignment or to find a feasible approach. But soon, eager discussions started as the team projected the breakability of their current design. It was very interesting to listen to their reasoning.

After two hours, the first phase ended with complete implementations of the simple use cases. All teams were confident to be prepared for the extensions that would happen now. But as soon as the moderator revealed the additional use cases for the API, they went quiet and anxious. Nobody saw this new requirement coming. That’s a very clever move by Jaroslav Tulach: The second assignment resembles a real world scenario in the very best manner. It’s a nightmare change for every serious implementation of the first phase.

The second phase

But the teams accepted the new assignment and went to work, expanding their implementation to their best effort. The discussions revolved around possible breaches with every attempt to change the code. The burden of an API already existing was palpable even for bystanders.

After another two hours of paranoid and frantic development, all teams had a second version of their implementation and we gathered for a retrospective.

The retrospective

In this discussion, all teams laid down arms and confessed that they had already broken their API with simple means and would accept defeat. So we called off the third phase and prolonged the discussion about our insights from the two phases. What a result: everybody was a winner that day, no losers!

Some of our insights were:

  • Users as opponents: While designing an API, you tend to think about your users as friends that you grant a wish (a valid use case). During the API Design Fest, the developers feared the other teams as “malicious” users and tried to anticipate their attack vectors. This led to the rejection of a lot of design choices simply because “they could exploit that”. To a certain degree, this attitude is probably healthy when designing a real API.
  • Enum is a dead end: Most teams used Java Enums in their implementation. Every team discovered in the second phase that Enums are a dead end in regard of design evolution. It’s probably a good choice to thin out their usage in an API context.
  • The most helpful concepts were interfaces and factories.
  • If some object needs to be handed over to the user, make it immutable.
  • Use all the modifiers! No, really. During the event, Java’s package protected modifier experienced a glorious revival. When designing an API, you need to know about all your possibilities to express restrictions.
  • Forbid everything: But despite your enhanced expressibility, it’s a safe bet to disable every use case you don’t want to support, if only to minimize the target area for other teams during an API Design Fest.

The result

The API Design Fest was a great way to learn about the most obvious problems and pitfalls of API design in the shortest possible manner. It was fun and exhausting, but also a great motivator to read the whole book (again). Thanks to Jaroslav Tulach for his great idea.