Designing a great API

Several years ago I worked on a payroll package developing a core engine that required an API to let third parties write calculations, validations and security gates that would execute as part of its regular operation.

We were a small team and I had many conversations with another developer tasked with building a payroll using the API I would provide. Some methods here, classes there, the odd helper function and I had an API and then we had a mini payroll running.

Then they showed me the code they had written and that smug grin dropped off my face. It was awful.

Perhaps this other developer wasn’t as great as I’d thought? Looking at the code made me realize they had done the best anyone could with a terrible API. I’d exposed parts of the core payroll engine with hooks when it needed a decision. Its job was to run the payroll — a very complex task that involved storage, translation, time periods, users and companies. That complexity and context had leaked out.

Unfortunately it’s not a unique story — many APIs are terrible to use. They’re concerned with their own terminology, limitations and quirks because they are exposed sections of an underlying system developed by those responsible for the underlying system.

If you want others to have a good experience with your product you have to put yourself in their shoes. Whether it’s a UI or an API makes no difference.

You are not the user

That’s the real difference between writing the classes that form your regular implementation and those that make up your public API.

We had time to fix our payroll API. Instead of refining and polishing here and there we took the 20 or so snippets developed for the mini payroll and pruned, cleaned and polished until they looked beautiful. They scanned well and made sense to payroll developers unfamiliar with our package.

When a third developer familiar with payroll but unfamiliar with our package developed the necessary code for a fully-functional jurisdiction in record time with minimal assistance we knew we had hit our goal.

Sure implementing that new API was hard work. Instead of simple methods sticking out of the engine we had a facade over our engine but it was justified. They were two different systems for two different types of user with distinct ideas about what the system was and how it was going to be used.

Code First

Years later I found myself on a team of three people tasked with putting a new API on top of Entity Framework for configuring models with code. The .NET world would come to know as this API as Code First and it was Microsoft’s first fluent API if I recall. I was determined to use my experience and avoid another complex API surface littered with terminology and leaky abstractions. Parts of EF already suffered from that problem.

For the first few weeks of that project we didn’t write any of the code that would in fact become Code First.

Instead we decided who our user was — in this case a C# developer who likes writing code, knows LINQ and some database concepts but doesn’t know Entity Framework as people who did were already using Model First or Database First.

Then we wrote tiny sample app configurations and tried to find simpler and simpler ways to describe them in code. We’d often start on a whiteboard with a scenario and write the complete mapping. We’d then try and find conventions that would remove the need for most of it and then try to write succinct code to configure the rest. As the newest guy to the team I’d fight to keep EF terms away from the main API surface in order to reduce that barrier to entry and help drive adoption.

Finally we’d hit the computer and develop stub C# classes and methods to make samples compile and let us try the IntelliSense. This isn’t always necessary but if you want to develop a fluent API or provide lots of type-safety such as Code First’s relationship mapping it’s highly recommended.

We’d then revisit the samples later and see if they could be read as easily as they were written and figure out what problems people were likely to run into and whether we could solve them without too much noise. Sometimes this meant having more than one way to do things such as chaining the fluent methods or allowing a bunch of properties to be set (solved with an extension method class providing the fluent API) and how users could manage larger models.

We finally ended up with succinct code like this with IntelliSense guiding you along the way and preventing you from even being able to specify invalid combinations. The HasMany prompts the properties on Customer and it won’t show you WithRequired unless it is valid. In the case of required-to-required it will ensure that the WithRequired specified which end is principle and dependent. In short it guides you through the process and results in highly readable code.

Entity<Customer>().HasMany(c => c.Orders).WithRequired(o => o.Customer).WillCascadeOnDelete();

This process took a little longer but given the amount of use the API will get that time will be saved by users countless times over.

Code First went down well with both the target audience and existing EF users and inspired the simpler DbContext interface that became the recommended way of accessing EF and now in 2024 is the primary way everyone uses EF and it’s modern EF Core replacement.

[)amien

PS. Martin Fowler has some great guidance in his book Domain Specific Languages.