What makes a good API?

Joshua Tauberer’s Blog: “There comes a time in every dataset’s life when it wants to become an API. That might be because of consumer demand or an executive order. How are you going to make a good one?…
Let’s take the common case where you have a relatively static, large dataset that you want to provide read-only access to. Here are 19 common attributes of good APIs for this situation. …
Granular Access. If the user wanted the whole thing they’d download it in bulk, so an API must be good at providing access to the most granular level practical for data users (h/t Ben Balter for the wording on that). When the data comes from a table, this usually means the ability to read a small slice of it using filters, sorting, and paging (limit/offset), the ability to get a single row by identifying it with a persistent, unique identifier (usually a numeric ID), and the ability to select just which fields should be included in the result output (good for optimizing bandwidth in mobile apps, h/t Eric Mill). (But see “intents” below.)
Deep Filtering. An API should be good at needle-in-haystack problems. Full text search is hard to do, so an API that can do it relieves a big burden for developers — if your API has any big text fields. Filters that can span relations or cross tables (i.e. joins) can be very helpful as well. But don’t go overboard. (Again, see “intents” below.)
Typed Values. Response data should be typed. That means that whether a field’s value is an integer, text, list, floating-point number, dictionary, null, or date should be encoded as a part of the value itself. JSON and XML with XSD are good at this. CSV and plain XML, on the other hand, are totally untyped. Types must be strictly enforced. Columns must choose a data type and stick with it, no exceptions. When encoding other sorts of data as text, the values must all absolutely be valid according to the most narrow regular expression that you can make. Provide that regular expression to the API users in documentation.
Normalize Tables, Then Denormalize. Normalization is the process of removing redundancy from tables by making multiple tables. You should do that. Have lots of primary keys that link related tables together. But… then… denormalize. The bottleneck of most APIs isn’t disk space but speed. Queries over denormalized tables are much faster than writing queries with JOINs over multiple tables. It’s faster to get data if it’s all in one response than if the user has to issue multiple API calls (across multiple tables) to get it. You still have to normalize first, though. Denormalized data is hard to understand and hard to maintain.
Be RESTful, And More. ”REST” is a set of practices. There are whole books on this. Here it is in short. Every object named in the data (often that’s the rows of the table) gets its own URL. Hierarchical relationships in the data are turned into nice URL paths with slashes. Put the URLs of related resources in output too (HATEOAS, h/t Ed Summers). Use HTTP GET and normal query string processing (a=x&b=y) for filtering, sorting, and paging. The idea of REST is that these are patterns already familiar to developers, and reusing existing patterns — rather than making up entirely new ones — makes the API more understandable and reusable. Also, use HTTPS for everything (h/t Eric Mill), and provide the API’s status as an API itself possibly at the root URL of the API’s URL space (h/t Eric Mill again).
Never Require Registration. Don’t have authentication on your API to keep people out! In fact, having a requirement of registration may contradict other guidelines (such as the 8 Principles of Open Government Data). If you do use an API key, make it optional. A non-authenticated tier lets developers quickly test the waters, and that is really important for getting developers in the door, and, again, it may be important for policy reasons as well. You can have a carrot to incentivize voluntary authentication: raise the rate limit for authenticated queries, for instance. (h/t Ben Balter)
Interactive Documentation. An API explorer is a web page that users can visit to learn how to build API queries and see results for test queries in real time. It’s an interactive browser tool, like interactive documentation. Relatedly, an “explain mode” in queries, which instead of returning results says what the query was and how it would be processed, can help developers understand how to use the API (h/t Eric Mill).
Developer Community. Life is hard. Coding is hard. The subject matter your data is about is probably very complex. Don’t make your API users wade into your API alone. Bring the users together, bring them to you, and sometimes go to them. Let them ask questions and report issues in a public place (such as github). You may find that users will answer other users’ questions. Wouldn’t that be great? Have a mailing list for longer questions and discussion about the future of the API. Gather case studies of how people are using the API and show them off to the other users. It’s not a requirement that the API owner participates heavily in the developer community — just having a hub is very helpful — but of course the more participation the better.
Create Virtuous Cycles. Create an environment around the API that make the data and API stronger. For instance, other individuals within your organization who need the data should go through the public API to the greatest extent possible. Those users are experts and will help you make a better API, once they realize they benefit from it too. Create a feedback loop around the data, meaning find a way for API users to submit reports of data errors and have a process to carry out data updates, if applicable and possible. Do this in the public as much as possible so that others see they can also join the virtuous cycle.”