What is an Open API?

I was reading a document this week that referred to an “Open API”. It occurred to me that I hadn’t really thought about what that term was supposed to mean before. Having looked at the API in question, it turned out it did not mean what I thought it meant. The definition of Open API on Wikipedia and the associated list of Open APIs are also both a bit lacklustre.

We could probably do with being more precise about what we mean by that term, particularly in how it relates to Open Source and Open Data. So far I’ve seen it used in several different ways:

  1. An API that is free for anyone to use — I think it would be clearer to refer to these as “Public APIs”. Some may require authentication, some may only have a limited free tier of usage, but the API is accessible to anyone that wants to use it
  2. An API that is backed by open data — the data that is extracted by the API is covered by an open licence. A Public API isn’t necessarily backed by Open Data. While it might be free for me to use an API, I may be limited in how I can use the data by API terms and/or a non-open data licence that applies to the data
  3. An API that is based on an open standard — the data available via an API might not be open, but the means of accessing and querying the data is covered by a specification that has been created by a standards body or has otherwise be openly published, e.g. the specification of the API is covered by an open licence. The important thing here is that the API could be (re-)implemented in an open source or commercial product without infringing on anyone’s rights or intellectual property. The specification of APIs that serve open data aren’t necessarily open. A commercial vendor may provide a data publishing service whose API is entirely proprietary.

Personally I think an Open API is one that meets that final definition.

These are important distinctions and I’d encourage you to look at the APIs you’re using or the API’s you’re publishing and considering into which category they fall. APIs built on open source software typically fall into the third category: a reference implementation and API documentation are already in the open. It’s easy to create alternate versions, improve an existing code base, or run a copy of a service.

While the data in a platform may be open, lock-in (whether planned or otherwise) can happen when APIs are proprietary. This limits competition and the ability for both data publishers and consumers to choose other vendors. This is also one reason why APIs shouldn’t be the default for open government data: at some level the raw data should be portable and useful outside of whatever platform the organisation may choose to deploy. Ideally platforms aimed at supporting open government data publishing should be open source or should, at the very least, openly licence their API documentation.

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