API; what and why?

API; what and why?

Let’s start right at the start and figure out what an API is and why you should consider using one.
The term API stands for ‘Application Programming Interface’. In the world of web development the term ‘API’ is synonymous with online web services which client apps can use to retrieve and update data. These online services have had several names/formats over the years such as SOAP, however the current popular choice is to create a REST (or RESTful) API.
We’ll come onto REST and why REST is now preferred later, but first let’s examine why you would even bother with an API.
Let’s consider a modern application which may include several mobile apps on various platforms and usually some kind of web application too. Without an API, a basic architecture may look like this where each client app has its own embedded business logic.

Notice that each client app has its own embedded business logic (probably written in several different languages) which connects directly to the database to get, update and manipulate data. This local business logic means that the client apps can easily become complex and all need to be maintained in sync with each other. When a new feature is required, you’ll have to update each app accordingly. This can be a very expensive process which often leads to feature fragmentation, bugs and can really hold back innovation.
Now let’s consider the same architecture with a central API which hold all the business logic.

Each app uses the same API to get, update and manipulate data. All apps have feature parity and when you need to make a change you just make it in one place in line with the ‘Don’t Repeat Yourself’ (DRY) principle of software development. The apps themselves then become relatively lightweight UI layers.