APIs are mostly created to be used by other programs, which means that regular testing techniques can be applied to it, but only if you keep in mind its purpose and rules it follows.
In order to start API testing you need to have a basic understating of what it is, what it does and why do you need it.API (Application Programming Interface) - is a set of complete structures, protocols, functions, classes and constants that are provided by application (library or service) to be used in 3rd party applications.
Modern APIs are often presented as web-services that provide certain information to users (other apps or people). Usually this data in structured so that both sides can know what to expect, how to read and use provided data.
In most cases APIs are built according to REST API ideology (Representational State Transfer API) which features following principles:
1. Client-server architecture2. Statelessness3. Cachability4. Layered system5. Uniform interface
There is a ton of info you need to read through to understand what it means, but to be short - this is an ideology that allows building a complicated application with simple and usable interface.
Usually you need a specific key to be able to use an API which is used to let server identify a user but it's not a must and open APIs may not have it at all or provide it by request. For two-side authentication of separate services OAuth protocol is usually used.
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