Partner APIs are designed specifically for communication between business partners. As an example, Apple and AT&T could develop APIs that help iPhones communicate with AT&T's communication infrastructure.
Partner APIs don’t have to be on such a scale. Business partners can develop an API that makes it easy for their systems to communicate with each other. An example of this could be APIs used to automate emails between two organizations.
Additionally, partner APIs can use open or closed software in order to operate, meaning you could have a partner API that's open and free to use.
Partner APIs are often useful for small and medium businesses, just as often as they aid large enterprise organizations. It’s important to remember this. Overlooking this type of API could limit how well you leverage technology for your company.
A composite API combines multiple APIs into a single service. Generally speaking, APIs make frequent requests in order to facilitate communication between devices. Since common devices are complicated and utilize many different APIs, the total sum of communication can require a large number of digital trips back and forth between devices in order to function properly.
Composite APIs specifically look for ways to combine API functions and requests to make communication more efficient.
As an example, composite APIs can organize the communication for email automation triggers. A retail business might have abandoned cart emails that automatically send after an online shopping cart sits un-purchased for a specific amount of time. This system requires multiple communication trips between user devices, servers, and more; thus, the composite APIs are appropriate for consolidating communication resources.
Internal or private
Internal and private APIs are the same thing. These are APIs built around internal logic for a specific business or group. Such APIs are hosted on internal servers (usually) and are developed internally as well.
Private APIs typically serve very specific functions that wouldn’t make sense in another organization. For example, a logistics company might use APIs to facilitate warehouse communication. They may function on customized warehousing API protocols; therefore, they wouldn’t be useful to another business, even in the same industry.
The main point here is that internal APIs are highly customized.
Comparing API architectures
Above, we discussed API types, but there’s more that goes into the discussion. It’s also important to understand the architectures that impact API design. When considering how many APIs there are in the world, knowing about the most common architectures can help set a sense of scale.
Below are the 3 most common API architectures. To be more specific, they aren’t necessarily architectures. In some cases, they're better described as design philosophies or paradigms. Regardless, you'll often develop your APIs according to one of these 3 larger systems.