Types of APIs: What Are the Differences?

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APIs, which stand for Application Programming Interface, are an essential part of computer development. The majority of the most popular web applications rely on these software interfaces. They allow programs to communicate with each other and share functionality and data. This means that developers do not have to reprogram existing functionality, as they can access it via the API. Furthermore, they do not even need to understand how the API works to implement it. They can just use its interface. However, there are actually several types of APIs with very specific use cases. Find out about these API types and their differences in this article.


API types by use case       

Web APIs


Web APIs are programming interfaces that are accessible via the HTTP protocol. These APIs define the endpoints and the accepted request and response formats. They are mainly used to communicate with the web browser – for example to display notifications or store temporary data. This type of API is ubiquitous on the Internet. They are used to integrate external resources into a site, such as Facebook comments, posts or videos.


Web APIs are often confused with web services, which are also means of communication. The difference is that web services facilitate exchanges between two machines on a network, whereas APIs serve as interfaces to applications. However, the web service is also designed to have an interface in a machine-processable format. This is most often described in an XML notation: the Web Service Description Language (WSDL).


Web services therefore incorporate APIs, but they generally do not contain all the specifications, and cannot perform all the tasks that a full API can perform. In addition, the web service necessarily needs a network to function, whereas not all APIs are web-based.



System APIs


A system API is a layer or interface built on top of a system to manage, among other things:

●      connection parameters;

●      instructions from a user application or hardware such as a keyboard or touch screen;

●      the operation and internal variables of a system.


Unlike web APIs, system APIs are usually available, outside the network, from a library, SDK or other “local” service. However, the line between these two types of APIs has become quite blurred, as many web APIs have replaced the basic system APIs.


APIs by protocol type



SOAP stands for Simple Object Access Protocol. It is an information exchange protocol that uses an XML data format to declare requests and define the types of responses expected. SOAP uses the Remote Procedure Call (RPC) model, in which methods and functions receive parameters and return results.


Public and private APIs typically use SOAP for their interface. They are designed to create, retrieve, update and delete records such as accounts, passwords or other custom objects.

SOAP APIs can then be used with any programming language. The receiver and sender of a request can even use different languages. Finally, SOAP APIs support web services. However, the strict rules and rigid standards of this protocol lead developers to favour REST APIs.




A RESTful API, also known simply as a REST API, is a software interface that conforms to the constraints of the REpresentational State Transfer (REST) architectural style. This architecture determines how the API functions.


A RESTful API breaks down a transaction into a series of small modules, each of which handles a sub-part of the transaction. This modularity offers a lot of flexibility, but also a certain complexity for the design. This is why several companies such as Amazon S3, OpenStack Swift or Cloud Data Management Interface (CDMI) offer ready-made REST API templates for developers.


REST calls are “stateless”, i.e. each request must include all the information needed to process it and there is no need for a session to save data on the server side. As a result, any request can be directed to any instance of a component. Stateless modules can be easily deployed in case of failure and are scalable. This makes REST APIs particularly effective for web and cloud computing uses.




RPC stands for Remote Procedure Call. It is a network protocol that provides the ability to call procedures remotely. Basically, RPC APIs allow a piece of code to be executed on a remote computer or server. This type of RPC API is the simplest and oldest. It is based on two encoding formats: XML (then called XML-RPC) or JSON (JSON-RPC).


RPC APIs are similar to REST APIs, but more tightly coupled with the underlying system, which makes their maintenance, architecture changes and upgrades much more complex.


Types of APIs by availability

Public APIs


Public APIs, also known as open APIs, are, as the name implies, open to other developers and third party companies with minimal restrictions. They may require registration or the use of an API or OAuth key, but some are completely transparent.


These open APIs make it easier to access an organisation’s data and services. For example, the Google Maps public API enables tracking and geolocation functionality in external applications, such as carpooling or food delivery.


Partner APIs


Partner APIs are APIs that are open only to the business or strategic partners of their owner. They are therefore not publicly available, but usually require specific authorisation and strong authentication mechanisms to access them.


For example, if a company wants to share its client data with an external company, it can provide a partner API that is intended only to connect to the internal client database. No other use of the API will be possible.


Internal APIs


Internal APIs, also known as private APIs, are exposed only to the applications and machines of their owner. They are therefore intended to be used only within the same company. For example, they can be used to facilitate collaboration and optimise productivity (in particular by encouraging the use of the same services between development teams).


Composite APIs


Composite or mixed APIs combine two or more service and data APIs to create a sequence of related or interdependent operations. They allow developers to access various endpoints in a single call. Composite APIs are therefore particularly useful in microservice architecture models – where developers often need information from multiple services to perform a single task.


And how about you, developers or other IT professionals, what types of APIs do you work with most in your projects? What do you think of the ones presented? Feel free to share your experiences in the IT forum!

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