Types of APIs & Popular REST API Protocol

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Application Programming Interfaces (APIs) are everywhere, and with good reason. These bits of software join apps and microservices together despite different programming languages, operating systems, and data formats. An API can integrate services that otherwise can’t communicate with each other. They lay the groundwork for the most successful digital transformation strategies and composable enterprises. In this article, we’ll give an overview of the different types of APIs and what that might mean for your organization.  

Here are the key takeaways about the different types of APIs:

  • APIs act as bridges, enabling the exchange of data and functionality between applications.
  • APIs offer functionalities like data access, modularity, ecosystem expansion, and platform independence.
  • The main types of APIs include Open, Partner, Private, and Composite APIs.
  • RESTful API is an architectural style characterized by client-server separation, HTTP interface, and statelessness.
  • SOAP, XML-RPC, JSON-RPC, and Thrift are other popular API protocols with unique features and use cases.

What are APIs?

At its core, an API is a set of rules and protocols that defines how different software components or systems can interact with each other. It acts as a bridge, facilitating the exchange of data and functionality between applications, regardless of their underlying technologies, programming languages, or platforms. APIs define a standardized way for developers to request specific actions or retrieve specific data from a system or service.

APIs act as intermediaries, allowing applications to communicate with each other by sending and receiving requests and responses. Just as a waiter serves as an intermediary between customers and the kitchen in a restaurant, APIs serve as intermediaries between applications, enabling them to exchange information and perform various tasks.

Functionality and Benefits of APIs

APIs offer a wide range of functionalities and benefits that drive the growth and efficiency of modern software development. Here are some key aspects to consider:

  1. Data Access and Integration: APIs provide a means to access and integrate data from external sources, such as social media platforms, weather services, payment gateways, and more. By leveraging APIs, developers can tap into a wealth of information and services, enriching their own applications and enhancing the user experience.
  2. Modularity and Code Reusability: APIs promote modularity and code reusability, allowing developers to leverage existing functionalities instead of reinventing the wheel. By integrating APIs, developers can save time and effort by utilizing pre-built components, libraries, and services, accelerating the development process and fostering innovation.
  3. Ecosystem Expansion and Collaboration: APIs enable the creation of ecosystems and foster collaboration between developers, businesses, and third-party services. By exposing their APIs to external developers, companies can encourage the development of integrations, extensions, and plugins that enhance their core offerings. This collaborative approach leads to a thriving ecosystem of interconnected applications that benefit users and drive business growth.
  4. Platform Independence: APIs provide a platform-independent means of communication. They allow applications to interact with each other irrespective of their underlying technologies, operating systems, or programming languages. This flexibility empowers developers to create diverse software solutions that can seamlessly integrate and interoperate with various systems.

APIs are the backbone of modern software development, enabling applications to interact, share data, and provide powerful functionalities. They offer a standardized and efficient way for systems to communicate with each other.

Types of APIs

Most of the APIs you’ll encounter can be broken down into four main types: Open API, Partner API, Private API, and Composite API.  

Open API

Also called public APIs, these are your completely public access APIs, with no restrictions on who can use them. Businesses often favor these as they allow third parties to introduce new and innovative ideas to improve user experience. Sometimes businesses will encourage this type of innovation by tailoring their APIs to developers.   The developer community generally looks favorably upon making APIs public due to the number of integrations that will be developed around it. Many API integrations make both the API and the application sharing its data more useful and valuable.   Facebook released an excellent example of an open web API. You’ve likely used at least one app before that allowed you to make an account with your Facebook credentials rather than a separate login, even though Facebook doesn't own the site you logged into. In this case, the company used Facebook’s API as part of its login process.   Another example is Google Maps. Their open API not only allows Maps integration with third parties. Game developers can use realistic terrain in their designs.  

Partner API

These types of API is not open to the public, and access is restricted through the use of specific licenses. Business partners often make use of these APIs. For example, you will see them used between a business and its client as part of a paid online service. Another example of a partner API is a recurring subscription service.   If a business wants greater control over who can access its data and dictate how it can be used, then partner APIs are a good choice. An example of this model in the real world is Pinterest's API submission program. If you are a developer wanting to access Pinterest data services by way of APIs, you must submit a request to Pinterest outlining how you will use the API. If Pinterest deems your request appropriate, you will be granted partner API access.  

Private API

Also called internal APIs or enterprise APIs, private APIs are types of APIs that are used only within a single company. These are often used to integrate different company services and streamline workflows. A company intranet could make use of a private API. As private APIs aren't publicly released, they typically aren't documented in any way that's available to the general public. But most private APIs would be of little use to the developer community anyway, as they are typically constructed to facilitate data transfers between internal systems. Companies typically use these types of APIs to build a scalable architecture. Building systems around APIs is an example of a microservice-based architecture. Using this methodology ensures that companies don't get bogged down developing applications in monolithic architecture, which is usually a less nimble and agile framework. As the company adds more systems, these can easily integrate with the existing infrastructure via APIs.  

Composite API

Composite APIs are a combination of data and service are types of APIs that are used to speed up the execution of certain tasks and improve performance. A composite API can enable these calls to run together as a single service if a task uses multiple API endpoints. An example of this could be a shopping cart for an Ecommerce web application.   The advantage that some companies see in composite APIs is that they can be used to collect data from several different endpoints. This reduces the overall number of API calls a server has to make, reducing workload and making for faster systems. In the microservice-based architecture, composite APIs are more commonly used because of their ability to make calls to several private APIs at once.   As you can see, the various categories of APIs all serve different scenarios. Typically, business needs or project requirements will decide which type of API will be best suited for a particular task.

API Architecture Types

Another way to understand and categorize APIs is by their architecture. An API's architecture effectively governs what information an API can share and how it does that sharing. Here's a detailed look at some of the most popular APIs in use today.  

RESTful API Protocols

REST is short for “representational state transfer” and refers to an architectural style of API. The REST architecture has a set of characteristics that includes:   Client-Server Separation: This defines the roles of the client and server in an API exchange. In essence, the client makes a request, and the server responds. Servers can't make requests, and clients can't issue responses.   HTTP Interface: All requests and responses must use the HTTP protocol, with requests formatted in a specific way. This is done to ensure compatibility between any pair of clients and servers, regardless of platform or operating system.  Stateless: Each request and response is made independent of every other exchange. No client data is stored on the server, and the server doesn't remember anything from past interactions. The original "Web 1.0" version of the World Wide Web was entirely stateless; as such, there were no usernames or passwords to log in with or cookies to keep track of your interactions with the server. The only activity that took place was the client (your web browser) making an HTTP request for an HTML page from the server. REST APIs behave in a very similar stateless fashion.  

SOAP (Simple Object Access Protocol)

SOAP (short for Simple Object Access Protocol) APIs are among the original web services, dating back to Web 1.0 days. However, SOAP APIs are still widely used by enterprises and government systems due to SOAP's strict, more defined security protocols. Most businesses today would benefit more from the flexibility, simplicity, and reduced bandwidth REST provides.   


XML stands for “Extensible Markup Language,” and RPC stands for “Remote Procedure Call.” XML-RPC uses XML to encode API calls and HTTP to transport them.   The use of the XML format is designed to be both human and machine-readable, which could be useful if you have minimal code knowledge and are willing to learn more. WordPress is one company that makes use of XML-RPC in its platform.  


This is a similar protocol to XML-RPC except for the use of JSON instead of XML. JSON is short for “JavaScript Object Notation” and is similar in that it’s relatively simple to read by humans. It provides maximum browser compatibility and is simpler to implement and use.   Unlike REST and SOAP, both XML-RPC and JSON-RPC can initiate processes on the server. That means they can run scripts, launch applications, start data transfers, and other actions on the server. Not surprisingly, this raises some security concerns. For this reason, you most often see the RPC-style APIs used in internal systems, where there is no concern about external threats.  


Initially designed by Facebook, the Thrift protocol was eventually made open source and is now part of Apache’s software lineup. It can work with JSON or binary notation, among other languages. The API protocol is actually just one part of Thrift, as it supports its own compiler to convert various languages into what is known as a "stub code." The stub code looks very similar to XML or JSON but has several features unique to other API protocols. This language independence is a critical feature that attracts developers and DevOps teams to use Thrift.

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Frequently Asked Questions: Types of APIs

What are the main types of APIs?

The main types of APIs are:

  • Open API: Publicly accessible APIs with no restrictions.
  • Partner API: Restricted APIs for business partners and clients.
  • Private API: Internal APIs used within a single company.
  • Composite API: Combine data and services to streamline tasks.

What is the significance of Open APIs?

Open APIs encourage innovation by allowing third parties to introduce new ideas and integrations, improving user experiences. They can enhance the value and usefulness of both the API and the applications using its data.

How do Partner APIs work?

Partner APIs are restricted and require specific licenses for access. They are commonly used between businesses and their clients for paid online services. The company has greater control over data access and usage with partner APIs.

What are Private APIs used for?

Private APIs, also known as internal or enterprise APIs, are used within a single company to integrate different services and streamline workflows. They are designed to facilitate data transfers between internal systems.

How do Composite APIs improve performance?

Composite APIs combine data and services, reducing the number of API calls a server needs to make. This improves performance by reducing workload and enabling faster systems. They are often used in microservice-based architectures.

What is the RESTful API protocol?

RESTful API (Representational State Transfer) is an architectural style for building APIs. It has client-server separation, uses HTTP as its interface, and is stateless. It provides a flexible and scalable approach to API design.

What is the advantage of using RESTful APIs?

RESTful APIs offer compatibility between different clients and servers, regardless of platforms or operating systems. They are widely adopted, well-documented, and simplify communication and data transfer between applications.

How do SOAP APIs differ from RESTful APIs?

SOAP (Simple Object Access Protocol) APIs are older and have stricter security protocols. While they are still used in enterprise and government systems, RESTful APIs are more popular today due to their simplicity, flexibility, and reduced bandwidth requirements.

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