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What is an API?

An API, which stands for application programming interface, is a set of protocols that enable different software components to communicate and transfer data. Developers use APIs to bridge the gaps between small, discrete chunks of code in order to create applications that are powerful, resilient, secure, and able to meet user needs. Even though you can't see them, APIs are everywhere—working continuously in the background to power the digital experiences that are essential to our modern lives.

Here, we'll give a high-level overview of how APIs work before reviewing the different types of APIs and how they are used. We'll also discuss some common use cases for APIs—and offer a few real-world API examples that can help you get started.

Diagram What is an API Postman. Illustration.

How do APIs work?

APIs work by sharing data between applications, systems, and devices. In order to better understand this process, it can be useful to think of APIs like restaurants. In this metaphor, the customer is like the user, who tells the waiter what she wants. The waiter is like an API, receiving the customer's order and translating it into easy-to-follow instructions for the kitchen—sometimes using specific codes or abbreviations that the kitchen staff will recognize. The kitchen staff is like the API server because it creates the order according to the customer's specifications and gives it to the waiter, who then delivers it to the customer.

Once that metaphor makes sense, you can go a level deeper and start reviewing the different components of an API, starting with the API client. The API client is responsible for assembling requests in response to user actions and sending them to the appropriate API endpoint. Endpoints are Uniform Resource Identifiers (URIs) that provide access to specific resources in a database. For instance, if a user wants to see all of the products at an e-commerce store, the API client will send a GET request to the /products endpoint.

The API server receives the request, confirms that it's valid, retrieves or manipulates the appropriate data, and returns a response to the client. In the case of a successful GET request to the /products endpoint, the response would include all of the products in the store's catalog, as well as a 200 status code.

Diagram Different types of APIs Postman. Illustration.

What are the different types of APIs, and how are they used?

There are many different types of APIs and ways to categorize them. For instance, you can categorize APIs by who has access to them. This organizational framework includes:

  • Private APIs: Private APIs, also known as internal APIs, are used to connect different software components within a single organization, and they are not available for third-party use. For instance, a social media application might have a private API that handles the login workflow, another private API that handles the feed, and yet another private API that facilitates communication between users. Some applications may include dozens or even hundreds of private APIs.
  • Public APIs: Public APIs provide public access to an organization's data, functionality, or services, which third-party developers can integrate into their own applications. Some public APIs are available for free, while others are offered as billable products. For instance, an e-commerce application may incorporate a public payment API, such as Stripe, to handle payment processing without having to build that functionality from scratch.
  • Partner APIs: Partner APIs enable two or more companies to share data or functionality in order to collaborate on a project. They are not available to the general public and therefore leverage authentication mechanisms to ensure they are only used by authorized partners.

You can also categorize APIs according to their architectural style, of which there are many. The most frequently used architectural styles are:

  • REST: As discussed above, REST is the most popular API architecture for transferring data over the internet. In a RESTful context, resources are accessible via endpoints, and operations are performed on those resources with standard HTTP methods such as GET, POST, PUT, and DELETE.
  • SOAP: SOAP, which stands for Simple Object Access Protocol, uses XML to transfer highly structured messages between a client and server. SOAP is often used in enterprise environments or legacy systems, and while it includes advanced security features, it can be slower than other API architectures.
  • GraphQL: GraphQL is an open source query language that enables clients to interact with a single API endpoint to retrieve the exact data they need, without chaining multiple requests together. This approach reduces the number of round trips between the client and server, which can be useful for applications that may run on slow or unreliable network connections.
  • Webhooks: Webhooks are used to implement event-driven architectures, in which requests are automatically sent in response to event-based triggers. For instance, when a specific event occurs in an application, such as a payment being made, the application can send an HTTP request to a pre-configured webhook URL with the relevant event data in the request payload. The system that receives the webhook can then process the event and take the appropriate action.
  • gRPC: RPC stands for Remote Procedure Call, and gRPC APIs were originated by Google. In gRPC architectures, a client can call on a server as if it were a local object, which makes it easier for distributed applications and systems to communicate with one another.
Private, partner, and public APIs. Illustration.

What are some common API use cases?

APIs are extremely versatile, and they support a wide range of use cases that includes:

Integrating with internal and external systems

One of the most common reasons developers turn to APIs is to integrate one system with another. For instance, you can use an API to integrate your customer relationship management (CRM) system with your marketing automation system, which would allow you to automatically send a marketing email when a sales representative adds a new prospective customer to the CRM.

Adding or enhancing functionality

APIs let you incorporate additional functionality into your application, which can improve your customers' experience. For instance, if you're working on a food delivery application, you might incorporate a third-party mapping API to let users track their order while it's en route.

Connecting IoT devices

APIs are essential to the Internet of Things (IoT) ecosystem, which includes devices such as smart watches, fitness trackers, doorbells, and home appliances. Without APIs, these devices would not be able to connect to the cloud—or to one another—which would render them useless.

Creating more scalable systems

APIs are used to implement microservice-based architectures, in which applications are built as a collection of small services that communicate with one another through private APIs. Microservices are managed, deployed, and provisioned independently of one another, which enables teams to scale their systems in a reliable yet cost-efficient way.

Reducing costs

APIs help organizations reduce operational costs by automating time-intensive tasks, such as sending emails, pulling reports, and sharing data between systems. They can also reduce development costs by enabling teams to reuse existing functionality, instead of reinventing the wheel.

Improving organizational security and governance

APIs power many workflows that are essential for organizational security. For instance, single sign-on (SSO), which enables users to use one username and password for multiple systems, is made possible by APIs. APIs are also used to enforce and automate corporate governance rules and policies, such as a requirement that expenses be approved before employees are reimbursed.

This list is far from exhaustive, and it will keep growing as developers continue to create innovative solutions that change the ways we live, work, and interact with one another.

Private, partner, and public APIs. Illustration.

What are some real-world examples of APIs?

If you're looking for real-world examples of APIs, a good place to start is a public API catalog, such as Postman's Public API Network. The Public API Network, which supports a community of over 25 million developers, is a searchable, highly organized library of APIs that makes it easy to find the API that's right for you. You can use the network's search functionality if you're looking for something specific, or browse by category if you want to get inspired. Some companies that have published great examples of APIs on the Public API Network include Salesforce, Notion, Discord, Pinterest, and DoorDash.

Other common questions about APIs

Who works with APIs?

While developers are most likely to work with APIs as part of their job, Postman's State of the API report found that many non-developers, such as product managers, business analysts, and customer support professionals, work with APIs, as well.

Which industries use APIs?

APIs are used extensively in the technology industry, as they are the primary building blocks of applications and other digital services. Financial institutions also rely on APIs to facilitate their customers' transactions, and healthcare providers use APIs to manage patient data and keep it secure.

What is the API-first strategy?

The API-first strategy is an approach to software development in which applications are designed and built as a collection of internal and external services that are delivered through APIs. APIs are the building blocks of these applications, and the API-first strategy helps teams prioritize their quality, security, and performance.

What are some tools that can help you build and integrate APIs?

API development is an iterative and collaborative process, so it's important to leverage the appropriate tooling to ensure everything runs as smoothly as possible. For instance, teams should use a source control management tool like GitHub or BitBucket to keep track of API changes, and a CI/CD pipeline such as Jenkins or CircleCI will help them automate the API testing and deployment processes. It's also essential for teams to use an API platform that integrates with these tools, which will reduce friction and augment existing workflows.

How do you build an API?

The API development process can vary widely according to the API's purpose, language, and scope. Nevertheless, every new API will need to be designed, implemented with an API development framework, and thoroughly tested to ensure it's working as expected. Learn more about how to build an API.

The API-First Transformation is here

Kin Lane's book The API-First Transformation shows business leaders how to create an API-first strategy, assemble the right technology, and put solutions to work.

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