API documentation

What is API documentation?

API documentation is a set of human-readable instructions for using and integrating with an API.

API documentation includes detailed information about an API's available endpoints, methods, resources, authentication protocols, parameters, and headers, as well as examples of common requests and responses. Effective API documentation improves the developer experience for private, partner, and public APIs, but it also offers distinct benefits for each API type. For instance, private API documentation improves cross-team collaboration, while public API documentation makes it easier for leaders to understand a third-party API's intended use case and determine whether it will help advance their organization's business goals. Teams that prioritize API documentation typically see higher rates of API adoption, fewer support tickets, and—in the case of public APIs—increased revenue.

Here, we'll start by discussing the role that API documentation plays in an API-first world. Then, we'll review the key components of API documentation, as well as some API documentation best practices. Finally, we'll explore how the Postman API Platform enables producers to create API documentation that sets their consumers up for success.

Why is API documentation critical in an API-first world?

API-first is a development model in which applications are conceptualized and built by composing internal or external services that are delivered through APIs. This approach not only enables teams to build highly performant applications that are powered by an intricate web of microservices, but also complements the API-as-a-Product strategy, in which APIs are offered as billable products to third-party consumers. An increasing number of organizations are therefore adopting the API-first strategy to help them systematically develop high-quality APIs that advance business objectives in myriad ways.

API documentation plays a crucial role in ensuring the success of any API—whether it's private or public. For instance, internal API documentation facilitates cross-team collaboration, reduces code duplication, and streamlines the onboarding process for new employees. These benefits ensure that every team is able to work efficiently towards the end goal of delivering excellent software to users. In contrast, public API documentation helps potential consumers understand and experiment with an API, which leads to increased adoption and, by extension, revenue. In fact, Postman's 2022 State of the API report indicates that documentation is one of the top four things leaders consider when deciding whether to integrate with a third-party API.

What are the most common types of API documentation?

There are many different types of API documentation, and each one plays an important role in helping consumers use an API effectively. Four of the most common types are:

• Reference documentation: Reference documentation typically provides a rundown of every endpoint, including its methods, parameters, and accepted data types. This type of documentation also describes—in plain language—what each endpoint is intended to do.
• Tutorials: Some API documentation is presented in the form of tutorials, which provide step-by-step instructions for using the API. These tutorials are often focused on a specific use case that the API is intended to support, and they may also cover common workflows that are required to get started, such as authentication.
• Examples and code samples: Sample-based documentation provides examples of common API requests and responses. This type of documentation is often provided in several programming languages, and it helps the reader better understand what to expect from the API.
• Release notes: Release notes include updates on important changes to an API, such as new features, bug fixes, or security patches. Release notes are an important resource for an API's consumers, as some changes may affect their own codebase.

What should be included when creating API documentation?

Every API is different and therefore requires documentation that is tailor-made for its consumers. Nevertheless, the following components can serve as an initial checklist for creating high-quality API documentation:

Authentication instructions

Authentication helps keep an API's data safe and secure, and it is the first hurdle that a developer must cross when using a new API. If an API's authentication process is too difficult or poorly documented, the developer might become frustrated and decide to try a different API. API documentation must therefore include a clear explanation of the available authentication methods and provide thorough, step-by-step instructions for obtaining and using authentication credentials.

Detailed information about every endpoint, operation, and resource

API documentation should offer a comprehensive overview of every API endpoint and operation, including parameters, headers, and request and response bodies. It should also thoroughly explain the relevant data models, including their required attributes and any default, minimum, and maximum values. These details help ensure full coverage of every possible use case and empower consumers to construct complex requests that might otherwise be prone to errors.

Examples of common requests and responses

Examples are a crucial part of effective API documentation, as they help consumers understand endpoint behavior under a variety of conditions. Producers should include example requests in several client languages, as well as example responses, which enable consumers to troubleshoot issues they might encounter. Examples can also be used to guide new users through a sequence of API calls that are involved in a specific workflow, which provides important insight into how an API can support sophisticated use cases.

Terms of Use

Public API documentation should include a Terms of Use, which is a legal agreement that helps producers ensure their API's data and functionality is not abused by consumers. It should also include information on the API's rate limits, which dictate how many API calls a consumer can make in a given period of time. Rate limits help protect an API from denial-of-service (DoS) attacks, as well as any other activity that may negatively affect its performance. Consumers who exceed their rate limit will be temporarily unable to execute any additional calls, which may lead to user-facing downtime.

How do you write API documentation?

Writing API documentation is a multi-step process that requires familiarity with the API's functionality, empathy for its consumers, and a willingness to iterate. To ensure your documentation is effective, you should:

• Understand the API: Anyone who is writing API documentation needs to not only understand the API's purpose, but also be familiar with its endpoints, methods, parameters, accepted data types, and authentication mechanisms. This will help ensure that the documentation is accurate and complete.
• Know your audience: API documentation is consulted by a wide range of audiences that may have different levels of technical knowledge. It's therefore important to identify your primary audience—and understand their needs—to ensure your documentation is useful.
• Provide detailed instructions for the most common use cases: While you should strive to thoroughly document the API's complete functionality, you should pay special attention to the most common use cases. Additional details, such as code samples and example requests, will help consumers get up and running with these use cases quickly.
• Review, test, and verify the documentation: Everyone makes mistakes, so it's critical to thoroughly test your documentation before publishing it. This process should involve a thorough walk-through of every use case and request, as well as an additional review by a stakeholder who was not directly involved in the documentation writing process.
• Continuously update the documentation: APIs evolve quickly, and outdated documentation can confuse consumers and erode their trust. It's therefore essential that you methodically review your documentation whenever you ship new code—and make updates as necessary.

What are some best practices for creating API documentation?

API documentation is an essential deliverable that has a significant impact on consumers, and its quality can be directly correlated with the overall success of the API. It is therefore crucial for teams to adhere to the following best practices when writing API documentation:

Tell a compelling story

Every API plays a unique role in the software landscape of its producers and consumers, and API documentation is responsible for telling its story. Documentation readers should be able to learn who the API is meant for, how they can use it, and how it can help them achieve their goals. This “big picture” provides important context for more technical implementation details, which can be useful as developers begin to understand the possibilities of a given API.

Keep the documentation up-to-date

Many API development teams ship code changes several times a week, which puts their documentation at risk of falling out of date. Outdated documentation erodes consumers' trust, especially when updates are not backward compatible. It's therefore essential for teams to systematize the process of updating their documentation to ensure it always reflects the current state of their API in production. They should also capture updates in a changelog, which is a dated record of every change to an API's resources and functionality.

Write for a range of audiences

API documentation is an important resource for a wide range of software and business professionals. Developers will consult an API's documentation to learn how to interact with it, while CTOs may use documentation to help them understand an API's pricing and evaluate whether it will help them meet their business goals. Documentation writers must therefore keep this diverse audience in mind. For instance, they must thoroughly describe the API's functionality without relying too heavily on technical language or obscuring the larger purpose that the API serves.

What are some examples of API documentation?

Postman's Public API Network is a global, centralized API catalog, where producers can share their APIs and API documentation with a community of over 30 million developers. Teams that publish their API documentation on the Public API Network can include detailed descriptions and tutorials, example requests and responses, and environment variables, which can increase API adoption and reduce ticket volumes. Some teams that have published excellent API documentation in the Public API Network include Stripe, Notion, PayPal, Amplitude, Salesforce, and DoorDash, and this list just scratches the surface. Explore the Public API Network to see more.

Why use Postman for API documentation?

The Postman API Platform includes several features that enable teams to make effective documentation a core part of their API workflow. With Postman, you can:

• Automatically generate API documentation: Postman enables users to automatically generate API documentation for any OpenAPI 3.0 definition, as well as for any collection they create. Postman API documentation includes information about each path, operation, and data model, while collection documentation includes sample code in various client languages, as well as key-value pairs for request parameters, headers, and bodies.
• Keep API documentation up-to-date: Postman automatically updates documentation to reflect the latest changes to a definition or collection, which helps teams ensure that their consumers always have the most up-to-date information about their API.
• Enhance collection documentation with more details: API producers can add more information to collection documentation, such as descriptions, text, tables, and images, in the visual Postman editor or classic Markdown editor. These details provide additional context that can help consumers better understand an endpoint's purpose or functionality.
• Use variables to connect documentation to specific environments: Postman users can create and save variable values for specific environments—and share those environments as part of their documentation. This can be especially useful for teams who want to offer a testing environment to go along with their API's documentation, which enables potential consumers to experiment with the API without incurring costs or having to go through the full authentication process.
• Publish documentation alongside other public API artifacts: API producers can publish documentation alongside their workspaces and collections in the Postman API Network, which enables consumers to more easily discover, explore, and begin working with the API. Producers can also publish their documentation to public domains.