API design

What is API design?

API design is the process of making intentional decisions about how an API will expose data and functionality to its consumers. A successful API design describes the API's endpoints, methods, and resources in a standardized specification format.

The API design process benefits both consumers and producers by ensuring that APIs support business objectives while remaining easy to use, adaptable, testable, and well-documented. API design should occur early in the API lifecycle in order to achieve alignment among key stakeholders and to help teams identify issues before they become ingrained. API design is also an important part of an effective API governance strategy, as it helps teams standardize API patterns that can be reused across their organization.

Here, we'll explore the relationship between API design and the API-first development model, the key stages of API design, the role of mocking in API design, and how the Postman API Platform can help your organization implement mature API design processes.

How does API design support the API-first development model?

API-first is a development model in which applications are conceptualized and built with services that are delivered through APIs. Whereas companies that take a code-first approach might view APIs as afterthoughts, API-first companies design their APIs before they develop their applications. This strategy enables consumers and producers to collaborate on API definitions before the implementations are built out, which improves both the quality and usability of APIs.

What are some API design approaches?

There are a few different API design approaches that are commonly used, including:

Inside-out API design

With the inside-out approach, the API provider's pre-existing backend system serves as the starting point. The internal implementation details and capabilities of the backend are then exposed through the API. This approach provides granular control, direct access to internal processes, and rich functionality but may lead to tightly coupled APIs and increased complexity for consumers.

Outside-in API design

Outside-in API design focuses on designing APIs based on the needs and requirements of external consumers, abstracting away internal complexities. It prioritizes simplicity, ease of use, and flexibility, allowing consumers to interact with the API without knowledge of the underlying implementation. This approach often produces loosely coupled APIs that are easier to maintain and evolve.

Agile API design

With the agile API design approach, developers start work on the API without a completed spec. It emphasizes flexibility, collaboration, and responsiveness to change throughout the API development process. Developers must iterate quickly to deliver value to consumers while adapting to evolving requirements and priorities. This approach encourages close collaboration between developers, stakeholders, and end-users to ensure that the API meets their needs.

What are the key stages of API design?

There are four key steps in the API design process that every organization should follow. Each step requires collaboration between stakeholders—such as business leaders, developers, consumers, and partners—to ensure that the API meets all relevant needs. Collaborating at every step of the API design process also helps developers avoid building unnecessary functionality. These steps are:

Step 1: Determine what the API is intended to do

The first step in the API design process is for all stakeholders to agree on the API's business use case. An API that is responsible for an authentication workflow will have different requirements than an API that allows a user to browse a product catalog, so it's important to align on the use case before making any other decisions. The use case may also have implications on the type of architecture you choose. For instance, a gRPC-based architecture might make the most sense for an API that connects internal microservices, while a GraphQL API would be well-suited for a service that relies on disparate data sources. Once in agreement, stakeholders should clearly outline their goals for the API by describing—in natural language—exactly how it will meet specific needs.

Step 2: Define the API contract with a specification

Once all stakeholders are aligned on the API's use case, you will need to decide which resources are required, how their data should be formatted and structured, how they should relate to one another, and which methods should be available on their associated endpoints. It's also important to determine the desired levels of abstraction and encapsulation in your API, which will help you strike a balance between reusability and legibility.

These decisions should be captured in an API definition, which is a human- and machine-readable representation of an API's intended functionality. API definitions adhere to API specifications, such as OpenAPI and AsyncAPI, which provide a standardized format for API definitions and lay the foundation for API contracts, documentation, mocks, and tests.

Step 3: Validate your assumptions with mocks and tests

Once you've completed your API definition, you can use it to generate mock servers. Mock servers return sample data in response to requests, which enables you to confirm that your API will work as you intend it to. Mocks can also be used alongside API tests, which can be run manually, on a schedule, or automatically within CI/CD pipelines. Testing and mocking during the API design process will help you catch and remediate any issues before they find their way into your consumers' codebases, when they are much more difficult to fix.

We'll discuss mock servers in more detail in a later section.

Step 4: Document the API

The last step in the API design process is to write documentation. This step, which involves defining key details about every resource, method, parameter, and path, helps validate the design and ensure that consumers are able to start using your API as quickly as possible. Documentation might also include examples of API requests and responses, which give consumers crucial insight into how a particular API supports common business needs. Some tools can automatically generate documentation from an API definition, so teams don't have to worry about their documentation becoming outdated.

What is the role of mocking in API design?

Mocking, which involves setting up mock servers to return sample data in response to API requests, is a crucial part of the API design process. Mocks can be introduced as soon as your definition is complete, which means they can be used alongside tests to validate design choices and confirm that your API will work as expected. Mocking not only enables API consumers to start integrating an API while it is still in development, but also reduces pressure on API producers to publish an implementation that is buggy or incomplete. This benefit allows producers and consumers of internal APIs to work concurrently, which significantly reduces the time to market.

What are some common API design patterns?

An API design pattern is a standardized solution to common design problems encountered when creating APIs. An effective API design pattern provides a structured approach to designing reusable and maintainable APIs. Some popular API design patterns include:

• Request-response: This is the most basic and widely used pattern in API design. In this pattern, a client sends a request to the server, and the server processes the request and sends back a response. This pattern is synchronous and works well for performing CRUD operations on resources.
• Pagination: This pattern is used to handle large datasets by breaking them into smaller, more manageable chunks. Common approaches include using limit-offset (e.g., ?limit=10&offset=20) or cursor-based pagination, where the client uses a cursor to fetch the next set of results. This helps improve performance by loading data incrementally.
• Rate limiting: Rate limiting controls the number of requests a client can make to the API within a specified time period, which helps protect APIs from abuse. This pattern can be implemented using tokens, quotas, or sliding windows, and typically involves returning appropriate HTTP status codes and headers to inform clients about their rate limit status.
• API authentication and authorization: This API design pattern ensures that only authenticated and authorized users can access specific API endpoints. Common methods include API keys, OAuth 2.0, and JSON Web Tokens (JWT). This pattern involves secure transmission of credentials, token management, and defining access controls to protect sensitive data and functionality.
• Webhooks: This API design pattern allows APIs to push real-time updates to clients by making HTTP requests to a client-defined URL when certain events occur. Unlike the traditional request-response cycle, webhooks are asynchronous and are used for notifications, event-driven workflows, and integrations with external systems. Clients register their URLs with the API provider, which then sends POST requests with event data to those URLs.

What are some API design best practices?

Every API is different and will therefore require a unique approach to API design. Still, there are several best practices that you should always keep in mind—regardless of the API's architecture, language, or use case. For instance, it's important to:

• Prioritize consistency: Consistency is a key ingredient of a successful API design, so it's crucial for leaders to establish an API governance strategy that promotes organization-wide standards. For instance, every API in an organization's portfolio should use consistent naming conventions for every method, endpoint, and resource.
• Gather input from every stakeholder: API design issues often stem from poor communication. Each stakeholder is likely to have domain-specific knowledge that may impact the API's design and implementation, and they should be included in every conversation.
• Understand the API's context and constraints: Teams must have a clear understanding of an API's context and constraints before they make any design decisions. For instance, it's important to consider competing project timelines, limitations in the underlying system, and expected traffic volumes. This knowledge will enable teams to make informed decisions that set the API up for success.

Other common questions about API design

What is API-first design?

API-first design is an approach to API design that prioritizes the quality of the API's interface contract. The resulting APIs are reusable, secure, efficient, intuitive, and aligned with the organization's goals.

What makes a well-designed API?

A well-designed API is easy to understand, use, and maintain. It should follow consistent style conventions, include built-in security mechanisms for authentication and data encryption, and reliably handle large volumes of traffic.

How can I design an API?

To design an API, you must first have a clear understanding of the API's intended use case. You should then define the API's contract with a specification, validate your assumptions with mocks and tests, and clearly document every resource, method, parameter, and path. It's also important to collaborate with other stakeholders throughout the entire process.

How long does it take to design an API?

API design is a highly iterative process that varies in duration according to the API's use case and requirements. It can take anywhere from a few weeks to several months.

What is RESTful API design?

RESTful API design is the process of designing an API that follows the principles of Representational State Transfer (REST), which is the most popular API architecture today. In a RESTful architecture, resources are identified by URIs (Uniform Resource Identifiers), and the client interacts with those resources with standard HTTP methods such as GET, POST, PUT, and DELETE.

Why use Postman for API design?

The Postman API Platform, which was ranked the best tool for API design by G2, comes equipped with a robust suite of features that can help teams implement mature API design processes. With Postman, you can:

• Generate and edit API definitions: Postman enables you to import an existing API definition or generate a new one from scratch. Postman supports OpenAPI, RAML, Protobuf, GraphQL, or WSDL definitions, so you can choose the specification that works best for you.
• Document your API: Postman automatically generates documentation for any collection, as well as for any OpenAPI 3.0 definition. Your documentation will update automatically as you make changes, ensuring that your documentation remains useful to consumers.
• Build, run, and automate API contract tests: Postman provides a library of customizable code snippets that can be used to create API contract tests. You can run tests manually within Postman—or use Newman or the Postman CLI to run them automatically within your CI/CD pipeline.
• Simulate API behavior with mock servers: Postman enables you to set up mock servers that will return sample data in response to requests—even if your API isn't in production yet. This allows producers to validate their assumptions without rushing development, while giving consumers a jump start in the integration process.
• Collaborate across your organization: Collaboration is central to Postman's DNA, and Postman users can leverage team workspaces to easily collaborate on API projects. Postman also supports commenting across the platform, which reduces friction by allowing teams to communicate in the same place where they do their work. Finally, Postman is intuitive and user-friendly, which makes it accessible to developers and business stakeholders alike.