An API is a contract between different parts of a software system. In a world of microservices, mobile apps, and third-party integrations, the quality of API design determines how easy a system will be to develop and maintain over the long term. A well-designed API is intuitive for developers, resilient to change, and flexible enough for a variety of use cases. A poorly designed API becomes a constant source of problems, requiring workarounds and creating technical debt.
In this article, we discuss API design principles and practices that help create usable, reliable, and long-lasting interfaces.
Choosing a paradigm: REST, GraphQL, or something else
Before getting into the design details, you need to choose an overall approach to organizing the API. REST and GraphQL are the dominant paradigms in modern web development, each with its own advantages and limitations.
REST (Representational State Transfer) organizes an API around resources—the entities an application works with. Each resource has a unique identifier (URL), and operations on resources are expressed through HTTP methods: GET for reading, POST for creating, PUT or PATCH for updating, and DELETE for deleting. This model works well for systems with a clear domain model and predictable data operations.
GraphQL takes a fundamentally different approach. Instead of multiple endpoints for different resources, there is a single endpoint that accepts queries in a special language. The client specifies exactly what data it needs, and the server returns just that—no more and no less. This addresses the under-fetching and over-fetching problems common with REST, but adds complexity on the server side and requires client developers to have a deeper understanding.
The choice between REST and GraphQL depends on the characteristics of the project. For public APIs with a broad audience and diverse clients, REST is often preferable because of its simplicity and universality. For internal APIs with known consumers and complex relationships between data, GraphQL can provide a better developer experience and performance.
Other approaches also exist: gRPC for high-performance inter-service communication, WebSocket for real-time two-way communication, and Server-Sent Events for server push notifications. Often, the best solution combines several approaches for different parts of a system.
Resource naming and structure
In a REST API, resource names and URL structure are important for understanding and using the API. Good names make an API self-documenting: developers can guess an endpoint's purpose from its URL.
Resources are named with plural nouns. The user collection is available at /users, and a specific user at /users/123. This convention is well established and expected by developers. Verbs in URLs are a sign of procedural thinking, which is incompatible with the REST resource model.
Nested resources reflect relationships between entities. Posts by a specific user are available at /users/123/posts, and comments on a post at /posts/456/comments. However, deep nesting makes URLs more complex and can create caching problems. If a resource has its own identifier, it is often preferable to expose it at the top level.
Operations that do not fit the CRUD model require a creative approach. The “cancel order” action can be represented as creating a cancellation resource, POST /orders/123/cancellation, or as updating the status with PATCH /orders/123 and a body containing the new status. Both approaches are acceptable; the choice depends on the operation's semantics in the specific domain.
Error handling and HTTP status codes
HTTP provides a rich set of status codes that should be used appropriately. Correct response codes let clients understand the result of a request without analyzing the response body and respond accordingly.
2xx codes indicate success. 200 OK is a general success response. 201 Created means a resource was created successfully, usually with a Location header pointing to the new resource. 204 No Content indicates success with no response body, which is typical for DELETE.
4xx codes indicate a client error. 400 Bad Request means the request is invalid and cannot be processed. 401 Unauthorized means authentication is required. 403 Forbidden means authentication succeeded, but access is denied. 404 Not Found means the resource does not exist. 409 Conflict indicates a state conflict, such as an attempt to create a duplicate. 422 Unprocessable Entity means the request is syntactically valid but semantically invalid (validation errors).
5xx codes indicate a server error. 500 Internal Server Error is a general server error. 502 Bad Gateway indicates an error when communicating with an upstream service. 503 Service Unavailable means the service is temporarily unavailable.
The error body should provide enough information to understand and fix the problem. A machine-readable error code lets the client respond programmatically. A human-readable message explains the problem to the developer. For validation errors, it is important to specify the affected fields and the reasons for the errors.
API versioning
Any successful API eventually needs changes: new features, fixes to design flaws, and adaptation to changing requirements. Versioning lets you make changes without breaking existing clients.
A version in the URL is the most explicit and common approach. The path /v1/users clearly indicates the first version of the API. When a new version is created, the old one continues to work at /v1/, while the new one is available at /v2/. Clients can migrate at their own pace.
A version in the header is an alternative approach that keeps URLs clean. The client specifies the desired version in the Accept header or a custom header. This approach is more elegant in theory, but less convenient in practice: it is harder to test in a browser and harder to share links.
Regardless of the approach you choose, a version support policy is important. Clients should know how long an old version will be supported, when they should stop using it, and what the migration schedule is. Too many parallel versions create a maintenance burden; deprecating versions too quickly frustrates consumers.
Semantic versioning helps communicate the nature of changes. The major version increases for breaking changes. The minor version increases when functionality is added without breaking compatibility. The patch version increases for bug fixes.
Authentication and authorization
API security starts with reliable authentication and granular authorization. Without them, user data is at risk and the system is open to abuse.
OAuth 2.0 and its extension, OpenID Connect, have become standards for API authorization. They let users grant an application limited access to their data without sharing their credentials. Access tokens have a limited lifetime and set of permissions, minimizing the consequences of compromise.
JWT (JSON Web Tokens) are often used as access token format. They contain encoded information about the user and their permissions, signed by the server. The advantage of JWT is that a token can be validated without contacting a database or authentication service. The disadvantage is that a token cannot be revoked before it expires without additional infrastructure.
API keys are a simpler mechanism suitable for machine-to-machine communication and public APIs with rate limits. The key is sent in the header of each request and identifies the calling client.
Rate limiting—the restriction of the number of requests—protects an API from abuse and ensures fair resource allocation among clients. Limits can be set globally or for individual endpoints and user categories.
Documentation and developer experience
Even a perfectly designed API is useless if developers cannot understand how to use it. Documentation is an integral part of an API and deserves as much attention as the code.
The OpenAPI Specification (formerly Swagger) has become the standard for describing REST APIs. It allows endpoints, parameters, data formats, and response codes to be formally described in a machine-readable format. Interactive documentation, client libraries, and server stubs can be generated automatically from the specification.
Interactive documentation lets developers experiment with an API right in the browser. The ability to send a test request and see the actual response speeds up learning and debugging.
Usage examples are a critically important part of documentation. An abstract description of parameters is not enough; developers need concrete examples of requests and responses for typical scenarios. Examples should work—copying and pasting them should produce the expected result.
Getting started guides take a new developer from registration to their first successful request. They lower the barrier to entry and create a positive first impression.
Performance and scalability
An API should remain responsive under load and use resources efficiently on both the client and server sides.
Pagination is essential for endpoints that return collections. Without pagination, a request for a list of a million items will overload both the server and the client. Cursor-based pagination is preferable to page-based pagination for large datasets that change frequently.
Server-side filtering and sorting let clients retrieve only the data they need. Transferring an entire collection for filtering on the client is inefficient and does not scale.
Compressing responses with gzip or brotli significantly reduces the amount of data transferred, especially for JSON responses with repetitive structures.
Caching at different levels—from HTTP caching with correct headers to server-side caching of query results—is critical for performance under load.
Conclusion
API design is the art of balancing many requirements: simplicity and power, stability and evolution, security and ease of use. A good API grows with the product while remaining understandable and predictable.
Investing in API quality pays off many times over. Developers are your clients, and their experience with the API affects integration success, development speed, and their overall impression of your product.
Test your APIs not only for functionality, but also for usability. Gather feedback from developers and take it into account when planning changes. Use reChecker to check the correctness of your API responses and the overall technical health of web services.