Introduction to API Design

So far, you've learned how to understand and use existing APIs. But what if you need to create your own API? How do you design an API that's clear, consistent, and easy for other developers to use?

API design is the process of planning and creating the structure, endpoints, and data models that define how your API will work. Just like designing a building requires blueprints before construction, designing an API requires careful planning before implementation.

In this article, we'll explore the fundamental concepts and principles of API design, focusing on RESTful APIs—the most common type of API you'll encounter.

1. What Is API Design?

API design is the process of defining:

What resources your API will expose (like users, orders, or products)

What operations can be performed on those resources (create, read, update, delete)

How data is structured (what fields exist, what types they are)

How clients interact with your API (endpoints, methods, parameters)

Think of API design like creating a menu for a restaurant:

The menu (API specification) tells customers what's available

The items (resources) are what you can order

The descriptions (schemas) explain what each item contains

The ordering process (endpoints) defines how to place an order

A well-designed API is like a well-organized menu: clear, consistent, and easy to understand.

2. RESTful API Design Principles

Most modern APIs follow REST (Representational State Transfer) principles. REST is not a technology or standard, but rather a set of architectural principles that make APIs predictable and easy to use.

Resource-Based Design

REST APIs are built around resources—things you can interact with. Resources are typically nouns (like users, orders, pets), not verbs.

Good examples:

/users — a collection of users

/users/{id} — a specific user

/orders — a collection of orders

Avoid:

/getUsers — uses a verb

/createUser — uses a verb

/userData — unclear naming

Use HTTP Methods to Express Operations

Instead of putting actions in the URL, REST APIs use HTTP methods to express what you want to do:

HTTP Method	Meaning	Example
GET	Retrieve data	`GET /users/{id}` — get a user
POST	Create data	`POST /users` — create a user
PUT	Replace data	`PUT /users/{id}` — update entire user
PATCH	Partially update	`PATCH /users/{id}` — update some fields
DELETE	Remove data	`DELETE /users/{id}` — delete a user

Notice how the same path (/users/{id}) can be used with different methods to perform different operations. This is a key REST principle.

Stateless

Each API request should contain all the information needed to process it. The server shouldn't need to remember previous requests. This makes APIs:

Easier to scale — any server can handle any request

More reliable — if one server fails, another can take over

Simpler to understand — each request is independent

Uniform Interface

REST APIs use a consistent, predictable structure:

Standard HTTP methods (GET, POST, PUT, DELETE)

Standard status codes (200 OK, 404 Not Found, etc.)

Standard data formats (usually JSON)

Consistent URL patterns (/resource/{id})

This consistency makes REST APIs easy to learn and use.

3. API Design Best Practices

Beyond REST principles, here are some best practices that will make your API more professional and developer-friendly:

Naming Conventions

Use clear, consistent names:

Use plural nouns for collections: /users, /orders

Use lowercase with hyphens or underscores: /user-preferences or /user_preferences

Be descriptive but concise: /users is better than /u or /user_accounts_management

Example from Pet Store User API:

✅ Good:  POST /users
✅ Good:  GET /users/{id}
❌ Bad:   POST /createUser
❌ Bad:   GET /getUserById/{id}

URL Structure

Keep URLs hierarchical and logical:

/users/{id} — get a specific user

/users/{id}/orders — get orders for a user

/users/{id}/preferences — get user preferences

Avoid deep nesting:

✅ Good:  /users/{id}/orders/{orderId}
❌ Bad:   /users/{id}/orders/{orderId}/items/{itemId}/details/{detailId}

Versioning

APIs change over time. Versioning allows you to update your API without breaking existing clients.

Common approaches:

URL versioning: /v1/users, /v2/users

Header versioning: Include version in request headers

Query parameter: /users?version=1

For most APIs, URL versioning is the simplest and most common approach.

Error Handling

Always return meaningful error messages with appropriate HTTP status codes:

Status Code	Meaning	Example
200	Success	Request completed successfully
201	Created	Resource created successfully
400	Bad Request	Invalid input data
401	Unauthorized	Authentication required
403	Forbidden	Insufficient permissions
404	Not Found	Resource doesn't exist
422	Unprocessable Entity	Validation errors
500	Internal Server Error	Server error

Good error response:

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Email address is required",
    "field": "email"
  }
}

4. API-First vs Code-First

When building an API, you have two main approaches:

Code-First Approach

Write the code (backend implementation)

Generate API documentation from the code

Share the API with clients

Pros:

Faster initial development

Documentation is always in sync with code

Cons:

API design is constrained by implementation

Harder to get early feedback

Frontend and backend teams can't work in parallel

API-First Approach (Recommended)

Design the API (endpoints, schemas, responses)

Create the specification (OpenAPI/Swagger)

Share with team for feedback

Generate mock APIs for frontend development

Implement the backend based on the specification

Pros:

Better API design (not constrained by implementation)

Teams can work in parallel (frontend uses mocks)

Early feedback and validation

Clear contract between frontend and backend

Cons:

Requires upfront planning

Specification needs to be maintained

Apidog advocates for the API-First approach because it leads to better-designed APIs and smoother team collaboration.

5. The Complete API Design Process

Designing an API is a step-by-step process. Here's the typical workflow:

Analyze Requirements

Understand what the API needs to do

Identify users and use cases

Determine business rules

Identify Resources

What are the main entities? (users, orders, products)

What are the relationships between them?

Design Data Models

What fields does each resource need?

What are the data types?

What are the validation rules?

Plan Endpoints

What operations are needed? (CRUD)

What are the URL paths?

What HTTP methods will be used?

Define Schemas

Create detailed data models

Define request and response structures

Specify validation rules

Set Up Authentication

How will users authenticate?

What endpoints require authentication?

Create the Specification

Document everything in OpenAPI format

Add examples and descriptions

Validate and Test

Review the design

Test with mock APIs

Get feedback from team

Iterate and Refine

Make improvements based on feedback

Update the specification

In the following chapters, we'll walk through each of these steps in detail, using the Pet Store User module as our example.

6. Key Takeaways

Understanding API design principles is essential before you start creating your own API:

API design is about planning the structure, endpoints, and data models before implementation.

REST principles provide a consistent, predictable way to design APIs:

Resource-based (nouns, not verbs)

HTTP methods express operations

Stateless requests

Uniform interface

Best practices include:

Clear naming conventions

Logical URL structure

Versioning strategy

Meaningful error handling

API-First approach leads to better designs and enables parallel development.

Design is a process that involves analysis, planning, design, validation, and iteration.

Now that you understand the fundamentals of API design, you're ready to start the actual design process. The first step is to create your API project in Apidog, which will serve as your workspace for all design work.

Let's begin with Creating Your First API Project.

Introduction to API Design

1. What Is API Design?#

2. RESTful API Design Principles#

Resource-Based Design#

Use HTTP Methods to Express Operations#

Stateless#

Uniform Interface#

3. API Design Best Practices#

Naming Conventions#

URL Structure#

Versioning#

Error Handling#

4. API-First vs Code-First#

Code-First Approach#

API-First Approach (Recommended)#

5. The Complete API Design Process#

6. Key Takeaways#