APIs (Application Programming Interfaces) allow different software applications to communicate with each other. To ensure these communications are clear and efficient, we use API contracts. An API contract defines the rules for how APIs should behave like a set of promises between the API provider and its users.

1. Understand the Purpose of Your API
What does the API do? Before anything else, know the main function of your API. For example, will it provide weather data or process payments?
2. Define the API’s Users (Clients)

  • Who will use the API? 

Identify the users or systems that will communicate with your API. This helps determine what they need from it.

3. Choose the API’s Format

  • How will data be exchanged? APIs often use JSON (JavaScript Object Notation) or XML (Extensible Markup Language) to send data. JSON is more popular 

4. Outline Endpoints

  • What are the paths for communication? 

An endpoint is the URL where a user interacts with the API. Each endpoint represents a different function.

  • For example: GET /users might retrieve a list of users, while POST /users might add a new user.

5. Describe the HTTP Methods

  • How do users request information? 

APIs typically use HTTP methods:

  • GET: To retrieve data.
  • POST: To send or add new data.
  • PUT: To update existing data.
  • PATCH: To partially update existing data (only changes specific fields).
  • DELETE: To remove data.
  • Example:
    • GET /books: Get a list of books.
    • POST /books: Add a new book.

6. List Request and Response Parameters

  • What information should the client send, and what will the API return? Clearly define:
    • Request Parameters: Data the client needs to send, such as an ID number or name.
    • Response Format: The structure of the data returned by the API, usually in JSON format.

Example Response:
{

  “id”: 123,

  “name”: “John Doe”,

  “email”: “john.doe@example.com”

}

7. Error Handling

  • What happens if something goes wrong? Define error codes and messages for situations like:
    • Invalid input (e.g., missing required data).
    • Unauthorised access (e.g., wrong API key).
    • Server errors (e.g., system down).

Example error response:
JSON
Copy code
{

  “error”: “Invalid request”,

  “message”: “User ID is missing”

}

8. Authentication

  • How will users access the API? Decide if the API needs an authentication mechanism, such as API keys or OAuth tokens, to protect access.

9. Document the API Contract

  • Where can users find the details? Write clear documentation explaining how to use the API. Tools like Swagger or Postman can help you create interactive API docs.

Example of a Simple API Contract

Here’s a basic example of what a simple API contract could look like:

  • GET /books
    Retrieves a list of books.

Response:
JSON
[

  {“id”: 1, “title”: “1984”, “author”: “George Orwell”},

  {“id”: 2, “title”: “The Great Gatsby”, “author”: “F. Scott Fitzgerald”}

]

  • POST /books Adds a new book.

Request:
JSON
{

  “title”: “The Catcher in the Rye”,

  “author”: “J.D. Salinger”

}

Response:
JSON

{

  “id”: 3,

  “title”: “The Catcher in the Rye”,

  “author”: “J.D. Salinger”

}

Conclusion

Creating an API contract helps everyone understand how the API works. It makes sure that the API communicates clearly and works as expected. By following these steps, you’ll have a clear plan for building APIs that are easy for developers to use and understand.

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