API Calls and How do They Work? A Complete Guide

APIs, or Application Programming Interfaces, serve as a set of rules and protocols that enable different software applications to communicate with one another. They play a pivotal role in modern software development by allowing developers to access and leverage the functionality and data of other applications or services. The concept is pretty simple, but what goes on behind the scenes? In short, the back and forth amongst applications occurs through API calls. 

What are API Calls? 

API calls are specific requests made by one software application to another. These requests serve as a means for the requesting application to access and utilize the functionality and data offered by the target application or service.  

API calls are essential for enabling communication and interaction between different software systems, allowing them to exchange information, execute tasks, and integrate seamlessly. These calls typically consist of a structured set of instructions, parameters, and data, which are processed by the target API to perform a particular operation, retrieve information, or trigger a specific response. 

Read More: How to Build an API 

Technicalities of an API Call 

Before we move on to how you can make an API call, let’s understand the technicalities of API calls:  

Client Application: 

The first component of an API call is the client application, which is the software that intends to make use of an API. It can be a web application, a mobile app, or any program that requires data or services from another source. 

API Endpoint: 

The client application needs to know the API’s endpoint(s). An endpoint is a specific URL (Uniform Resource Locator) that represents a particular resource or action provided by the API. For example, an API for weather data might have an endpoint like https://api.weather.com/current-weather. 

HTTP Request: 

To communicate with the API, the client application sends an HTTP request to the API endpoint. The request consists of several parts: 

HTTP Method: This specifies the type of action the client wants to perform. Common methods include:  

1. GET (retrieve data) 
2. POST (create data) 
3. PUT (update data) 
4. DELETE (remove data). 

Headers: These contain additional information about the request, such as the content type, authentication details, and more. 

Query Parameters or Request Body: Depending on the API and the specific endpoint, data can be sent as query parameters in the URL or as a JSON/XML payload in the request body. 

API Server: 

The API server is the software that listens for incoming requests at the specified endpoints. When a request is received, the server processes it based on the endpoint, method, and data provided. 

Request Processing: 

The API server processes the request by carrying out the intended action such as retrieving data from a database, performing calculations, or interacting with other services. 

Business Logic: 

The API often contains business logic, which defines how the data or service should be provided. For example, if you’re using a payment processing API, it might validate the payment data, charge the user, and return a response. 

Response Generation: 

After processing the request, the API server generates an HTTP response, which includes several components: 

1. Headers: These contain additional information about the response, such as content type and caching directives. 
2. Response Body: This is where the data or result of the request is included. It’s typically formatted as JSON, XML, HTML, or some other structured format. 
3. HTTP Status Code: It indicates the outcome of the request.  

HTTP Response: 

The API server sends the HTTP response back to the client application. 

Client Processing: 

The client application receives the response and processes it based on the HTTP status code and the data provided in the response body. 

Error Handling: 

If an error occurs during the API call (e.g., due to invalid input or server issues), the API server will return an appropriate status code (e.g., 400 for bad request or 500 for internal server error). The client application should handle and report these errors gracefully. 

Subsequent Actions: 

Depending on the nature of the client application, it may take further actions based on the API response, such as displaying data to the user or using the retrieved data in its own processes. 

How to Make an API Call? 

Now that you have a basic understanding of the terms and how API call works, let’s see how you can make one:  

Read API documentation thoroughly

When preparing to make API calls, it’s essential to thoroughly review the API documentation to ensure that you understand how to use the API effectively and adhere to any requirements or limitations. Here are the key pointers you should read in the API documentation before making the call: 

1. Understand the authentication method required to access the API. The documentation explaina whether you need API keys, OAuth tokens, or another form of authentication. Learn how to obtain and use these credentials. 
2. Identify the available API endpoints and their purposes. The documentation should provide the base URL and specific endpoint paths for each resource or action you want to access. 
3. Check for any rate limits or usage quotas imposed by the API. Understand how many requests you are allowed to make within specific time intervals. Be aware of any cost implications or the consequences of exceeding these limits. 
4. Check if the API documentation mentions versioning. Understand the API version you are working with and how to specify the version in your requests. Be aware of any upcoming changes or deprecated features. 
5. Determine if the API provider offers a sandbox or testing environment where you can experiment with the API without affecting production data. 

Choose the Right HTTP Method

As explained earlier, the HTTP method you will choose for your API call will be based on the action you want to perform. These HTTP methods, often referred to as CRUD operations (Create, Read, Update, Delete), provide a standardized way for clients to interact with APIs. What method you choose, depends on the action you want to perform. When working with RESTful APIs, these methods map to the basic operations that can be performed on resources, contributing to a clear and consistent API structure. 

Construct the API URL 

Build the URL for the API call by combining the base API URL and the specific endpoint you want to access. Here’s a breakdown of the considerations when making an API URL: 

1. The base URL is the starting point for the API. It typically includes the protocol (http or https) and the domain or server where the API is hosted. For example, the base URL is “https://api.example.com.” 
2. Next, you need to specify the endpoint, which comes right after the base URL and is often represented by a path. For example, if you want to retrieve user profiles, the endpoint might be “/user-profiles.” 
3. Then you need to add the query parameters that provide additional information to the API request. They are included in the URL after a question mark “?” and separated by “&”. For instance, if you want to retrieve a specific user profile with an ID of 123, you might include a query parameter like “?user_id=123.” The final URL would look like: https://api.example.com/user-profiles?user_id=123 
4. If the API requires authentication, you might need to include an authentication token or API key in the request, you can do it through headers or as part of the URL (e.g., “?api_key=your_api_key”). 

Putting it all together, your URL might look like: https://api.example.com/user-profiles?user_id=123. 

Set Up the Request 

Create an HTTP request in your programming language or API client. Set the following elements in your request: 

• HTTP Method: Set the method corresponding to your intended action (e.g., GET, POST, PUT, DELETE). 
• Headers: Include headers like “Content-Type” (specifying the data format of your request) and any required authorization headers. 
• Query Parameters or Request Body: Include any data or parameters necessary for the API call. Depending on the API, data may be sent as query parameters in the URL or in the request body as JSON or another format.  Receive the Response 

Here are some common types of response codes that you should know about: 

2xx Success

• 200 OK: The request was successful. 
• 201 Created: The request resulted in the creation of a new resource. 
• 204 No Content: The request was successful, but there is no new information to send back (often used for DELETE requests). 

3xx Redirection

• 301 Moved Permanently: The requested resource has been permanently moved to a new location. 
• 302 Found (or 307 Temporary Redirect): The requested resource has been temporarily moved to a new location. 
• 304 Not Modified: Used for caching purposes; the requested resource has not been modified since the last request. 

4xx Client Errors

• 400 Bad Request: The server could not understand the request. 
• 401 Unauthorized: The request lacks proper authentication credentials. 
• 403 Forbidden: The server understood the request, but it refuses to authorize it. 
• 404 Not Found: The requested resource could not be found on the server. 
• 405 Method Not Allowed: The method specified in the request is not allowed for the resource identified by the request. 

5xx Server Errors

• 500 Internal Server Error: A generic error message indicating that the server encountered an unexpected condition. 
• 501 Not Implemented: The server does not support the functionality required to fulfill the request. 
• 502 Bad Gateway: The server, while acting as a gateway or proxy, received an invalid response from an upstream server. 
• 503 Service Unavailable: The server is currently unable to handle the request due to temporary overloading or maintenance. 

429 Too Many Requests: 

• The user has sent too many requests in a given amount of time. 

How to Secure APIs from Invalid Calls? 

So, what happens when you are on the other end that is you are receiving API calls?  You need to protect your APIs from unnecessary calls as Invalid calls may be attempts by malicious actors to exploit vulnerabilities in your system. Plus, they consume system resources, leading to degradation in performance or even denial of service.  

Here is how you can protect your APIs from unwanted API calls: 

Authentication: 

• Use strong authentication mechanisms such as API keys, OAuth tokens, or JWT (JSON Web Tokens). 
• Implement multi-factor authentication for additional security. 

Authorization: 

• Enforce proper access controls to ensure that authenticated users only have access to the resources they are authorized to use. 
• Implement role-based access control (RBAC) to manage permissions effectively. 

HTTPS (SSL/TLS): 

• Always use HTTPS to encrypt data in transit and prevent eavesdropping. 
• Use the latest and most secure versions of SSL/TLS protocols. 

Input Validation: 

• Validate and sanitize all input data to prevent injection attacks such as SQL injection, cross-site scripting (XSS), and other common exploits. 

Rate Limiting: 

• Implement rate limiting to prevent abuse and protect against brute force attacks. Limit the number of requests a client can make within a specific timeframe.

API Keys:

• Use API keys to control access and track API usage. Rotate keys regularly and revoke access for compromised keys. 

Token Expiration: 

• Set expiration times for tokens to limit their validity period. Refresh tokens should be used to obtain new access tokens. 

Logging and Monitoring: 

• Implement robust logging to record API requests and responses. Monitor logs for suspicious activity and unauthorized access. 
• Set up alerts for unusual patterns or security incidents. 

CORS (Cross-Origin Resource Sharing): 

• Configure CORS settings to control which domains are allowed to access your API. This helps prevent cross-site request forgery (CSRF) attacks. 

API Gateway: 

• Use an API gateway for centralized management of API security, including authentication, authorization, and monitoring. 

Web Application Firewall (WAF): 

• Implement a WAF to protect against common web application attacks, such as SQL injection, XSS, and other security threats. 

Security Headers: 

• Utilize security headers like Content Security Policy (CSP), Strict-Transport-Security (HSTS), and others to enhance the security of your API. 

Regular Security Audits and Penetration Testing: 

• Conduct regular security audits and testing to identify vulnerabilities and address them proactively. 

API Versioning: 

• Implement versioning for your APIs to ensure backward compatibility. This allows you to to deprecate and retire outdated versions with security vulnerabilities. 

Parting Words 

As technology continues to evolve, the role of APIs becomes increasingly pivotal in shaping the interconnected digital landscape. Whether you’re designing APIs for internal use or exposing them to external developers, a thoughtful and well-documented approach is key. 

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