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What is an API: A Beginner's
Guide to Application
Programming Interfaces


StackHawk|April 5, 2024

Gain a better understanding of APIs and why they matter from the basics to real-world applications and security tips.

Whether you're working directly with tech or indirectly, you've likely heard the term "API". So, what is an API, and why is it important? Let's use the most common metaphor for how an API works: the waiter example. Imagine you're in a cozy cafe, sitting at one table while your friend is at another. You want to share the news but can't just walk over. Instead, you write a note and pass it to the waiter, who ensures it reaches your friend. In this scene, the waiter acts like an API or Application Programming Interface—a critical intermediary enabling different computer programs to interact and share information seamlessly, much like the two friends in the example.

Understanding API Basics: The Building Blocks of Our Digital World

The Wizards Behind the Curtain

To delve into specifics, what is an application programming interface (API) exactly? An API is a foundational framework comprising a series of commands and protocols that allow communication between software entities. Much like the rules of a board game, which ensure all participants play in accord, understanding their respective roles and boundaries, APIs set forth a uniform method for software components to engage with one another. They enable these components to share data, perform tasks, and operate cohesively within various digital contexts.

Why APIs Matter

Despite their low profile, web service APIs are critical in the tech ecosystem. They are developed by various creators, from individual coders to tech giants. They empower your weather app to predict rain, enable social games to tell you which friends are online, and streamline online shopping. By facilitating smooth dialogue between different systems, web APIs bring together the vast web of online services and apps we depend on.

The Role of APIs

APIs are like the connectors that help different online services talk to each other. They allow various programs to work together smoothly. For instance, they can link a customer service system to a marketing tool or connect a project management app to a time-tracking service, improving teamwork.

But APIs don't just help inside a company. They also let businesses enhance their offer by connecting with external partners online. This could mean using another company's web service to add new features or data to what they already provide, making their services even better for customers.

The Anatomy of APIs: Calls, Requests, and Responses

APIs communicate in a process similar to initiating and having a conversation:

  • Making the Call: Just as you might call a friend to suggest grabbing coffee, an app makes an API call to start an interaction. This is the digital equivalent of reaching out to initiate a conversation.

  • Sending a Request: Once the call is connected, you ask your friend, "Do you want a coffee?" In the realm of APIs, this translates into a "request." The requesting app outlines what it needs in a format the recipient understands, thanks to the universally recognized HTTP (Hypertext Transfer Protocol) standard that web APIs use.

  • Receiving a Response: As your friend replies to your coffee query, the targeted app or API endpoint responds to the request. This "response" could affirm the action, deny the request, or inform you of an error, much like a friend's confirmation, decline, or suggestion to meet elsewhere.

Although this is a very simple example, it outlines the basics of the API call and the request and response that are part of the overall interaction. Now, we will look at how you determine "who" to call, the equivalent of the phone number in the example above: the API endpoint itself.

Who Do You Call?

API endpoints are crucial in the process of different online services communicating with each other. Think of them as specific web addresses (URLs) where requests for information or actions are sent and from where responses or results are received. Each endpoint is a unique address that guides the data flow to the right place within an API. These generally look just like web URLs we use in the browser, since the data you receive back in your browser can also come from an API.

For example, if you’re using an app to check the weather when you select your city, the app sends a request to the weather service’s API endpoint. This endpoint knows how to fetch the weather data for your city. Then, it sends this data back to the app, which displays the weather to you. Endpoints ensure that requests for data or actions reach the exact location in the system that can handle them, making the interaction between different services seamless and efficient.

API Functionality and Communication

The Basics of Chatting Between Apps

Web APIs, or Application Programming Interfaces, use a specific rulebook to define how different applications communicate, similar to how we use text messages to send messages back and forth. This conversation between apps occurs through a simple yet effective process called a client-server model, which can be broken down as follows:

The Client: Imagine this as one app that initiates the conversation, similar to how you might text a friend to ask a question. This app makes a request when it needs something. For instance, when you use a weather app on your phone (the client) to check the forecast, you're essentially asking it to find and display the latest weather information.

The Server: On the other side of this conversation is the server, like the friend who receives your text and responds. The server is another application that waits for requests. When it receives one, like the weather app on your phone asking for the latest forecast, it knows how to fetch that information or perform the needed action. Once it has the information, the server sends it back to the client app, which presents the results to you.

Making Sure Everyone Speaks the Same Language

A common language is necessary for these digital interactions to be effective. Web APIs utilize HTTP, the standard web protocol that facilitates these exchanges. Think of HTTP as the grammar rules ensuring apps communicate effectively, irrespective of origin.

HTTP headers specify the message's format, commonly in JSON (JavaScript Object Notation) or XML (Extensible Markup Language) sent to the web API, and how responses should be formatted. With its user-friendly and concise structure, JSON is ideal for straightforward data sharing. In contrast, XML offers a more detailed, hierarchical approach suitable for complex data exchanges.

The Magic of RESTful APIs

Regarding versatility, taking a quick look at REST APIs makes sense. As one of the most common types of APIs, REST APIs stand out for their ability to "speak" various digital languages; on top of JSON and XML, they can also use:

Plain Text: Simple and direct, perfect for basic messaging.

HTML: The foundation of web pages, enabling rich text formatting.

YAML: A human-friendly data format ideal for configuration files.

The flexibility of REST APIs empowers developers to select the most fitting communication method, enhancing app interaction efficiency.

Asking Nicely and Getting What You Asked For

APIs need to articulate their needs in a universally understood manner, achieved through the request-response model. This model mirrors a typical question-and-answer interaction. RESTful APIs streamline this process with intuitive commands that come in the form of HTTP verbs. Below are some of the most common that you'll come across:

GET - Retrieves data by asking for the latest news updates.

POST - Submits new data, similar to posting a new photo on social media.

PUT - Updates existing information, like changing your profile bio.

DELETE - Removes data, such as deleting an outdated post.

REST APIs facilitate myriad functions through these mechanisms that make our digital experiences richer, more connected, and incredibly seamless. From social media interactions to securing the next meal through a delivery app, APIs are the hidden gears powering the digital world's connectivity.

Understanding the Different Types of APIs

By Audience and Access Level

Depending on who will use an API or have access to it, there are some sub-types that APIs can fall into. APIs can be broadly categorized based on who can access them and how they're used based on the following categories:

Private APIs are the backbone of internal operations within companies, safeguarding sensitive data while ensuring seamless inter-departmental communication.

Partner APIs open doors to strategic business-to-business (B2B) collaborations, offering a secure yet flexible way for companies to integrate and leverage each other's resources and capabilities.

Public APIs democratize company services access, inviting external developers to innovate and develop applications that enhance or complement the original platform. This broad access can amplify a company's influence and potentially unlock new revenue streams.

By Use Case

On top of the categories above, there are also some other descriptors for APIs to be aware of and what they mean. These include the following:

Open APIs stand out for their unrestricted access, fueling innovation across the digital ecosystem by enabling developers worldwide to create and expand upon existing platforms.

Internal APIs, synonymous with private APIs, optimize organizational efficiency by facilitating the smooth flow of information and functionalities within a company.

Composite APIs efficiently bundle multiple API requests into one, minimizing the number of calls made and accelerating processes that depend on data from numerous sources. Composite APIs are especially useful for streamlining client and server interactions, reducing the number of round-trip calls needed to perform related operations.

API Architecture: The Blueprint of APIs

APIs Hero Image image

The style of an API architecture plays a critical role in how it functions and interacts with other systems. Certain types are more flexible than others; some technologies help with specific use cases. Types of API architectures include:

REST (Representational State Transfer): RESTful APIs utilize HTTP methods in a flexible, stateless model, ideal for building scalable web services. They adhere to RESTful architectural constraints, simplifying communication over the web.

SOAP (Simple Object Access Protocol): SOAP APIs prioritize security and standardized protocols, employing XML to ensure robust data exchange across diverse platforms. This makes them a preferred choice for enterprise-level applications, especially legacy ones.

GraphQL: Revolutionizes data retrieval by allowing clients to specify precisely what information they need, significantly reducing inefficiencies associated with data over-fetching or under-fetching.

gRPC: RPC APIs focus on high-efficiency communication, which is particularly beneficial in microservices architectures by enabling quick and compact binary data exchange.

WebSocket: WebSocket APIs provide a continuous connection for real-time interactions, enhancing experiences in applications that demand instant updates, like messaging apps or online gaming.

Webhooks offer a mechanism for apps to automatically notify each other about events, enabling responsive and synchronous workflows across different services.

Understanding the diverse types of APIs, from private, partner, and public APIs to REST APIs, simple object access protocol (SOAP) APIs, and GraphQL architectures, is essential for harnessing their potential. Whether enhancing internal operations with internal APIs, engaging in B2B integrations with partner APIs, or innovating on open platforms with public APIs, these tools are indispensable in crafting the interconnected digital experiences that define the modern web.

Securing Your API: Authentication and Authorization

Authentication and Authorization: The First Line of Defense

Securing an API involves establishing strong authentication methods right from the start. This typically includes utilizing an API key and deploying authentication tokens, which are crucial in ensuring secure and legitimate communication. An API key acts as a unique password for applications, enabling them to authenticate their identity with each request they make. This key, generally obtained and managed via the API provider’s developer portal, is a fundamental access control layer by granting permissions based on predefined rules.

Authentication tokens take security a step further. They validate access requests and manage user sessions or specific privileges. These tokens are generated following a successful login, offering a more dynamic and secure approach by detailing more granular permissions aligned with the user's role. This method significantly boosts security by facilitating temporary and revocable access, ensuring each session is secure and specific to the user’s access rights.

API endpoints act as critical gatekeepers within this security infrastructure, controlling access to the API’s underlying functionalities and data. Endpoints are essential in protecting the API from unauthorized entry by ensuring that only authorized users or applications can access sensitive information or perform specific actions.

Talk to a StackHawk Expert about how your AppSec program can be better

Rate Limits and Security Audits: Monitoring and Control

Implementing rate limits is an effective measure to prevent abuse of API calls and overloading of web API services. By capping the number of requests a user can make within a certain timeframe, APIs can maintain optimal performance while mitigating the risk of malicious attacks. If users can inundate the endpoint with traffic, the API and underlying services could be overwhelmed and experience a denial of service.

Another critical practice is regular security audits, which enable timely detection and remediation of vulnerabilities. These audits assess the API’s security posture, ensuring compliance with best practices and regulatory standards. At StackHawk, we believe API security should be part of the API development process from the outset. With tools like StackHawk, APIs can be tested for potential security vulnerabilities as they are being built. This means that before APIs are made available to users, they can be secured and tested.

The Role of API Gateways in Security

An API Gateway operates as a critical intermediary, offering a centralized platform for managing, securing, and analyzing API traffic. API gateways can help API developers with:

  • Performing user authentication and verifying the identities of those attempting to make an API call.

  • Managing rate limits to prevent service abuse.

  • Collecting and providing API usage analytics is invaluable for ongoing security assessments and optimizations.

As part of an API management tool, API gateways can give developers an easier path to creating secure and scalable APIs.

Fine-Grained Access Control: Defining Permissions

Making sure that users of the API are authenticated and authorized is also a major part of keeping endpoints and data secure. Effective access control is foundational to API security and involves:

  • Establishing user roles and permissions, determining what data and functionality are accessible to different users.

  • Implementing detailed access rules using standards like OAuth 2.0, OpenID Connect, and JSON Web Tokens (JWT). These protocols offer secure, token-based authentication mechanisms, enabling web APIs to grant access based on verifiable credentials.

In contrast to API keys, which have a broad and static scope, tokens offer a dynamic and revocable means of access control. Their temporary nature and specific scope make them more secure, minimizing potential data exposure risks.

By embracing these strategies—leveraging API keys and tokens for authentication, utilizing API Gateways for centralized management, and enforcing granular access control—developers can significantly bolster the security and integrity of their APIs, protecting both their data and their users.

Implementing APIs: Integration and Development

API integration is a cornerstone of modern software development, laying the groundwork for systems to communicate and exchange data efficiently. This process begins with acquiring an API key (if required), setting up an HTTP API client or a way to call the API endpoint, such as an SDK, and diving into the unique syntax needed for integrating the API into your project. Let's briefly look at few things to consider when integrating with an API.

Goals and Compatibility

The journey of API integration involves clear goal-setting and compatibility checks to ensure seamless communication between your software and the API. This includes understanding the API’s capabilities, limitations, and how it fits within your project’s architecture. It is crucial to understand what the API offers and how your app will use it.

Maintenance and Scalability

Keeping an API integration up to date requires vigilance in managing API version changes, ensuring secure authentication practices, adept error handling, and planning for future growth. Tools for automated testing and monitoring play a vital role in maintaining an API's health and effectiveness over time. Adhering to rate limits imposed by the API provider is crucial to avoid service interruptions or penalties and ensure a reliable service for the end-users of your application.

API Documentation

API documentation is a collective term for the materials, including references, tutorials, examples, functions, classes, return types, arguments, and requirements, that enable developers to use and integrate an API effectively. When it comes to picking a third-party API, look for documentation that provides clear explanations and up-to-date examples. Good documentation is one of the signals that the API has been well-built.

When it comes to your own APIs, developing documentation for an API endpoint should follow best practices, such as adhering to API design standards with tools like OpenAPI specifications. Solutions like Swagger, Postman, and hosted platforms benefit various types of documentation, such as static or interactive documentation. Companies like Stripe and Twilio are recognized for setting benchmarks in the industry with their robust and user-friendly API documentation, so they can be good examples to follow.

Leveraging API Libraries and SDKs

API libraries and Software Development Kits (SDKs) significantly reduce the complexity of developing and integrating APIs. These resources bundle APIs with additional tools and frameworks, streamlining the development process. Whether using Apple’s iOS SDKs for crafting mobile applications or Microsoft’s .NET SDK for building enterprise-level software, SDKs equip developers with native and simple ways to use API functionalities directly in their code.

In summary, effective API integration and development hinge on understanding and leveraging the available tools, maintaining clear and accessible documentation, and utilizing libraries and SDKs to simplify the development process. Combining all of the examples and thoughts above, whether integrating with another API or trying to make your API more accessible, is an excellent way to ensure seamless and straightforward API integration.

Real-World API Examples and Applications

APIs are crucial in various applications, such as mapping apps, weather services, and social media platforms. These real-world examples showcase the versatility and importance of APIs in our everyday digital experiences. The practice of offering free APIs initially and then monetizing access to valuable digital assets is known as the API economy. API marketplaces are platforms where developers can buy and sell APIs, contributing to the monetization aspect of the API economy.

For instance, mapping apps leverage APIs for features like displaying maps, providing directions, traffic warnings, and identifying points of interest. Travel booking sites integrate APIs by sourcing data from hotels and airlines to present users with real-time flight and accommodation information. APIs aid companies in establishing new business partnerships, launching innovative services, and tapping into new market segments.

Let's take a quick look at some common APIs that are used extensively on the web and in mobile applications.

Google Maps API

The Google Maps API is a perfect example of an API that enhances application functionality. It allows developers to incorporate customized maps into websites or applications using Google’s geographic data, enabling features like real-time traffic updates, street view, and detailed location information.

Industries such as travel and delivery services utilize the Google Maps API to enhance their services. Some of the key features and benefits of the Google Maps API include:

  • It can aid in itinerary creation for travelers

  • It can display routes for deliveries

  • It offers real-time map updates, ensuring that users can always access the most current map data.

These features make the Google Maps API a valuable tool for businesses in these industries.


The WeatherAPI is another example of how APIs provide real-time data in applications. It provides real-time weather data that can be used in websites and applications to display current conditions and weather forecasts. This information can be crucial in various contexts, from planning a picnic to scheduling a flight.

Social Media APIs

Social media platforms like Facebook and Twitter offer APIs that allow third-party applications to integrate with their services. These APIs enable features such as:

  • Logging in to third-party websites using Facebook, Twitter, or Google profile credentials

  • Simplifying the user experience and streamlining the login process

  • Reducing the need for users to remember multiple passwords for different services


APIs are indispensable in crafting the interconnected digital experiences we've come to expect. Their role in modern software development extends from simplifying complex interactions between systems to enabling instantaneous data sharing and enhancing user engagement across platforms. As digital solutions evolve, the strategic implementation of APIs will remain a key driver in developing innovative, efficient, and user-centric applications. Remember, each time you enjoy a frictionless digital service, multiple APIs are likely working tirelessly behind the scenes.

Are you planning to build your own web API? In that case, StackHawk can help ensure they are secure and ready for customers. With StackHawk's DAST (Dynamic Application Security Testing) platform, developers can test their APIs as they build them to uncover potential security issues. Once an issue is found, StackHawk even shows users potential fixes so that they can fix and confirm that the vulnerability is remedied. Want to try it out for yourself? Sign up today for a free trial and begin your API build journey with security in mind.

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StackHawk  |  April 5, 2024

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