Sign In

What is an API? A Comprehensive Guide to Application Programming Interfaces

What is an API

In 2025, almost 90% of developers use APIs, and the reason is that they help run everyday tools like apps, websites, and cloud software. Furthermore, APIs (Application Programming Interfaces) are more than just ways to share data. They are the backbone of modern software, helping apps, services, and devices talk to each other safely and smoothly.

The evolution of REST APIs and SOAP APIs has empowered organizations to adopt API-first development and microservices architectures, accelerating digital transformation. Beyond the fundamentals, APIs introduce a new paradigm of collaboration between API producers (providers) and API consumers (developers, partners, or internal teams). This dynamic relationship requires clear acceptance criteria, effective API governance, and robust API testing protocols. 

This article explains the meaning of API in simple terms. It covers what they are, how they work, the main types, and the key terms you should know.

What is an API?  

An API, or Application Programming Interface, is a set of rules and protocols that enables one software application to interact with another. Think of it as a contract that defines how different programs communicate, request services, and exchange data in a consistent, secure, and predictable way. 

API definition in simple terms

An API acts as a messenger or translator between two separate systems. It specifies: 

  1. What data is needed 
  2. How to request it 
  3. What format the response will be in 
  4. How errors will be handled 

APIs are used in all kinds of technology, from mobile and web apps to cloud services and smart devices. They help developers build on existing services, connect with third-party platforms, and automate complex workflows. 

Restaurant menu analogy

A popular way to explain APIs is by using a restaurant menu example. The menu is the API, showing what can be ordered. The user asks the waiter (API), who gets the meal from the kitchen (backend) and brings it back. The user doesn’t need to know how the kitchen works, only how to place the order.

Types of APIs and communication styles

APIs can be grouped and understood in different ways depending on how they are accessed and the purpose they serve.

Types of APIs by access and purpose

  • Open APIs (Public APIs): Available to developers, partners, or the general public. Designed for broad consumption with thorough documentation. Example: GitHub API, OpenWeather API 
  • Internal APIs (Private APIs): Used exclusively within an organization for communication between internal systems, microservices, and applications. 
  • Partner APIs: Exposed only to specific, authorized third parties such as business partners, vendors, or clients. 
  • Streaming APIs:Enable real-time data delivery using protocols like WebSockets or Server-Sent Events (SSE). Rather than waiting for clients to poll for updates, streaming APIs push new information to consumers as soon as it is available. This makes them ideal for live feeds, notifications, or financial data. Example: Twitter Streaming API, live sports score feeds.

Types of API protocols by communication format

  • REST APIs: The most popular style, leveraging standard HTTP methods (GET, POST, PUT, DELETE) and resource-oriented URLs. Valued for simplicity, scalability, and statelessness. 
  • SOAP APIs: It shares data using XML and follows strict rules. Big companies use it when they need everything to be very safe and work without errors. 
  • GraphQL APIs: Allow clients to specify exactly what data they need, reducing over-fetching and under-fetching with a single endpoint. 
  • WebSockets/Server-Sent Events (SSE): Enable full-duplex, real-time communication between client and server. These protocols are ideal for applications requiring instant updates or interactive features. Use case: Chat applications, live dashboards, online gaming 

How to choose the right API type and style 

The right API type and protocol depend on the audience, the use case, and system requirements such as performance, security, and scalability. For instance, REST APIs are suitable for simple web services, GraphQL supports flexible frontend needs, gRPC is designed for high-performance microservices, and WebSockets are effective for real-time applications.

How APIs work

To understand how an API works, let’s look at one of the most common protocols: the REST API. A REST API call follows a few simple steps:

  • Endpoint: A unique web address (for example, /users or /orders) that points to the resource.
  • Request: A message sent using HTTP methods such as GET, POST, PUT, or DELETE.
  • Response: The data returned, usually in JSON format, along with a status code like 200 OK or 404 Not Found.
  • API key: A code included in the request header to identify and secure the user or application.

What is Rate Limiting? 

API providers use rate limiting (capping) to control how many requests a client can make in a certain time, such as per minute or per hour. This method prevents overload (too many requests), ensures fair use (equal access), and keeps performance steady. For example, an API may allow 1,000 requests per hour for each user. If the client crosses (exceeds) this limit, the API returns an error (often “429 Too Many Requests”) and tells the client to slow down.

Authentication methods beyond API keys

There are four types of authentication methods:

  • OAuth 2.0: Industry-standard protocol for delegated, token-based access, widely used for third-party integrations.
  • Bearer Tokens: Short-lived tokens (often from OAuth 2.0) passed in Authorization headers for granular access control.
  • Certificate-based Authentication (mTLS): Digital certificates for high-security scenarios like banking or B2B APIs.
  • Basic Authentication: Username/password in headers (rarely recommended for modern APIs).

Example of API Call Walkthrough 

Example of API Call Walkthrough

API design principles

Good APIs follow certain principles that make them easy to use, secure, and reliable. Below are the key practices, design standards, and patterns that help developers create APIs with real business value.

Core Principles of API Design 

  • Consistency: Use uniform naming conventions, URL structures, and response formats across all endpoints. 
  • Clarity: Endpoints, parameters, and data models should be self-explanatory with clear documentation. 
  • Error Handling: Send clear error messages and use standard codes so users know what went wrong. 
  • Security: Always check who’s using the API, make sure only the right people get access, and use safe connections like HTTPS. 
  • Versioning: When the API changes over time, use version numbers (like /v1/, /v2/) to keep things organized. 
  • Documentation: Provide comprehensive, up-to-date documentation with examples and authentication requirements. 

Standards and schema-driven design

  • OpenAPI Specification: Standardizes API definitions in a machine-readable format, enabling automated documentation, testing, and client code generation. This ensures consistency across your API ecosystem. 
  • Schema-Driven Design: Use a set format (called a schema) to show what data should look like. This helps teams understand each other better, check data easily, and use tools to speed things up. 
  • Consumer-Driven Development: Involve both API producers and consumers from the start to define clear acceptance criteria. This collaborative approach ensures APIs deliver exactly what’s needed, reducing rework and improving adoption. 

API design patterns for business value

  • Backend for frontend (BFF): Builds dedicated API layers for each type of client such as web, mobile, or IoT. This ensures each client gets only the data and logic it needs, improving performance and user experience.
  • API gateway: Acts as a single entry point for all API traffic. It centralises security, monitoring, and policies, simplifying integration for clients while giving teams better control.
  • Façade and proxy APIs: Provide modern interfaces over complex or legacy systems. This allows teams to modernise gradually without disrupting existing users.
  • Event-driven APIs: Enable real-time, asynchronous communication through events and message brokers. These are especially valuable in microservices architectures and for applications needing instant updates.

API development lifecycle 

The API Development Lifecycle is the step-by-step process of creating, testing, and managing an API. In simple words, it’s like building a product in stages:

  • Planning: Identify the goals, requirements, and target users of the API to ensure it solves the right problems.
  • Design: Carefully outline the structure, endpoints, and data formats of the API to make it clear and easy to use.
  • Development: Write the actual code for the API, implementing all planned features and following best coding practices.
  • Testing: Thoroughly test the API for functionality, security, and performance to catch and fix any errors before release.
  • Documentation: Prepare detailed and user-friendly instructions so developers can easily understand and use the API.
  • Deployment: Move the API to a live server or cloud platform so it’s available for real users to access.
  • Monitoring: Continuously track the API’s usage, performance, and errors to quickly respond to any issues that arise.
  • Maintenance: Regularly update the API with bug fixes, improvements, and new features to keep it reliable and up-to-date.
  • Retirement: Clearly announce the API’s end-of-life, assist users with transitioning, and safely remove the API when it’s no longer needed.

Successful APIs require ongoing collaboration between API producers (teams building and exposing APIs) and API consumers (developers, partners, or internal teams integrating with those APIs). Producers, therefore, need to understand real-world use cases, pain points, and the future needs of consumers. Meanwhile, consumers should actively share requirements and provide feedback.

Best practices for API development

  • Security-first design: Start with safety by adding login, permissions, and data protection from the very beginning.
  • Test-driven development: Write tests before or during building to make sure everything works as expected.
  • Environment management: Keep API keys, database passwords, and tokens safe by using secure tools.
  • Input validation: Always check user inputs to stop bad data or harmful attacks.
  • Meaningful error handling: Give clear error messages so it is easy to find and fix problems quickly.
  • Performance optimisation: Use caching, set limits, and monitor the system so it stays fast and can handle more users.

Types of API testing

APIs can be tested in many ways to check their functionality, performance, security, and overall reliability. Below are the main categories of API testing.

Functional testing

  • Unit testing
  • Integration testing
  • End-to-end (E2E) testing

Non-functional testing

  • Performance testing (load, stress, scalability)
  • Security testing
  • Usability testing
  • Reliability testing
  • Compatibility testing

API-specific testing

  • Endpoint testing
  • Request/response testing
  • Authentication/authorization testing
  • Error handling testing
  • Documentation testing

Automated testing

  • API automation testing
  • CI/CD testing
  • Regression testing

Other testing types

  • Exploratory testing
  • Fuzz testing
  • Compliance testing

Best practices for API testing  

  • Shift left: Integrate all types of testing early in development 
  • Test positive and negative cases: Ensure APIs succeed when they should and fail gracefully when they must 
  • Contract-driven testing: Keep API specifications and tests synchronized 
  • Security and performance are non-optional: Make them standard parts of your test suite 
  • Choose developer-friendly tools: Select platforms supporting scripting, reusable tests, and easy CI/CD integration 

API security 

Advanced security threats 

  • Broken object level authorization (BOLA): Attackers gain access to objects they should not by changing object IDs in requests. This is one of the most common API vulnerabilities.
  • Excessive data exposure: APIs return more data than needed, which can leak sensitive fields that consumers should not access.
  • Security misconfiguration: Issues such as outdated software, open ports, unnecessary HTTP methods, default credentials, or detailed error messages that expose system information.
  • Sensitive data in logs: Storing credentials, tokens, or personally identifiable information (PII) in plaintext logs by mistake.

Comprehensive security best practices 

Authentication and Authorization

  • Implement strong authentication such as OAuth 2.0, JWT (JSON Web Token), mTLS (Mutual Transport Layer Security), or API keys as needed.
  • Apply least-privilege and role-based access controls (RBAC).
  • Validate permissions on the server side and never trust client-side checks.
  • Rotate and revoke credentials regularly.

Data Protection: 

  • Enforce HTTPS across all APIs, including production.
  • Validate and sanitize user inputs by defining what is allowed.
  • Limit data exposure by returning only authorized fields.
  • Encrypt sensitive data at rest and in transit.

Operational Security: 

  • Implement rate limiting to prevent brute-force and DDoS (Distributed Denial of Service) attacks.
  • Monitor for unusual patterns, spikes, and potential abuse.
  • Return generic error messages to clients and log detailed errors internally.
  • Remove or secure deprecated endpoints and unused features.

API Governance 

API governance helps teams build reliable, secure, and consistent APIs while reducing risks and improving collaboration.

  • Reusable Assets: Shared schemas, style guides, code templates, and best practices accelerate development and reduce duplicated effort across teams. 
  • Automated Quality Checks: Linters, policy engines (like Spectral or Open Policy Agent), and CI/CD integrations catch issues before they reach production, maintaining quality without slowing development. 
  • Clear Ownership and Accountability: Knowing who owns and maintains each API streamlines support, change management, and incident response. 
  • Enhanced Collaboration: Consistent standards and documentation enable teams to easily integrate, review, and extend each other’s APIs across organizational boundaries. 
  • Inventory and Risk Management: Modern governance gives real-time visibility into all APIs. It covers environments such as development, testing, staging, and production. In addition, it shows authentication types and data sensitivity levels. Finally, it links producer and consumer relationships, helping teams improve security, ensure compliance, and respond quickly to incidents.

How to manage API Dependencies and changes 

  • Dependency Mapping: Governance tools map API dependencies, enabling impact analysis and reducing the risk of breaking changes cascading through your system. 
  • Change Management: Versioning policies, deprecation timelines, and automated changelogs help manage breaking changes while keeping consumers informed and supported. 
  • Lifecycle Visibility: Governance dashboards show API status, usage patterns, compliance levels, and health metrics—supporting audits, continuous improvement, and strategic decision-making. 

How to maintain and evolve APIs

APIs need regular care to stay reliable and useful. This includes checking performance, listening to feedback, and updating versions in a planned way.

Keep track of API health

  • Watch uptime, speed, errors, and security alerts.
  • Collect feedback from developers and users.
  • Notice patterns in usage and adoption.
  • Make sure there is clear ownership when issues come up.

Manage API versions

  • URI versioning: Add version numbers in the URL (for example, /v1/users, /v2/users).
  • Header versioning: Use custom headers to handle versions.
  • Share updates early: Provide migration guides to help users adjust.
  • Automate testing: Run tests on all supported versions to ensure reliability.

Summing up

In 2025 and upcoming years, the AI will set the new ways to manage APIs. First, API-first development will make APIs products from the start. Next, AI-driven automation will handle documentation, testing, and code. In addition, low-code tools will empower business users, while self-healing systems will fix issues automatically. At the same time, security will improve with context-aware AI/ML threat detection, developers will gain instant onboarding through live sandboxes, and automated governance will enforce standards consistently.

By learning the basics of what is an API today, developers build the foundation to adapt to these changes and create secure, resilient technology for the future.

Share the Post: