The developers'

API Guide

Everything you need to know about building, purchasing and integrating with APIs.

Types of APIs

APIs come in various forms, each with its own strengths and purposes. Understanding these differences is essential for picking the right API for your project and ensuring your application is effective, scalable, and reliable. Let’s look at the different types of APIs, exploring what makes each unique and how they fit into modern software development.

This table provides a high-level overview of various API types, their descriptions, and examples.

API typeDescriptionExamples
Library-based APIsPart of software libraries, language-specific, range from basic utilities to complex GUI componentsJava API for Android app development, .NET libraries used in Windows applications
Operating system APIsProvide interfaces for OS interaction, manage hardware resources and processesWindows API for Windows OS interaction, POSIX API for UNIX-like systems
Database APIsEnable interaction with database management systems, facilitate data querying and manipulationSQL API for relational databases, Oracle’s OCI for Oracle databases
Hardware APIsAllow communication with hardware devices, direct control over hardware functionsIoT device APIs for smart home systems, Printer APIs for document processing
Cloud APIsProvided by cloud service platforms, enable interaction with cloud-based resources and servicesAmazon Web Services (AWS) API, Microsoft Azure API for cloud-based solutions
HTTP APIs (web API)Facilitate client-server communication over the web, use standard HTTP methods, lightweight and flexibleWeb services, mobile apps, IoT devices, social media platforms, content management systems
REST APIs (web API)Use HTTP requests for data operations, stateless and separate client-server concernsWeb services accessible via the web, social media APIs, cloud services
SOAP APIs (web API)Use service interfaces to expose business logic, high security, suitable for enterprise-level organizationsEnterprise-level services like banking, healthcare, where security and transactions are critical
GraphQL APIs (web API)Clients request only needed data, reduce data transfer over the network, support multiple responses in one requestComplex data-driven web and mobile applications, ad-hoc queries by the client
Open APIs (public APIs)Accessible by third-party developers, typically for external users, require API keysTwitter API for tweets, Stripe API for payments, providing data or services to external developers
Internal APIs (private APIs)Designed for internal use within an organization, enhance integration between systemsLinking HR systems to internal employee directories, improving internal efficiency and data security
Partner APIsAvailable to strategic business partners, require specific entitlementsIntegration of supply chain systems, service expansion to partners, creating revenue channels, controlled data sharing with trusted partners
Composite APIsCombine different data and service APIs, access multiple endpoints in one callAggregating various services in a single call for smoother user experience, commonly used in microservices architectures
JSON-RPC and XML-RPC APIsEncode data as JSON or XML for remote procedure calls, send lists of commandsRemote procedure calls with simple request/response model, suitable for basic request/response scenarios
Synchronous and asynchronousSynchronous: Request-response model, Asynchronous: Non-blocking, no immediate response requiredSynchronous: Essential data retrieval, web page loading, Asynchronous: Background tasks, real-time data streams
Library-based APIsPart of software libraries, language-specific, range from basic utilities to complex GUI componentsJava API for Android app development, .NET libraries used in Windows applications

See below for more detailed information on each API type, including their specific characteristics and use cases.

1. Library-based APIs

Library-based APIs are part of software libraries, providing pre-written code to help developers with common programming tasks, thus speeding up the development process.

Examples: The Java API is integral to Android app development; .NET libraries are used in various Windows applications.

Characteristics: Library-based APIs are language-specific and offer functionalities ranging from basic utilities to complex graphical user interface (GUI) components.

2. Operating system APIs

Operating system APIs offer interfaces for applications to interact with and perform operations on the underlying operating system.

Examples: Windows’ API allows applications to interact with Windows operating systems; POSIX API is used for UNIX-like systems.

Characteristics: These APIs manage hardware resources, file systems, and process controls, providing essential services like memory management and device control.

3. Database APIs

Database APIs enable applications to connect and interact with database management systems, simplifying data querying and manipulation.

Examples: SQL API for relational database interactions; Oracle’s OCI (Oracle Call Interface).

Characteristics: They facilitate database operations like data querying, manipulation, transaction management, and concurrency control.

4. Hardware APIs

Hardware APIs allow software applications to communicate with hardware devices, from simple peripherals to complex machinery.

Examples: IoT device APIs for smart home systems and printer APIs for document processing.

Characteristics: They provide direct control over hardware functions and are crucial in real-time data processing and device management.

5. Cloud APIs

Cloud APIs are provided by cloud service platforms, enabling applications to interact seamlessly with cloud-based resources and services.

Examples: Amazon Web Services (AWS) API; Microsoft Azure API.

Characteristics: These APIs allow access to cloud infrastructure and services, supporting scalable and distributed computing solutions.

6. Web APIs

Web APIs enable communication over the web using HTTP/HTTPS protocols, allowing web applications to interact with servers and other services. Each type of web API offers unique advantages suited to different requirements. 

For a deeper understanding, let’s look at the specifics of each:

Characteristics: They typically support XML or JSON data formats and are designed for stateless operations (especially REST), making them scalable and flexible for web-based interactions.

Additional types of APIs:

Open APIs (public APIs)

Key characteristics:

  • Accessible by any third-party developers
  • Typically designed for external users to access data or services
  • Usually come with limited access and require API keys for usage tracking

Use cases: 

  • Providing external developers with data (e.g., Twitter API for tweets), or services (e.g., Stripe API for payments).


  • Broad accessibility can lead to widespread adoption
  • Potentially large ecosystem of third-party applications and services


  • Security and rate limiting are critical to prevent abuse
  • Requires comprehensive documentation and developer support

Internal APIs (private APIs)

Key characteristics:

  • Designed for use within an organization
  • Facilitate improved integration and connectivity between internal systems
  • Enhance productivity and facilitate the creation of a unified work environment

Use cases: 

  • Interconnecting an organization’s internal systems, such as linking HR systems to internal employee directories


  • Can improve internal efficiency and streamline operations
  • Tighter control over API usage and data security


  • Internal APIs may become bottlenecks if not well-managed
  • Can create dependencies between different systems within the organization

Partner APIs

Key characteristics:

  • Available to strategic business partners
  • They are not open to the public and require specific entitlements to access
  • Enable companies to extend their services to other business partners safely

Use cases: 

  • Enabling business partners to integrate with supply chain systems or add a service to their offerings (e.g., Amazon’s Alexa voice service)


  • Facilitates B2B collaborations and can create new revenue channels
  • Allows for controlled sharing of data and services with trusted partner


  • Requires management of partner permissions and access controls
  • Often demands service level agreements (SLAs) to guarantee uptime and performance

Composite APIs

Key characteristics:

  • Combine different data and service APIs
  • Allow developers to access several endpoints in one call
  • Useful for microservices architectures where a user may need information from several services to perform a single task

Use cases: 

  • Aggregating various services in a single call for a smoother user experience is often used in microservices architectures


  • Reduces server load by reducing the number of calls
  • Improves speed and performance of applications


  • Complex error handling since multiple services are involved
  • Can become a single point of failure if not implemented with fault tolerance in mind


Key characteristics:

  • Remote procedure calls that encode their data as JSON or XML
  • Allow for sending a list of commands to a system which returns with the responses

Use cases: 

  • Remote procedure calls where a simple request/response model is sufficient


  • Simple request structure with flexibility in data types
  • Language agnostic with the ability to bypass CORS policies in web applications


  • Not as feature-rich as REST or SOAP
  • Potentially less secure if not properly managed

Synchronous and asynchronous API

Key characteristics:

  • Synchronous APIs:
    • Operate on a request-response model
    • The client sends a request and gets an immediate response from the server
  • Asynchronous APIs:
    • Allow for operations that do not require an immediate response
    • The client can continue processing other tasks while waiting for the response

Use cases:

  • Synchronous APIs: Ideal for scenarios where immediate data retrieval is essential, such as web page loading where the response is needed for the next operation
  • Asynchronous APIs: Suitable for background tasks, long-running operations, or when handling real-time data streams


  • Synchronous APIs:
    • Simplicity and ease of understanding
    • Immediate feedback and direct communication flow
  • Asynchronous APIs:
    • Non-blocking, allowing for better resource utilization
    • Suitable for complex, time-consuming operations or real-time updates


  • Synchronous APIs:
    • Can lead to inefficient resource use, as the client has to wait for the response
    • Not ideal for operations that require long processing times
  • Asynchronous APIs:
    • More complex to implement and manage
    • Requires handling callbacks or event loops, which can be challenging regarding error handling and tracking

Syncronous API vs Asynchronous API

Explore the differences between synchronous and asynchronous APIs and how to choose the right one.

GraphQL APIs

Key characteristics:

  • Allow clients to request only the data they need
  • Reduce the amount of data that needs to be transferred over the network
  • Clients can get multiple server responses in a single request

Use cases: 

  • Applications that require flexible, ad-hoc queries by the client (e.g., complex data-driven web and mobile applications)


  • Clients have the power to request exactly what they need, reducing over-fetching of data
  • Facilitates the development of faster and more dynamic applications


  • Can be complex to set up on the server side
  • Performance issues with large queries or deep nesting