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Web API Overview

API stands for Application Programming Interface. It comprises a set of functions that can be accessed and executed by another software system.

Why we need APIs?

APIs are used to integrate new applications with existing software systems. This increases development speed because each functionality doesn't have to be written from scratch. You can use APIs to leverage existing code.

What is API Testing?

Many web applications are designed based on a three-tier architecture model.

These are:

  • Presentation Tier – User Interface (UI)

  • Logic Tier – Business logic is written in this tier. It is also called Business Tier. (API)

  • Data Tier – Here information and data are stored and retrieved from a Database.

Ideally, these three layers (tiers) should not know anything about the platform, technology, and structure of each other.

We can test UI with GUI testing tools and we can test logic tier (API) with API testing tools. The logic tier comprises all of the business logic and it has more complexity than the other tiers and the test executed on this tier is called API Testing. API testing tests the logic tier directly and checks expected functionality, reliability, performance, and security.

In GUI testing we send inputs via keyboard texts, button clicks, drop-down boxes, etc., on the other hand in API testing we send requests (method calls) to the API and get output (responses). These APIs are generally REST APIs or SOAP web services with JSON or XML message payloads being sent over HTTP, HTTPS.

Different web API's type

Here are some of the most common types of web API's:

Public API's

Public APIs (or Open APIs) function through a set of tools, routines, and protocols for developing software applications. They allow for access to application data while shielding developers from having access to shared source codes. This means companies can have the benefit of making their services available while preserving proprietary ownership over developer output.

The usage of public APIs are used for creation of applications and services.

Partner API's

Partner APIs are specialized interfaces that enable organizations to access data and service offerings across businesses (B2B). This streamlined approach helps businesses to create unique features within their own applications or services by utilizing a partner's resources.

With the ability to leverage existing technologies, companies can effectively reduce costs and allocate time more efficiently. By integrating valuable data and services, companies can enhance customer experience using partner APIs.

Internal API's

As the name implies, an internal API is a type of API intended for use internally by the organizations own developers. These APIs facilitate the transmission of data between different components of a system, enabling process automation.

As an exclusive operation and service for large organizations, it's common for internal APIs to be proprietary systems. They enable various applications and systems within an organization to communicate with each other, developing a layer of abstraction regarding modifications or maintenance.

Composite API's

A composite API is multiple APIs into one interface and provides developers with a unified view of the data from different sources. This integration simplifies data access and utilization by easing interactions with complex applications, commonly seen in connecting various cloud services.

Composite APIs offer developers efficient coding practices, as they don't have to write separate code for every individual API. The composite API can streamline the development process and provide cost-effective solutions by offering a unified interface for accessing multiple APIs.

Different types of API protocols

When it comes to programming interface integration between two parties using an API, strict guidelines known as API protocols are enforced to regulate interactions.

These rules is enhance efficiency when exchanging data within various applications through standardized communication modes. Some requirements outlined by the API protocols include specifying formatting for request/response exchanges or permissible kinds of data that can be shared, authentication procedures, or security measures for safe information transfer.

Following these standards guarantees consistent interactions while ensuring reliable performance.

Rest API's

A majority of APIs are powered by REST. Web applications commonly employ REST APIs. These are versatile interfaces for sending and retrieving data through HTTP requests in JSON, XML, and plaintext among other formats.

REST (Representational State Transfer) APIs represent an example of these interfaces since they use the principles underpinning the REST architectural design when communicating between clients and servers. This design includes predefined sets of different operations parameters available via standard HTTP methods such as GET, POST, PUT or DELETE while allowing scalability.

Through APIs, accessing structured data in JSON or XML format is facilitated while also guaranteeing security and accessibility to resources. Adhering to REST principles helps developers create dependable, user-friendly API systems that are capable of being used across devices and platforms.


Web services can also be built using SOAP APIs, which rely on XML messages to enable communication between two applications over a network.

SOAP has strict rules and rigid messaging standards that can make it more secure than protocols such as REST. These types of APIs are frequently used in enterprise applications, particularly for payment processing and customer management as they are highly secure in nature.

Creating web services that can be accessed by various clients without being restricted to a particular platform is doable thanks to SOAP APIs. Its secure nature is another important benefit that has made SOAP widely used for transferring highly sensitive information.

GraphQL APIs

If you're looking for a fast API solution, consider GraphQL. This approach eliminates the need for excessive queries from clients by leveraging its specialized query definition capabilities hence providing much-needed flexibility during data aggregation.

Unlike traditional REST APIs, developers can take advantage of an interface capable of on-demand "data querying" with ease. In essence, with GraphQL APIs, users request everything that they want from the backend service all in one API endpoint. This helps them avoid superfluous API calls. By enabling the retrieval of precise data, GraphQL APIs have the potential to minimize data requests and consequentially make applications more efficient. In addition, it enables developers to swiftly incorporate new fields or types in their applications, creating a rapid and streamlined development process.

Webhook APIs

The functionality of webhook APIs primarily revolves around facilitating real-time transmission of data and seamless integration between various web applications. An example of their usage would be sending push notifications to alert users about incoming emails. As soon as specific pre-configured events take place, these APIs automatically trigger messages or "hooks" that initiate communication between different software programs.

Webhook APIs enable applications to remain synchronized and respond to events instantly. These APIs are extensively used for event-based programming where the software can exchange real-time data. They can have varied purposes such as signaling a user after clicking on a link or updating an online store with new items without any manual intervention.


By using APIs, applications can execute remote functions seamlessly over long distances even when hosted on different machines. In distributed systems and microservices architectures where this kind of flexibility is crucially important, there is growing reliance on Remote Procedure Call (RPC) APIs whose implementation typically follows protocols like JSON-RPC or XML-RPC.

At its core lies the effective RPC protocol, designed to enable efficient data transfers among multiple interconnected computer nodes via networks characterized by various topologies. This interaction occurs via the standard but elegant client-server paradigm whereby servers respond to incoming requests issued by clients. Messages containing requests for a server to carry out a specific task and return a response, known as remote procedure calls, facilitate the communication process between the client and server. RPC APIs are often utilized to manage resources like databases on remote machines or to access web services.

There are other types of APIs as well, but these are some of the most common and widely used. The choice of API type will depend on the specific requirements of the application and the data being accessed.

API request methods

APIs utilize various request methods that define how a client app is allowed to interact with online systems. These particular approaches usually revolve around acquiring and manipulating information stored in those systems.

To be specific, there exist five popular kinds of API request methods: GET, POST, PUT, PATCH, and DELETE.

Consequently seeking information is made easy via GET requests, while creating fresh details may only be accomplished through POST logic. Updating older information must be done using PUT protocol, while PATCH may alter it if needed, and DELETE will remove any redundant files. Powerful, dynamic applications can be created by effectively utilizing a combination of methods distinguished by their unique purposes and applications.

Here is a high-level overview of several types of API methods:

  • GET method: retrieves information or data from a specified resource

  • POST method: submits data to be processed to a specified resource

  • PUT method: updates a specified resource with new data

  • DELETE method: deletes a specified resource

  • PATCH method: partially updates a specified resource

  • OPTIONS method: retrieves the supported HTTP methods of a server endpoint

  • HEAD method: retrieves only the headers of a response without the response body

  • CONNECT method: establishes a network connection to a resource, typically used for SSL/TLS tunneling

  • TRACE method: echoes the received request back to the client, for debugging purposes

API methods are typically used to interact with web services or web applications, allowing developers to access and manipulate data or functionality from a remote source.

HTTP status codes

In most cases APIs will respond to requests with the standard HTTP status code definitions. If the API fails to validate a request, it will respond with a validation error message (JSON or XML) describing the issue.

Here are some common HTTP status codes you may encounter:

200: "OK"

201: "Success"

204: "Success"

301: "Login required"

304: "Not Modified"

400: "There was a parsing error."

400: "Bad Request"

400: "There was a missing reference."

400: "There was a serialization error."

400: "There was a validation error" (see Validation errors)

401: "Login required"

401: "Unauthorized"

401: "You are not authorized to make this request"

403: "Forbidden"

403: "This service has not been enabled for your site."

403: "XSRF Protection Failure"

404: "The requested resource was not found."

409: "There was a conflict."

412: "The resource you are attempting to delete has dependencies, and cannot be deleted"

413: "Storage space exceeded."

429: "Too Many Requests"

500: "The service has encountered an error."

500: "Internal Server Error"

502: "Bad Gateway"

503: "Service Unavailable".

503: "There was a timeout processing the request".


Understanding the different types of web APIs, API protocols, API request methods & HTTP status codes is essential for developers looking to create modern web applications.

REST APIs are the most common type of protocol, while GraphQL is a newer type of protocol that allows for more efficient and targeted data retrieval. SOAP, RPC, and webhook API protocols also each have their place.

When it comes to API request methods, the methods used depend varying on the type of data to be sent and the desired action. Each method has its own strengths and uses, and developers should carefully consider their options.

In the end, the choice of web API type, protocol, and request method varies depending on your requirements and goals. By understanding the differences between these options, developers can choose the approaches best suited for their project and ultimately build more robust, effective applications.

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