What is API testing?

Modern applications are capable of advanced functionalities, thanks to integrations with other applications and various third-party services. To enable these integrations, application programming interfaces (APIs) serve as a portal to other applications or services and facilitate the data transfer required for these functionalities.

API testing is an integral part of software testing, which deals with validating APIs by checking if communication, data transfer, and other actions between services are handled efficiently and securely. API testing also checks if APIs work reliably when accessed multiple times.

Types of APIs

APIs can facilitate connections between applications and other services. This helps with secure data transfer, amongst other operations, such as updating, deleting, and more. APIs can belong to one of four types—public, private, internal, and composite. Based on the type of API, there can be restrictions on data transfer.

The API types are:

• Public/open API - Public APIs are available to developers, with little to no limitations on data access and its usage. Owing to the nature of such APIs, they often come with their own set of security issues. On the other hand, due to ease of access, they can be monetized as part of a business. For example, the APIs from Facebook and Google Maps are responsible for a part of their revenue generation.
• Private API - Private APIs are provided by businesses for use only by partners. These are generally a part of a subscription model. Each API provides the required data access based on the type of data being accessed.
• Internal API - Internal APIs are for use within an organization and cannot be accessed by outside developers. These APIs usually provide valuable data essential for functioning within the organization. 
• Composite API - Composite APIs are capable of consolidating multiple API requests into a single API call, in order to reduce the number of API calls and to access multiple API endpoints in fewer API calls. This is especially useful in the case of monetized APIs.

Private APIs are more secure when compared with open APIs, as they come with restrictions on who has access to them. Also, private APIs are more reliable and often rely on dedicated servers, so operations are performed much faster. Irrespective of the API types, secure and reliable data transfer is important, and APIs are capable of providing all the required customization to enable specific requirements.

API requests contain the parameters that define the action(s) to be performed. API responses provide the required data for CRUD—create, read, update, and delete—operations to be performed. For example, the API provided by Zoho Creator facilitates the modification of applications built on the development platform along with their associated data.

API testing and its importance

There are businesses that generate revenue from APIs-as-a-product, as their business model runs on providing APIs for various functionalities. There is also the API-first methodology in software development, which focuses on building software around APIs. Major businesses and even government organizations rely on secure APIs for their day-to-day operations, by integrating with third-party services. APIs play a pivotal role in software development and, therefore, warrant full-fledged testing.

Different types of testing are carried out under API testing, even if they don't fully rely on APIs for testing. All testing types under API testing don't necessarily deal only with data transfer. Some examples of API testing are functional testing, validation testing, integration testing, load testing, reliability testing, security testing, fuzz testing, and interoperability testing.

Automated API testing on Zoho QEngine

In QEngine, there are individual fields for all components of the API request—endpoint, header, body, and parameters—and a dropdown menu is available for the different API methods—GET, POST, PUT, PATCH, and DELETE. This test automation software allows for multiple API requests to be executed subsequently, one after the other, in a single place. Responses from previous requests can be stored in a variable and passed on as arguments for subsequent API requests, thereby enabling continuous testing.

Assertions in the API requests have multiple options to check the different components of the response. Zoho QEngine also supports major API authentication methods—API key, Basic Auth, and OAuth1 and OAuth2—to test the connections which enable reliable and secure data transfer. API requests can also be easily automated to save time, and parallel execution can further improve efficiency.

Once executed, all responses can be viewed, one after the other, in the same window. Furthermore, the results dashboard in Zoho QEngine provides an in-depth analysis of all API requests. Individual API responses can be analyzed for all components, to address any issues.