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Introduction to API testing in Katalon Studio

This article aims to provide an overview of API testing with candid answers to the What – When – Why- questions that hopefully shed light on this mysterious land hidden inside the boundary of testing – engineering realm.

You will also find potential challenges with API test implementation – best practices and how to choose right toolset to help you achieve API testing with a higher success chance.

What is API testing?

API testing is a type of software testing that involves testing application programming interfaces (APIs) directly and as part of integration testing to determine if they meet expectations for functionality, reliability, performance, and security. Since APIs lack a GUI, API testing is performed at the message layer and can validate application logic very quickly and effectively.

API testing is critical for automation testing and CI/CD process because it can cope with short release cycles and frequent changes especially the presentation layer without breaking the test outputs. API testing also requires less maintenance effort compare to UI automation testing which makes it a preferred choice for Agile and DevOps teams.

A side note, for Web and mobile applications, API often means Web services, and API testing refers to the automation test performed to the Web services.

Where is API testing performed?

Commonly, applications have three separate layers or tiers including Presentation Layer or user interface, Business Layer or application user interface for business logic processing, and Database Layer for modeling and manipulating data.

API Testing is performed at the most critical layer, the Business Layer, where business logic processing is carried out, and all transactions between User Interface and Database happen.

Why do we need to perform API testing?

The figure below shows three different layers of testing called the test pyramid initially coined by Mike Cohn in his book Succeeding with Agile. It has layers representing different types of testing. Despite its being overly simplistic, it offers us a general rule of thumb: it suggests how much testing we should focus on at each layer. As such, API and services tests in the second layer is an important testing activity that we should focus on.

Why we need perform API testing

Unit/component tests: This lowest level of testing brings the highest value and ROI. It is mainly performed by developers. The unit/component tests can attain between 70% and 80% of code coverage and require as much effort.

Business rules/Functional tests: This level of testing focuses on business rules of the application under test. The tests are designed to test against user stories to ensure that all implemented functions are working as expected. In Web and mobile applications, data shown in the user interface is often returned from servers via API/services. So API testing is performed to ensure the accuracy of API/services. Testing at this level may need about 20% of the total testing effort.

Workflow Tests (through the UI): functional UI testing is performed via the UI of the application to ensure that its features are built as expected. Due to the fact that automated functional UI testing is brittle, and any changes in the UI can break the tests, test automation teams should focus less on this level of testing.

API testing yields the highest ROI compared to all other testing type performed by testers as Unit test performed by developers mostly. It also offers a number of advantages over other kinds of testing:

Advantages of API testing

As discussed above, API testing is an important activity that testing teams should focus on. It offers a number of advantages over other kinds of testing:

  • Language independent: Data is exchanged via XML and JSON, so any language can be used for automation, independent from the languages used to develop the application. XML and JSON are typically structured data so the verification is fast and stable. There are also built-in libraries to support comparing data using these data formats.

  • GUI independent: We can perform API testing within the application prior to GUI testing. Early testing will get feedback sooner and improve the team's productivity. Core functionality can be tested to expose small errors and to evaluate a build's strengths.

  • Improved test coverage: Most API/services have specifications, allowing us to create automated tests with high coverage, including functional testing (happy cases, negative cases) and non-functional testing. With virtual user techniques, we rarely find API test cases that could not be automated.

  • Faster releases: It is common that executing a UI regression test suite takes 8-10 hrs while the same scenario with API testing takes only 1-2 hours. The flakiness of API testing is also lower than that of UI testing. All these allow us to release faster with API testing.

API testing types

API testing is generally categorized into common categories below:

API Testing Types

Validation testing

Validation testing is among the final steps and plays an essential role in the development process. Validation test helps verify the aspects of product, behavior, and efficiency. In other words, the validation testing can be seen as an assurance of the correct development against the stated user needs and requirements.

Functional testing

Functional testing includes the testing of particular functions in the codebase. These features are the representation of specific scenarios to make sure the API functions are handled well within the planned parameters.

End-to-end testing

UI testing is defined as a test of the user interface for the API and other integral parts. It is mainly about the UI functions, in which the interface is naturally graphical or rely on the command-line endpoint calls.

Generally, the test is more likely of the interface which ties into the API rather than the API testing itself. Although UI testing is not a specific test of API in term of the codebase, this still provides overview information about the health, usability, and efficiency of both front-end and back-end.

Load testing

Load testing is generally done after a specific unit, or the whole codebase has been completed to check if the theoretical solutions work as the practical plan. Load testing is performed to ensure the performance under both normal and at peak conditions. To do so, load testing takes on some various scenarios:

  • Baseline – tests the API against theoretical regular traffic that the API expects in ordinary usage;
  • Theoretical maximum traffic – this is to make sure during the full load periods, the solutions respond to those requests properly;
  • Overload test – to test the maximum capability according to the theory, then add 10-20% more on the peak traffic.

Runtime/Error detection

This testing type is related to the actual running of the API, particularly with the universal results of utilizing the API codebase. Generally, they focus on one of the below aspects:

  • Monitoring: The runtime of the compiled code is tested for different implementation errors, handler failures, and other intrinsic issues to help ensure the codebase does not contain any insecurity.
  • Execution Errors: The code needs to show the response to valid requests and marks the failure for invalid requests predictably and in a known way, just the same with valid requests.
  • Resource Leaks: The requests which are invalid or commonly illegal will be submitted to the API to test the memory, resource, data, or operation leaks/insecurities
  • Error Detection: The known failure scenarios will be used to test the code to make sure the errors are appropriately figured and resolved

Security testing

Security, Penetration and Fuzz testing are the three separate elements of the security auditing process. They are used to ensure the API implementation is secure from external threats.

Besides enclosing with penetration and fuzz testing, security testing also includes additional steps like the validation of encryption methodologies, and of the design of the access control for the API. It also contains the user rights management and validating authorization checks to access the resources.

Penetration testing

Penetration testing is considered the second test in the process if auditing. In this testing type, the users with limited API knowledge will try to attack to assess the threat vector from an outside perspective, which is about functions, resources, processes, or aim to the entire API and its components.

Fuzz testing

Fuzz testing is another step in the whole security audit process. In this testing type, a vast amount of random data (referred to as "noise" or "fuzz" ), will be input into the system with the aim to try a forced crash or any negative behavior. This fuzz test will help test the API in term of the limits to prepare for the "worst case scenarios."

API testing best practices

API Testing Best Practices


When starting to develop test scenarios, some of us are relatively common to jump right into some of the negative test scenarios (to verify invalid information, validation on special characters, uncommon input, etc.). These types of verification might cause crashes and other interesting problems. In other words, easy bugs might be found quickly with negative test scenarios. Unfortunately, it is not necessarily the best use of a tester's effort. It is suggested that mainline scenarios should be focus first with higher priority. A basic guideline is to identify the most common parameters and conditions that an end developer will use when calling the APIs and test these scenarios extensively.

Functional and non-functional testing priorities

API is an essential part of modern web applications, its non-functional aspects such as performance, security, connectivity will affect the system heavily. The behavior makes traditional testing mindset, which focuses on functional testing first, not suitable anymore. Functional and non-functional testing should be performed at the same time with equal priority.

End-points management

A typical product may have hundreds of API/Web services endpoints, which look very similar to each other, and might cause duplication issues to the testing project. How you organize the tests and the endpoints will influence the productivity, effectiveness, and maintenance of your tests.

Test endpoints could be grouped by path (including path parameters), which generally performance action on specific object types. For examples:

api/2/issue: affects to Jira issue
-- Create issue
POST /rest/api/2/issue
-- Create issues
POST /rest/api/2/issue/bulk
-- Get issue
GET /rest/api/2/issue/{issueIdOrKey}


api/2/group: affects to Jira Group
-- Create group
POST /rest/api/2/group
-- Get group DEPRECATED
GET /rest/api/2/group
-- Remove group
DELETE /rest/api/2/group

This method will help us preventing missing test endpoints, and test scenarios.

Endpoint as a template

This endpoint is a part of the data-driven approach, which relates to data models more than input data. One of the most challenging issues in API testing is test data mapping with data models, subsets of data models. For examples:

An API creating new Jira issue requires an input data model having required fields (such as Project Key, Summary) and non-required fields (such as Description, Assignee, etc.). All the combination of required and non-required fields should be tested to ensure that all the test scenarios are covered. For each combination, we then apply data-driven to test with different data set.

This practice suggests that we consider each combination as a test template for the endpoint, then apply data-driven approach for this test template to generate actual executable test scenarios.


The most important feature of any API test tools is the capability of the data-driven approach. Lacking data-driven ability will lead to test data hardcoded, duplicated test scripts, test verification, therefore causing massive effort at maintenance phase.

Together with data-driven, there are some important notes for test data that we should pay attention seriously: data types, blank, empty, null string. With RESTful web services, the input data is typically in JSON format, and missing value of a specific key is considered differently (null, empty) in some cases. To avoid these ambiguous test scenarios, using a subset of input data models are highly suggested.

Challenges in API testing

challenges api testing

Initial setup of API testing

In API testing, both manual and automated testing matter. While manual testing confirms whether the app works, automated testing helps to check how effectively the app performs under pressure. Setting up the test infrastructure get it working is considered among the most challenging tasks of the testing process, as it is not only particularly tricky but also a significantly motivation-killer. However, it will be a long-term benefit once the team gets through those challenging setup process.

To avoid these challenges, teams need to start the API testing in the early stage, specifically in the design phase and make sure to conduct the interval checking of APIs for 100% uptime.

Update the schema of API testing

The schema (data formatting which handles requests and responses for the API) should be maintained during the testing process. Any changes in the program that create additional parameters for the API calls must be reflected in the schema configuration.

In today's API economy, the changes in API helps facilitate the demand, but those changes can cause downtime. To avoid this issue, the team needs to test the API in beta and alpha environments, which is seen to reduce 90% of the chances of issues happen.

Testing parameter combinations

APIs handle communication between systems by assigning data values to parameters and passing these parameters through data requests. Testing all possible parameter request combinations in the API is necessary to detect the problems regarding the specific configurations. A more significant project would end up assigning two different values to the same parameter, or create instances where numerical values appear when text values should be. Adding parameter will aggressively increase the number of possible combinations.

This challenge can be solved by picking some applications that aren't very critical to daily operations to utilize the API and release it to those applications only. By doing this, testers can know how the API is utilized and whether there are any changes should be done for a general availability release.

Sequencing the API calls

It is a challenge for testing teams when API needs to be in a specific order to work correctly. For instance, a call to return a user's profile information goes through before the profile is created, the request will return an error. The process can become even more significantly difficult when involving more applications.

To help overcome this issue, a flowchart is recommended to visualize the API calls. Developers then can build and integrate API calls faster with less unexpected problems.

Validating parameters

Software testing team may find it challenging invalidating the parameters sent through API requests because of the sheer number of parameters and use cases for those parameters. Testers need to ensure all parameter data uses the correct string or numerical data type, fits within length restrictions, fits within the designated value range and pass other validation criteria.

To solve this problem, testing team can use the continuous synthetic API monitoring to detect the problem early. Besides, it should be combined with an APM solution to get the 360-degree view.

Tracking system integration

Ensuring the API testing system is working correctly with the data tracking system is another challenge in API testing. This integration is essential in bringing back the appropriate responses on whether a call is working properly. It is also useful in monitoring the API performance. Tracking system integration is a late stage in a whole process, which cause the team not to give it enough attention.

It can be improved by planning since the design phase to see how it will integrate with other systems. Each application has various requirements that need to be met. So, the team should not test in parallel with other critical integration systems. This can be achieved by implementing and adding load testing in the continuous delivery.

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