top of page

Basics of an API Testing for Beginners

What is an API testing?

API (Application Programming Interface) testing is the process of confirming that an API is working as expected or it is a type of software testing that aims to determine whether the APIs that are developed meet expectations when it comes to functionality, performance, reliability and security for an application.

Different phases of API Testing are below:

  1. API specification review: This initial step involves a thorough examination of the API documentation to comprehend its functionalities, limitations, and intended usage.

  2. Deciding what to test the API for: After gaining a solid understanding of the API, the next step is to determine the specific aspects that will be subjected to testing. This typically involves functionalities, error handling, performance, security, and load capacity.

  3. Setting up the test environment: In this stage, a simulated environment that replicates the real-world scenario where the API will be used is established. This environment should encompass necessary hardware, software, and any other dependencies.

  4. Combining application data: If the API interacts with other applications, the appropriate test data from those applications is incorporated at this point. This data could include customer information, product details, or anything else the API is likely to encounter during operation.

  5. Test execution & reporting: The actual testing of the API commences here. A battery of tests designed to assess the chosen functionalities is executed, and the outcomes are meticulously recorded.

  6. Review:  The results are meticulously examined to identify any discrepancies between the anticipated and actual outcomes.

What are the types of API testing?

There are several types of API testing. These are:

1) Smoke Testing This is done after API development is complete. Simply validate if the APIs are working and nothing breaks.

2) Functional Testing This creates a test plan based on the functional requirements and compares the results with the expected results.

3) Integration Testing This test combines several API calls to perform end-to-end tests. The intra-service communications and data transmissions are tested.

4) Regression Testing This test ensures that bug fixes or new features shouldn’t break the existing behaviors of APIs.

5) Load Testing This tests applications’ performance by simulating different loads. Then we can calculate the capacity of the application.

6) Stress Testing We deliberately create high loads to the APIs and test if the APIs can function normally.

7) Security Testing This tests the APIs against all possible external threats.

8) UI Testing This test the UI interactions with the APIs to make sure the data can be displayed properly.

9) Fuzz Testing This injects invalid or unexpected input data into the API and tries to crash the API. In this way, it identifies the API vulnerabilities.

What are the types of API testing tools?


Postman is the most developer-friendly tool. It is a proprietary testing tool owned by Postman Inc. Used for both manual and automation of API Testing, Postman provides the User Interface for performing all the actions for API Testing.

1.1. Key Features:

· Although it requires some coding effort, it anyways helps in automated testing

· Assists in exploratory testing

· It is compatible with Swagger and RAML (RESTful API Modeling Language) formats.

· It supports knowledge sharing within the team

· Run, test, document, and monitoring features

1.2. Limitations:

· Free Version Restrictions: Limited to basic API testing purposes.

· Suitable for tasks like validating API responses, status codes, and contracts.

· Performance Testing: Not equipped for performance testing.

· Lacks necessary insights and metrics for performance analysis.

· Code Reusability: Postman test scripts are challenging to repurpose.

2. Katalon

Katalon Studio is an automation tool for API, Web, Desktop testing and Mobile testing. It offers straightforward deployment by consolidating all frameworks, ALM integrations, and plugins into a single package. The capability of combining UI and API/Web services for multiple environments (Windows, Mac OS, and Linux) is also a unique advantage of Katalon Studio among the top API tools.

2.1. Key Features:

· Endorses REST, SOAP requests, and SSL client certificates

· Incorporates AssertJ support for crafting fluent assertions in BDD style

· Facilitates test import from Swagger (2.0 & 3.0), Postman, WSDL, and WADL

· Enables API test data setup through UI testing

· Applicable for both automated and exploratory testing

2.2. Limitations:

· Free Version Limitations: Katalon operates as a proprietary tool with restricted features in its free version.

· Limited Community Assistance: Due to a smaller user base, community support for Katalon is relatively limited.

· Finding solutions for intricate scenarios can be challenging.

· Absence of Scripting Language Support: Katalon lacks support for various scripting languages.

3. JMeter

JMeter is a freely available software designed for load and performance testing of applications. Operating on a cross-platform basis and functioning at the protocol layer, it serves as a versatile tool for developers. Notably, JMeter can be employed as a unit-test tool for JDBC database connection testing. With a plugin-based architecture, it facilitates the generation of test data and supports a Command-line mode, particularly beneficial for Java-compatible operating systems.

3.1. Key Features:

· Enable caching and offline playback of test results.

· Seamless integration with CSV files, allowing the team to swiftly generate unique parameter values for API tests.

· Integration of API tests into the CI pipeline facilitated through JMeter and Jenkins.

· Applicable for performance testing of both static and dynamic resources.

3.2. Limitations:

· User Interface Experience: JMeter’s user interface is often reported as not being user-friendly, causing challenges during test script execution.

· Memory Utilization: JMeter tends to use a significant amount of memory, leading to frequent performance issues when compared to alternative API automation tools.

· Scripting Complexity: JMeter’s scripting process for API testing is more intricate in comparison to other available API testing tools.

4. REST-assured

REST Assured is a freely available, open-source Java library utilized for the testing and validation of RESTful APIs. It stands out for its user-friendly approach in Java settings, making it a popular option among developers and testers dealing with REST services.

4.1. Key Features:

· Accommodates all the main HTTP methods like GET, POST, PUT, DELETE, facilitating interactions with a variety of API endpoints.

· REST Assured enables predefined configurations for requests, including base URI, headers, query parameters, and authentication, which can be reused in various tests.

· It offers the capability to examine various elements of API responses, such as status codes, headers, the body of the response, and timing.

· Comes with inbuilt capabilities for managing JSON and XML responses, which are prevalent in today’s APIs.

4.2. Limitations:

· Limited Support for SOAP APIs: Does not specifically cater to the testing of SOAP (Simple Object Access Protocol) APIs.

· Java Proficiency Required: To use REST Assured effectively, a solid understanding of Java programming is necessary.

5. Karate DSL

Karate DSL, an open-source API testing framework, utilizes Behavior Driven Development (BDD) and Gherkin syntax for API test automation. This approach, distinct from other BDD tools like JBehave or Cucumber, provides pre-defined step definitions, enabling easy test creation, even for those with minimal coding experience.

5.1. Key Features:

· Supports BDD Syntax

· It is beginner-friendly as it requires minimal coding experience.

· Integrates with the Gatling framework for performance checks.

· Features multi-thread parallel execution.

5.2. Limitations:

· Unique Scripting Language: May require learning new scripting syntax.

· Limited IDE Support: Absence of IntelliSense support in integrated development environments.

· Challenges in Error Troubleshooting: Identifying code errors can be complete.

6. SoapUI

SoapUI, a prominent API testing solution, is favored by many businesses for its ability to adhere to specific protocols and functionality needs. It supports a wide range of features and is compatible with REST and SOAP APIs. SoapUI specializes in API performance, security testing, and mocking, ensuring efficient and secure API operations.

6.1. Key Features:

· Evaluates API stability under high user loads.

· It provides insightful metrics analysis for easy decision-making.

· Generates test simulations and analyzes performance data.

· Includes checks for SQL injection and cross-site scripting.

· Mimics API service behaviors for thorough testing.

6.2. Limitations:

· Occasional Stability Issues: Some users report stability concerns, leading to crashes or erratic behavior during tests.

· Steep Learning Curve: The interface and features can be challenging for beginners in API testing.

· Reliance on WSDL for Web Services: SoapUI depends on Web Services Description Language (WSDL) for web service testing, which can be a limitation if WSDL files are not well-defined or available.

What are the methods of API testing?

There are mainly 4 types of API testing methods:

  • GET– The GET method is used to extract information from the given server using a given URI. While using GET request, it should only extract data and should have no other effect on the data.

  • POST– A POST request is used to create a new entity. It can also be used to send data to the server, for example, customer information, file upload, etc. using HTML forms.

  • PUT– Create a new entity or update an existing one.

  • DELETE– Removes all current representations of the target resource given by a URI.

What are the advantages of API testing?

·Finding bugs at an early stage of software development: Having access to the application without a user interface or users to engage with the system is a significant advantage of API testing. It provides QA engineers with early visibility into flaws and faults, enabling developers to address them before any impact on the interface occurs.

· Optimizing Time Efficiency & Reducing Testing Expenses: API testing stands out for its rapid result delivery, leading to early detection of flaws and ultimately lowering overall testing costs.

· Language Independence: Utilizing XML or JSON for data communication in API tests ensures language independence. This flexibility allows automation of API testing in the application using any core programming language.

· Enhanced Test Coverage: Automated API tests go beyond the limited scope of unit tests, which concentrate on individual components, functions, or methods within the source code.

Issues often arise at the intersection of two modules, where one level’s scope concludes, and another level begins.

API level testing is designed to validate the proper functioning of each system component, thereby improving the overall quality of the software and its interactions with users.

Happy Learning!

Thank You

64 views0 comments

Recent Posts

See All



bottom of page