Effective API Test Design: A Comprehensive Guide

Written by Andy in API Testing

If you are someone who needs to test software applications, you may have heard of API testing. It can be a daunting task to test APIs if you are not familiar with the technical aspects involved. However, there is no need to worry. In this comprehensive guide, we will take you through the step-by-step process of designing effective API tests in simple language, even if you don’t have much technical knowledge. By the end of this guide, you will have a clear understanding of how to test APIs and maximise the return on investment (ROI) for your software testing efforts. So, let’s get started!

Follow these 9 Steps (+1 bonus step) for effective API test design and implementation, and ensure your application is thoroughly tested for all all functional and non-functional aspects of your application.

STEP 1: Understanding the API

Before designing an API test strategy, it is essential to understand the function and purpose of the API. An API or Application Programming Interface is a software intermediary that allows two applications to talk to each other. It enables the exchange of data between applications without the need for direct communication.

To ensure an effective API test design, it is essential to review the API specification for duplication, missing, or wrong implementations. The specification is a document that outlines the API’s requirements, including endpoints, data structures, parameters, and methods.

Once the API’s function and purpose are understood, it is important to identify the types of testing required. The testing types may include functional testing, which checks if the API is working as expected, negative testing, which validates the API’s behavior in unexpected or invalid inputs, performance testing, which measures the API’s response time and throughput, and security testing, which checks for vulnerabilities or weaknesses in the API’s security.

Understanding the API’s function and purpose, reviewing its specification, and identifying the types of testing required are critical steps in designing an effective API test strategy. It ensures that the testing process is comprehensive and covers all aspects of the API’s functionality.

STEP 2. Selecting the Right Tools

Selecting the right tools for API testing is crucial for a successful test implementation. In this step, we’ll explore some of the best API testing tools available in 2023 and the criteria for selecting them.

It’s important to note that there are many API testing tools available, both open-source and commercial. Some of the most popular ones include:

  • Postman
  • SoapUI
  • Rest-Assured
  • Karate
  • JMeter
  • REST-Assured
  • RapidAPI
  • Newman
  • Katalan Studio
  • Tricentis Tosca

Each tool has its own unique features and capabilities, so it’s essential to evaluate them based on your testing needs.

When selecting a tool, the following criteria can be useful to consider:

  1. Ease of use: A tool that is easy to learn and use can save a lot of time and effort during the testing process. Consider the current skills of the team as well.
  2. Support for multiple protocols: APIs can use various protocols, such as REST, SOAP, and GraphQL. It’s crucial to ensure that the tool supports the protocols your API uses.
  3. Integration with CI/CD pipelines: API testing should be integrated with your organization’s CI/CD pipeline. Thus, it’s essential to ensure that the tool integrates seamlessly with your pipeline tools.
  4. Reporting and analysis capabilities: The tool should provide detailed reporting and analysis of test results for effective decision-making.
  5. Collaboration and version control: For teams, it’s crucial to have a tool that supports collaboration and version control.

By considering these criteria, you can select the right tool that fits your organization’s API testing needs.

STEP 3. Designing Effective Test Scenarios

The success of API testing depends heavily on designing effective test scenarios that ensure maximum coverage of the API’s functionalities. Here are some key elements to consider when designing your API test scenarios:

  1. Happy Path Scenarios A happy path scenario represents the ideal scenario where the API works as intended without any errors or exceptions. These scenarios cover the basic use cases of the API and serve as the foundation for testing other scenarios.
  2. Extended Positive Scenarios (Alternate Path) Extended positive scenarios cover more complex use cases of the API that require additional inputs or steps to complete successfully. These scenarios are variations of the happy path scenario and test the API’s ability to handle different inputs and conditions.
  3. Negative Path with Valid and Invalid Inputs Negative testing involves testing the API’s ability to handle invalid or unexpected input data. These test scenarios should include both valid inputs that are outside the expected range, as well as completely invalid inputs that the API should reject outright.
  4. Authorization/Permission Boundaries Authorization and permission boundaries refer to the access levels required to interact with the API. This is an important consideration when designing test scenarios, as it determines which test cases can be performed and which require additional permissions.
  5. Isolated API tests Designing Test Flows to Have APIs Tested in Isolation It is important to design test flows that allow APIs to be tested in isolation, without dependencies on other APIs or systems. This ensures that any errors or issues discovered during testing can be attributed to the correct API and resolved efficiently.
  6. API user flows Designing Longer Flows/Chains of Calls to Represent User Interaction with APIs In addition to testing individual API endpoints, it is also important to test the API’s ability to handle longer flows or chains of calls. These scenarios test the API’s ability to handle user interactions that may involve multiple API calls, such as authentication, retrieving data, and making changes to data.

While these criteria seem very normal for any test design, it’s combination of these with API specific validation targets (next section) that start to make the API test strategy more powerful.

STEP 4. Identifying Validation Targets

Arguably one of the most significant API specific step. When testing an API, it’s important to identify the areas that need to be validated to ensure the API functions correctly. The following are key validation targets:

HTTP response codes

Verify that the API returns the correct HTTP response codes for different types of requests. For example, a successful GET request should return a 200 OK code, while a failed request should return an appropriate error code.

Response headers

Verify that the response headers contain the expected values. This could include checking the content type, cache control, and any custom headers.

Response payload

Verify that the response payload contains the expected data. This could include checking the structure of the response, the data types, and the values returned.

End-to-end CRUD flows

Verify that the API can perform all CRUD (Create, Read, Update, Delete) operations correctly. This could include testing the creation of a new resource, retrieving the resource, updating the resource, and deleting the resource.

File management tests

Verify that the API can handle file uploads and downloads correctly. This could include testing the upload of a file and the download of the same file to ensure the file is not corrupted or altered during the process.

Database integrity tests

Verify that the API can interact with the database correctly. This could include testing that the API can retrieve, update, and delete data from the database without corrupting the data or causing any errors.

Error case coverage as per API design

Verify that the API can handle error cases correctly. This could include testing the API’s error handling capabilities when invalid data is submitted or when there are issues with the server or database.

Examples:

  1. When testing an API that allows users to submit a new product, validation targets could include verifying that the API returns a 201 Created response code, that the response headers contain the correct content type, and that the response payload includes the product information submitted.
  2. An API that allows users to upload a file, validation targets could include verifying that the API returns a 200 OK response code, that the response headers contain the correct content type and file name, and that the downloaded file matches the uploaded file in size and content.
  3. For an API that interacts with a database, validation targets could include verifying that the API can retrieve data from the database correctly, update the data without causing any errors or data corruption, and delete the data without leaving any remnants in the database.

STEP 5. Planning Non-functional API Checks

So far, we have discussed how to design effective functional API tests. However, it is also essential to plan non-functional API checks to ensure that the application is performing well under different conditions. Non-functional tests help to evaluate how well the system meets the expected requirements such as performance, security, and stability.

Here are some key areas to consider when planning non-functional API checks:

Overall Application State (if applicable)

One aspect of non-functional API testing is verifying the overall application state. This includes ensuring that the application is stable and functions as expected, with all necessary resources being available. If the application is dependent on other services or APIs, it’s important to test how it handles errors or service outages from these dependencies. For example, if the API being tested relies on a database, the test should include scenarios where the database is offline, busy or unresponsive. The testing should also ensure that the application fails gracefully and provides meaningful error messages to users.

Basic Performance Sanity

The second area to consider is basic performance sanity testing. This helps to verify that the application performs well under normal conditions, with acceptable response times and low error rates. Performance tests can help identify bottlenecks in the system and highlight areas that need optimization. The performance tests should be designed to simulate realistic user traffic to ensure that the system can handle the expected load without significant performance degradation.

Load Testing/Security Testing in the Application and User Context

Load testing and security testing are crucial components of non-functional API testing. Load testing helps to verify that the application can handle the expected load and scale efficiently under high traffic. It also helps identify performance bottlenecks, resource utilization, and other issues that may arise under high traffic. Security testing ensures that the application is secure and follows best security practices. It involves checking the API for vulnerabilities such as SQL injections, buffer overflows, and other security risks.

When planning non-functional API checks, it’s essential to consider the user context and the application’s intended usage. This ensures that the application performs optimally under real-world conditions, and any issues are caught and fixed before they impact the users. Non-functional testing should be an integral part of the testing process, and the tests should be planned and executed in parallel with functional tests.

STEP 6. Using Test Design Heuristics

Test design heuristics are guidelines and techniques that testers can use to design effective and efficient tests. One popular heuristic used in test design is SFDIPOT, which stands for Software Function, Data, Interface, Platform, Operations, and Time. SFDIPOT can help testers identify different areas of the system that need to be tested and ensure comprehensive test coverage. Here’s an overview of what each letter in SFDIPOT represents:

  1. Structure: What the product is comprised of including all executable as well as non executable software and hardware.
  2. Function: This refers to the application’s functionality, such as the business logic, user interface, and workflows. Testers should design tests that verify the correct behavior of the software functions, such as verifying that the application behaves as expected in different scenarios. This includes:
    • Application
    • Calculation
    • Time related
    • Security related
    • Transformations
    • Startup / Shutdown
    • Multimedia
    • Error handling
    • Interactions
    • Testability
  3. Data: Data refers to the different data types and structures used by the application, including inputs, outputs, and internal data. Testers should design tests that verify the correctness, accuracy, and completeness of the data used by the application. This includes:
    • Input / Output
    • Persistent
    • Preset
    • Independent / Interacting
    • Sequences / Combinations
    • Cardiniality
    • Big / Little
    • Invalid / Noise
    • Lifecycle
  4. Interface: This refers to the different interfaces used by the application, such as APIs, web services, and GUIs. Testers should design tests that verify the interoperability, compatibility, and security of the interfaces. Think in terms of:
    • User interfaces
    • System interfaces
    • API / SDK
    • Import / export
  5. Platform: The platform refers to the different environments and technologies used by the application, such as the operating system, hardware, and software libraries. Testers should design tests that verify the compatibility, stability, and performance of the application across different platforms. This also includes:
    • External Hardware
    • External Software
    • Embedded components
    • Product footprints
  6. Operations: Operations refer to the different actions performed by the application, such as installation, configuration, deployment, and maintenance. Testers should design tests that verify the correct operation of the application in different scenarios, such as upgrades and backups. Include cases related to:
    • Users
    • Environment
    • Common Use
    • Disfavoured use
    • Extreme use
  7. Time: This refers to the different time-related aspects of the application, such as response times, timeouts, and time-based functionality. Testers should design tests that verify the application’s performance, behavior, and correctness over time. Inclusions in this category to be:
    • Input / Output
    • Fast / Slow
    • Changing rates
    • Concurrency

Another important aspect of effective test design is the complete and correct use of assertions in your tests. An assertion is a statement that verifies the expected outcome of a test. It will vary how you assert based on the choice of tools. Key point to remember is that any validation targets (from Step 4 above) NOT checked will be marked and considered as a PASS for the specific tests. It is a critical component of automated testing, as it helps to identify failures and errors in the system. In simpler language – assertions are used to compare the expected outcome of a test with the actual outcome and report any differences. Incomplete assertion checks is one of the most common mistake I have noticed over the years working with hundreds of automation testers and thousands of automation checks.

STEP 7. Test Environment and Data Stability

The stability of the test environment and test data is critical for effectiveness of API (probably any form of) testing. It ensures that the tests are repeatable and can be run consistently without any variation in results. Here are some key steps to ensure test environment and data stability:

  1. Identifying test data required: The first step is to identify the types of data required for testing. This includes inputs, outputs, and any other relevant data. Testers should ensure that the data is representative of real-world scenarios and covers all possible test cases.
  2. Evaluating test data setup strategy: Once the data has been identified, testers should evaluate the best way to obtain the data during the test runs. This could involve generating data on the fly, using pre-existing data, or using mock data.
  3. Standardizing test data for runs: To ensure consistency, testers should standardize the test data. This could involve using pre-defined database backups, mock responses, or other techniques. Standardizing the data makes it easier to compare results across different test runs.
  4. Ensuring immediate repeatability of tests: To ensure immediate repeatability, testers should ensure that the test environment is stable and consistent. This could involve using virtualisation or containerisation to create a consistent environment for testing. Also designing test cases and cleanup in a manner that every test leaves the test data in the state it started for reliable repeatable execution.
  5. Integrating API testing with other software testing techniques: API testing should be integrated with other software testing techniques such as unit testing, integration testing, and acceptance testing. This helps to ensure that the application is thoroughly tested across different layers and components. Correctly leveraging various levels to mitigate the risk will compliment the API Testing efforts.
  6. Keeping test suites agile and flexible: Finally, testers should keep the test suites agile and flexible. This involves using techniques such as continuous integration and continuous testing to ensure that the tests are always up-to-date and can adapt to changes in the application.

By following these steps, testers can ensure that the test environment and data are stable and consistent, which is critical for effective API testing. Standardizing the test data, integrating with other testing techniques, and keeping the test suites agile and flexible can help to ensure that the application is thoroughly tested and ready for release.

STEP 8. Maximizing ROI

Maximizing return on investment (ROI) is crucial for any testing effort, including API testing. Here are some key steps to help maximize the ROI of API testing:

Including automation in CI/CD processes

One of the most effective ways to maximize ROI is to include API test automation in the continuous integration/continuous delivery (CI/CD) pipeline. This ensures that tests are run automatically on each code commit, and any issues are caught early in the development process.

Gated check-ins of impacting components

Another way to maximize ROI is to use gated check-ins for components that have a high impact on the application. This ensures that any changes are thoroughly tested before they are merged into the codebase.

Promoting tests from dev to UAT/staging/certification

Tests should be promoted from the development environment to the user acceptance testing (UAT), staging, and certification environments to ensure that the application is thoroughly tested in each environment before being released to production.

Using API tests for PVT and monitoring production

API tests should be used for production validation testing (PVT) and for monitoring production environments. This helps to ensure that the application is performing as expected in production and any issues are caught and addressed immediately.

By following these steps, testers can ensure that the API testing effort is optimized for maximum ROI. Including automation in the CI/CD pipeline, using gated check-ins for impacting components, promoting tests from dev to production, and using API tests for PVT and monitoring production environments can help to ensure that the application is thoroughly tested and any issues are caught early in the development process. This also helps development process highly coupled with automation checks and maintenance becomes a part of every activity rather than an after-thought allowing quality checks to shift left in the cycle.

STEP 9. Continuous Reporting

Simplest way to describe this step is – Report Everything, Everywhere and all at once!

Continuous reporting is essential to ensure that the testing effort is effective and that any issues are identified and addressed promptly. Here are some key steps to follow to establish continuous reporting for API testing:

  1. Setting up dashboards to provide product health in all stages: Dashboards can provide an at-a-glance view of the health of the application at various stages of the development process. This helps testers and developers to quickly identify any issues and prioritize their efforts accordingly.
  2. Detailed reports on failed tests with response codes and details: When a test fails, it is important to have detailed reports that provide the response codes and other relevant details. This helps to quickly identify the root cause of the issue and address it promptly.
  3. Connecting results with server-side logs for HTTP 500+ errors: In cases where the API returns an HTTP 500+ error, it is important to connect the test results with the server-side logs. This helps to identify the root cause of the issue and address it promptly.
  4. Continuously reviewing and updating the completed pipeline: Finally, it is important to continuously review and update the completed pipeline to ensure that it remains effective and efficient. This includes updating test cases as needed, reviewing test results regularly, and identifying areas where improvements can be made.

By following these steps, testers can establish an effective and efficient continuous reporting system for API testing. Dashboards provide an at-a-glance view of the health of the application, while detailed reports on failed tests with response codes and details help to quickly identify and address issues. Example – Connecting API results with server-side logs for HTTP 500+ errors is also essential for quickly identifying and addressing issues. Finally, continuously reviewing and updating the completed pipeline helps to ensure that the testing effort remains effective and efficient.

BONUS STEP 10. Industry Standards for API Testing

While there is no one-size-fits-all approach to API testing, there are some industry standards and best practices that testers can follow to ensure that their testing effort is effective and efficient. Here are some key points to keep in mind when it comes to industry standards for API testing:

  1. Overview of common API testing standards: There are several common API testing standards, including SOAP, REST, and GraphQL. Understanding the differences between these standards and when to use them is important for developing effective tests.
  2. Understanding industry guidelines and best practices: The industry provides several guidelines and best practices for API testing. These include things like designing test cases with specific inputs and expected outputs, testing for edge cases, and including performance and security testing in your API tests.
  3. Kaizen: Keeping a keen eye on changing technology landscape and continuous review of all the steps from time to time is key to ensure all the automation efforts don’t get abandoned when environments / infrastructure march ahead.

By following these industry standards and best practices, testers can enhance the efficacy and the efficiency of their API tests.

Conclusion

Effective API test design is an important part of software development. By following the steps outlined in this comprehensive guide, testers can develop API tests that are effective and efficient, and that help to reduce the risk of issues in the application.

Key takeaways from this guide include:

  • Understanding the purpose and goals of API testing
  • Planning API test cases based on business requirements and user needs
  • Designing API tests that are repeatable, maintainable, and scalable
  • Using automation to streamline the testing process and maximize ROI
  • Establishing continuous reporting to identify issues and address them promptly
  • Staying up-to-date with industry standards and best practices

In conclusion, effective API test design is a critical component of modern software development. By following the steps outlined in this guide, testers can develop high-quality API tests that help to reduce the risk of issues and ensure that the application performs as expected. By prioritizing API testing and following best practices, testers can help to ensure the success of their projects and deliver high-quality software to users.

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