API Testing Strategy: 2020 has already earned the title of “disruption decade”.

Most organizations around the globe are identifying new ways to survive and thrive in the new normal. Whether your organization has started the digital transformation journey years ago, or you have just started, APIs have become the glue of the digital landscape.

Don’t believe us?

Let the numbers speak.

A report by Markets and Markets indicated that the global API testing market size is expected to grow from USD 384.3 Million in 2016 to USD 1,099.1 Million by 2022, at a compounded annual growth rate of 19.7% in 5 years. 

Undoubtedly, investing more effort and time in API testing strategy is a must-have to dominate the market with good performance, reliability, and security.

APIs are everywhere

From online shopping to business applications, APIs are used everywhere. Currently, there are hundreds and thousands of APIs working for social messaging, payments, ecommerce, finance, and many other categories covering everything we do with the applications.

Real-world examples of API testing

  • Ordering in restaurants: You visit a restaurant, sit at your table, and place the order. Now, the waiter will come and take a note of your order. So, you are requesting the order from the waiter and the waiter is responding with the order he gets in the kitchen and serves you. In this case, the interface between you and the kitchen is the waiter. Similarly, API is an interface that carries out the application requests and ensures that it is getting done. 

  • Ride sharing app: Another example is UBER. Once the application is loaded, it provides you the ability to log in and continue with Google and Facebook. In this case, third-party applications like Google and Facebook are exposing a certain level of user information. Uber can also share locations by using Google Maps. It means that there is an agreement between Google/Facebook/Maps and Uber through API integration.

  • Travel booking: Another most common real-life example is the travel booking system. Take Make my trip for example. You enter the destination, select the dates, click on search, and you get the results from different airlines. How Make my trip is communicating with all these airline websites? That’s all through APIs. It enables the Make my trip site to expose some level of information from multiple airlines and display it to the end user.

What is API testing?

The Application Program Interface (API) is a set of protocols, processes, tools, and routines for building and maintaining critical software applications. It is a user interface that allows software applications to communicate with other applications and is used while programming the components of the graphical user interface (GUI). 

API testing strategy assures the quality of the application that includes sending API calls, getting the desired output, validating the response of the system against the defined input parameters, inaccuracy of data and data formats, error codes, and HTTP status codes. The testing process starts at the earliest stage and continues until the production release.

SOAP vs REST API Testing

APIs can utilize different architectures like SOAP and REST to transfer the data from the server to the client. Although REST is quite popular these days, it is important to understand both styles of communication. Here is a comparison table to highlight the key differences between the two API styles.

API Styles

SOAP (Simple Object Access Protocol)

REST (Representational State Transfer)


Standardized protocols with predefined rules

Architectural style with flexible guidelines and recommendations





API calls cannot be cached

API calls can be cached


Requires high-level computing power and bandwidth

Requires minimal resources


WS-Security with SSL support

Supports SSL and HTTPS


High extensibility, security, and standardized system

High scalability and flexibility, browser-friendliness, better performance


Less flexibility, poor performance, more complexity

Less security

Types of API Testing

APIs have become the center of software development and a source of connecting and transferring data and logic across disparate applications and systems. While there are specific types of tests that can be conducted to improve the efficiency of API testing strategy, most tests fit broadly into one of these categories. Let us explore the types of testing and their testing software stage.

Type of Testing

Stage of software

Unit testing and integration testing

Development Stage

Performance and Load Testing

As early as possible as these steps take time

Runtime error detection and security testing

Ongoing processes

Interoperability and fuzz testing

Testing stage

Validation Testing

User Acceptance Testing

Unit Testing

The tests are written to run with each build of the application automatically. It is written close to the code and should pass while running the build of the application.

Integration Testing

APIs work to integrate various parts of a system and all of this integration requires a relevant testing process. Integration testing ensures that the application takes the correct parameters with correct constraints while logically sanitizing the incoming traffic.

Performance testing

Performance testing is a non-functional process that is often overlooked. Performance testing takes everything into account including the traffic spikes, multiple heavy processes running simultaneously, and interfering with one another.

Load testing

Load testing is similar to performance testing as the primary goal is to emulate production without creating any traffic spikes. The production should be emulating a constant steady stream of traffic at a normal rate. It ensures that the APIs do not have any memory leaks or other similar defects that can cause issues after running for a prolonged time.

Runtime error detection

While conducting all of these tests, it is important to ensure that runtime error detection is enabled. This testing allows the APIs to report any defects that occur while it is in process.

Security testing

This testing is very important but needs to ensure that proper testing occurs based on risk analysis. Security testing focuses on several aspects of the API testing strategy. The entry points of APIs need to be taken into account along with the data flow and shadow APIs.

Interoperability testing

Working with third-party software providers cannot be taken for granted. It is important to create a section in the testing process that describes the overall testing process. The priority and severity of potential defects need to be discussed based on prior experiences and defines how API testing occurs when the APIs interact with third-party applications.

Fuzz tests

Before validating the application, it is important to fuzz all the endpoints of APIs. In the fuzzing process, the random data is sent to the API endpoints and the results are inspected carefully. The server should not crash from this unexpected traffic and should not display any unexpected behavior.

Validation Testing

The validation testing ensures that the application meets the business requirements. The test execution results need to match the expectations according to the test plan. The testers will subsequently guide the business users in user acceptance testing under which they ensure the stakeholders execute the pre-built testing scenario and report the deviations.

Test cases for API Testing

  • Validation of keys with the minimum and maximum range of APIs.

  • Verification of keys. If we have JSON, XML APIs, it is vital to verify that all the keys are coming.

  • Test cases to conduct schema validation of JSON, XML.

  • Verification of response data.

  • Verification of JSON schema validation, field type, and mandatory fields.

  • Validity of response headers and negative test cases response.

  • Verification of response HTTP status code.

  • Verification of the handling of APIs error codes.

  • Verification of data parameters and APIs.

  • Validation of response payload.

  • Data integrity test cases.

  • Verification of chaining request.

  • End to end CRUD flows.

  • File upload test cases.

Creating API Test scenarios and test matrices

In the scenario testing technique, the individual component tests are assembled into a single sequence. There are two common techniques to obtain the sequence.

  • Review the story of the user to identify the individual API calls.

  • Exercise the user interface to capture the traffic to the underlying APIs.

Scenario tests allow you to understand the occurrence of errors and defects by combing the different data points together. For example, a bank employs a series of services to call the financial profile, credit cards, available bank accounts, and recent transactions. Every API call works well individually, but when they are put together in sequence, they start failing. 

Scenario testing can validate expected behavior when APIs are being used in unexpected ways. To safeguard the application defects, you can create multiple scenario tests with different API combinations. The component tests are the backbone of scenario testing and help in reducing the time spent on the testing process.

  • Check out the most common test cases within the general test scenario groups.
    Basic positive tests (happy paths) – Checks the basic functionality and acceptance criteria of the API.

  • Extended positive testing with optional parameters – The positive tests are further extended to include additional functionalities and optional parameters.

  • Negative testing with valid and invalid input – In this group of tests, the application is expected to handle the problem scenarios gracefully with valid as well as invalid user inputs.

  • Destructive testing – It is a deeper form of negative testing where the robustness of the API is checked by intentionally breaking the API. For example, sending a huge payload body to overflow the system.

Test flows

API testing flows involve multiple input sources like databases, HTML, mobile, data files, Telnet/SSH, RESTful APIs, and more. A test automation platform like QAonCloud allows not only testing APIs manually but also to complete end-to-end API-based test flows, schedule and run tests automatically, get execution reports seamlessly. Let’s understand different types of test flows:

  • Testing requests in isolation – Executing a single API request and checking the response accordingly. Such basic tests are the minimal building blocks we should start with, and there’s no reason to continue testing if these tests fail.

  • Multi-step workflow with several requests – Testing a series of requests which are common user actions, since some requests can rely on other ones. For example, we execute a POST request that creates a resource and returns an auto-generated identifier in its response. We then use this identifier to check if this resource is present in the list of elements received by a GET request. Then we use a PATCH endpoint to update new data, and we again invoke a GET request to validate the new data. Finally, we DELETE that resource and use GET again to verify it no longer exists.

  • Combined API and web UI tests – This is mostly relevant to manual testing, where we want to ensure data integrity and consistency between the UI and API. We execute requests via the API and verify the actions through the web app UI and vice versa. The purpose of these integrity test flows is to ensure that although the resources are affected via different mechanisms the system still maintains expected integrity and consistent flow.

An API example and a test matrix

The below matrix explains the detailed test plan (for manual testing or test automation). The test cases should cover multiple test flows according to the resources, needs, and priorities of the API testing strategy. Let us assume that /users endpoint is a subset of our API, that includes the following API calls.

API Call


GET /users

List all users

GET /users?name=?{username}

Get user by username

GET /users/{id}

Get user by ID

GET /users/{id}/configurations

Get all configurations for user

POST /users/{id}/configurations

Create a new configuration for user

DELETE /users/{id}/configurations/{id}

Delete configuration for user

PATCH /users/{id}/configuration/{id}

Update configuration for user

API Testing Best Practices

  • Create API test cases for maximum possible input combinations of the API.

  • Group the API Test cases by test category.

  • Include the API declarations being called on the top of every test.

  • Prioritize the API function calls to make it easier for testers.

  • The selection of parameters should be mentioned explicitly within the test case.

  • Every test case should be independent and self-contained.

  • Avoid the test chaining process in the development lifecycle.

  • The sequencing of API calls should be well planned and performed.

  • Take special care of handling one-time call functions like Close Window, Delete, and alike.

  • Include the API workflows and involve all stakeholders in the creation of the requirements.

What is API Documentation?

API documentation is a technical content deliverable, that contains the instructions about using and integrating APIs effectively. It is a concise reference manual with all the information required to work with the API such as classes, functions, arguments, return types, functions, and more. The API documentation is made by using regular content creation, text editors, and maintenance tools. While documentation is the basis for a good experience of the developer, it also increases the adoption and usage of the application.

API Documentation Technical Tips

  • Standards – OpenAPI specification standard to help you generate documentation. It is language-agnostic and enables automated code generation for API endpoints.

  • Authentication – Explain in detail the security of credentials. A sample authentication code will be helpful.

  • Error handling – Document the communication of error information by API and ways to fix the problems.

  • HTTP requests and response details – List the status codes, types of content, and caching mechanisms.

  • Examples – Give examples of how the API call requests are made and what will be the subsequent action.

  • Parameters – Provide a clear description of every parameter and its values like formats, types, and rules.

Start Testing Your APIs with QAonCloud Today

Now when you know that API testing strategy is crucial for the overall success of API product lifecycle. Using the right method and tools allows you to test concurrently, making teams more efficient. Always remember that every application or product will have missing interactions, errors, or defects, but organizations need to provide a highly secure and scalable application. 

An effective API testing strategy is the most reliable way to achieve this goal. From protecting the users from malicious code to maximizing efficiency, QAonCloud can transform your overall application experience. We provide the most comprehensiveAPI testing automation  capabilities while ensuring that the APIs perform as intended, meet the desired timeframes and requirements, and team skills. 

Ready to get started with API testing and take the digital world by storm? Get a Free consultation to understand how we can help you take your business to new heights.