28 min read

What is API testing and why is it awesome?

An Application Programming Interface (API) is a tool used to facilitate efficient communication between software components.

Tim Davidson
Tim Davidson

An Application Programming Interface (API) is a tool used to facilitate efficient communication between software components. Think of it as a bridge that connects complex and distributed programs to form an ecosystem of applications that can exchange data to serve the needs of consumers.

API testing entails making several requests to an API endpoint and validating the response to guarantee the API's safety, reliability, and performance.

Usually, API testing is expensive and doesn't have an immediate return on investment. However, it's one of those mundane maintenance tasks that cost money when ignored.

With that out of the way, let's get stuck into unpacking the benefits of conducting API testing:

Benefits of API testing

Early detection of bugs

A typical application comprises three distinct layers: the presentation, business logic, and database. API testing is done on the business logic layer. So, once the logic is designed, you don't have to wait for the entire application to be developed to carry out the test. This allows your development team to find and fix bugs during the early development stages, saving you time and money in the long run - especially when building an enterprise-level application. Besides early detection of bugs, API testing allows you to validate your business logic early enough, contributing to fast product release.

API testing diagram

Effective use of time

API testing takes less time than functional GUI testing. A single UI-level test takes about seven minutes, while an API test usually takes less than fifteen seconds. The GUI test takes longer as it involves loading and rendering pages and UI components. Also, a developer may experience a few setbacks, such as a poor network connection and browser limitations during testing. On the other hand, API testing is quite straightforward as it involves only sending requests to an endpoint and verifying the response.

Language independence

During API testing, data is exchanged in XML and JSON formats. This method is independent of any programming language, thus allowing quality assurance engineers to choose any suitable language that supports the data transfer format.

Broad test coverage

API tests are aimed at ensuring all components of an application work as intended. As such, they have a broader scope than unit tests which are often limited to certain components within an application. Due to their comprehensive coverage, API tests help identify potential bugs and improve the overall quality of an application for improved user experience.

Easier to maintain

Changes in APIs are quite rare, neither are they subject to major releases or recent merges. Their changes are well-controlled and infrequent since they only happen when the business logic layer is updated. As such, API testing is inherently stable, making them easier to maintain than GUI, which is more dynamic due to changes in the UI field regarding browser, devices, and screen orientation upgrades.

Having understood the benefits of API testing, let's have a look at the various tests done on APIs

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    Types of API testing

    There are seven types of API testing:

    Functional testing

    As the name suggests, functional testing entails the assessment of certain functions in a codebase. The goal is to ensure that the API returns the expected output from a given input. It also ensures that errors are handled well if the output is outside the expected parameters.

    For example, let's say a user searches for the price of an equity in a trading app. The API should return the requested equity price if the input is error-free. If there was an error in the input, the user should be served with the appropriate handling instructions or a corrected response that's in line with the request/query. Functional testing works to cater to these scenarios.

    Load and performance testing

    Load testing focuses on checking whether a systems API can handle high loads. To achieve this, the API is tested against two main parameters -- the baseline and theoretical maximum traffic. When testing against the baseline traffic, regular-sized requests are sent to the API to simulate day-to-day usage, and its response time and throughput are evaluated. As for maximum traffic, QA engineers simulate numerous API calls to test if an API can scale to handle the requests and respond


    On the other hand, performance testing is similar to load testing, only that it tests the API against the unregular flow of requests (spiked traffic) instead of a constant steady flow of traffic.

    Security testing

    Security testing is designed to ensure that an API is safe from external threats. It focuses on authorization validation, access control, and validation of data encryption methodologies. Although the test is done independently, it consists of two other components -- penetration and fuzz testing.

    In penetration testing, an API's functions, processes, and resources are attacked by an outsider with little knowledge of how the API works. This helps reveal and assess security threats from a third-party perspective.

    Fuzz testing is the last step in the larger security audit test. Its stress test involves inputting an influx of data to see whether an API will withstand it or result in a forced crash.

    Runtime and error detection testing

    Unlike most API tests that focus on the results and behavior of an API, runtime and error detection test is concerned with the actual running of an API. This type of test is done while running the API is running to allow for error logging in real time.

    The test focuses on four main areas; monitoring, execution errors, resource leaks, and error detection.

    • Monitoring - the runtime of the API code is tested for handling failures and implementation errors to eliminate security threats that may stem from malfunction.
    • Execution errors - involve testing whether the API responds to valid requests in an expected and predictable way.
    • Resource leaks -- involve testing an API's memory, resource, and operational leaks by initiating invalid requests.
    • Error detection -- the system is subjected to known errors to see whether they're detected and handled correctly.

    Validation testing

    This test is done during the final stage of API development to ascertain that the API works as per user requirements. The test checks the following facets of an API:

    • Product -- this refers to the API itself. The goal is to check whether the API works based on product specifications.
    • Behavior -- seeks to determine whether the API is accessing the right data and storing it with respect to data integrity measures.
    • Efficiency -- checks whether the API is the most accurate and efficient way of accomplishing the task.

    UI testing

    It tests the user interface of an API and its constituent components. As such, the test leans more on the developer's experience of using an API's interface and less on the API's functionality. The results from this test give an overview of an API's usability and efficiency from a front and backend perspective.

    Integration and reliability testing

    Integration and reliability tests check the communication between APIs to ensure they're well connected and synced with other modules.

    Manual vs. Automated API testing

    Manual testing requires you to write your own code to test an API. As such, it's prone to human errors and difficult to maintain or update. On the other hand, automated testing involves using a tool such as Postman and Soap UI. This makes it more time and cost-efficient and is often preferred for dynamic and repeated testing designs. Also, automated testing offers more accuracy, broader test coverage, and fast feedback than manual testing.

    Ideally, manual testing is used to confirm whether an API is working, while automated testing is commonly used to test if an API can perform under pressure.

    How to go about API testing

    Here are some of the best practices in API automated testing

    Determine the API testing requirements

    Before starting an API test, you must lay out the testing boundaries and requirements. To determine these requirements, you must fully understand the API's target users, features and functions, and other connected APIs. You also need to specify the areas to be tested, types of tests to run, desired output, and a metric to measure whether the API has passed or failed a test.

    Set up an API test environment

    The best way to set up a test environment is through API virtualization. This process involves creating a virtual copy of your API that will be used for testing in place of your production API. The virtual copy mimics all components of your production API, including its endpoints, eliminating the need to configure a server to mirror a production environment.

    One of the most significant advantages of API virtualization is that it allows early testing. This way, development teams can perform comprehensive API tests frequently even when the production API is still under development. This helps solve bugs and other bottlenecks that would delay time-to-market delivery. Virtualization also saves you money, especially when working with third-party APIs that charge a premium for usage.

    Define the input parameter

    Parameters are resources in memory that store variables. Input parameters, in particular, pass information to an API to request it to perform a specific function with a known outcome. Therefore, these parameters are necessary for testing whether an API works as it should.

    Before testing your API, you should plan all the input parameters you intend to use.

    Choose the right testing tool

    Once you have everything ready, it's time to choose a testing tool for your API. However, the wide variety of testing tools available in the market makes it a bit challenging to choose the right one for your API. A good rule of thumb is to choose a tool that your team will find easier to learn and use. Most importantly, the testing tool should be compatible with the CI systems in your organization. Some popular automated API testing tools include SoupUI, Parasoft, Runscope, and Postman.

    Challenges in API testing

    Deciding the scope of your test

    One of the most frustrating challenges QA engineers face during API testing is deciding what not to test, especially when working with third-party applications. You should prioritize testing parts that apply to your application only to save time and money.

    For example, if you run an e-commerce website, you'll probably use a third-party API to process payments. In such cases, you don't have to run an API test on the payment processor since it's likely that the provider has done all the necessary tests. It would be best if you consolidated your effort on checking whether your app handles errors such as payment denial from the process and presents them correctly.

    Sequencing API calls

    API calls must appear in a specific order to achieve the desired results. However, sequencing these calls is challenging for many testing teams, especially when working with multi-threaded applications. It helps to create a flow chart of API calls to mitigate the sequencing challenge.


    While APIs are designed to facilitate communication between systems, API compatibility isn't guaranteed. Every system has a unique integration standard, making it difficult for some systems to connect with others seamlessly. The only solution to this problem is planning ahead of the API rollout. Take time to evaluate which systems are compatible with yours and which might require more integration time. Be sure to account for revenue losses or gain associated with pursuing the integration process.

    Planning for failure

    When working with third-party systems it's a good idea to prepare for failure. For example, assuming you run an online store and the payment processor fails. You're at risk of losing data and revenue if there's you don't have a failover plan. However, setting up a failover environment during testing is resource intensive in terms of time and technological knowledge.

    Frequently asked questions

    What is API testing?

    API testing is a type of software testing that evaluates the performance, security, and reliability of an application programming interface (API).

    Which language is used for API testing?

    You can use any programming language for API testing, provided it supports JSON or XML data formats.

    Wrapping up

    API testing helps ensure that your API function as expected even when faced with unprecedented errors. This helps improve the user experience for your customers. If you have questions about API testing, please leave them in the comments section below, and our team will respond in a day or two.

    Written by
    Tim Davidson

    Tim Davidson

    Tim is the face of the company. When you want to kick off a new project, or an update on your existing project, Tim is your man!

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