API Testing 101

API Testing 101

For many software developers, API testing evokes Many software developers find API tests evoking visions of endless trial and error - trying every possible combination until a flaw is found. But the truth is that API testing can be simple.

By taking a strategic approach and finding ways to automate the process, you can streamline your testing and ultimately create better products. So, where do you start?

Application Programming Interface; in simple terms, it's a way for different programs to talk to each other and exchange information. That's where API testing comes in. It involves checking that these exchanges are happening properly and accurately.

Of course, this can get a bit complicated- some protocols and formats need to be considered, not to mention different layers and types of API testing. But with the right approach and tools, API testing can lead to smooth and successful software development projects.

So, you must not write off API testing as merely another acronym: make it part of your software testing strategy for optimal results. This ultimate guide will walk you through everything you need about API testing and how best to implement it.

What is API testing, and what are its benefits?

Many believe that testing an API is just sending some input and checking the output. However, effective API testing requires a deeper understanding of the functionality and potential vulnerabilities of the system as a whole.

This involves conducting standard functional tests and considering security threats such as SQL injection or cross-site scripting attacks. In addition, performance and reliability must also be considered as an API that frequently crashes or has long response times can greatly impact the user experience for any application.

While it may initially seem a simple task, truly comprehensive API testing requires finesse and a unique understanding of the specific technology being tested and its place in the larger system.

Using specific methods and tools, testers can perform various tests on the API, such as functional testing to make sure it performs its intended task, performance tests to check for any lags or crashes, and security tests to ensure sensitive data is being properly protected.

So next time you encounter an error while using technology, don't blame the app itself - it could just be a case of faulty APIs that need serious testing.

Here are just a few of the benefits that come with effective API testing:

API testing can help to ensure that an application is functioning correctly

An API comes with a set of rules and protocols that allow software programs to communicate with each other. API testing can ensure that an application is functioning correctly by testing the functionality of the API.

API testing can help to ensure that an application is secure

API testing helps the developers check if the security measures ensure that unauthorized users cannot access sensitive data. Additionally, API testing can help to prevent denial-of-service attacks, which are a type of attack in which a malicious user attempts to overload a system by making numerous requests.

API testing can be used to test applications written in any programming language

API testing is not limited to applications written in a particular programming language. Any application that has an API can be tested using API testing tools. This makes API testing a useful approach for a wide range of applications.

API testing does not require a GUI

API testing does not require a graphical user interface (GUI). This makes it well-suited for testing server-side applications or web services. It can be easier and faster to test an application using an API than it would be using a GUI.

API testing can be performed manually or automated

API testing can be performed manually or automated. Automated tests can be run faster and with less effort than manual tests, making them well-suited for regression testing.

A wide variety of tools are available for performing API tests.

These tools vary in terms of features and price, so it is important to choose one that best suits the needs of your project. That means being aware of the capabilities, limitations, and pricing of various tools before making a choice as well as investing in it.

API Terminology

Did you know that a "wrapper" in API terminology has nothing to do with a sheet of paper or plastic? A wrapper, in this context, refers to a programming language that allows an API written in another language to be accessed and used by a different program.

And speaking of languages, "endpoint" doesn't refer to a destination but rather a specific URL that can be requested for data. We've also all heard of an "interface," but in the world of APIs, it refers to the methods and guidelines for accessing and utilizing the API.

So here are just a few of the many terms you should be familiar with if you want to start doing API testing:

SDK: A software development kit that provides tools and libraries for developers to build applications on top of an API.

REST: A type of API that uses simple HTTP requests to retrieve data from a server.

SOAP: A type of API that uses XML-based messages to communicate between two pieces of software.

WSDL: The Web Services Description Language, a standard used to describe the functionality of a SOAP-based web service.

UDDI: The Universal Description, Discovery, and Integration standard, used to discover and locate web services.

JSON: JavaScript Object Notation, a lightweight data-interchange format often used with RESTful APIs.

XML: Extensible Markup Language, a markup language used to encode data in a structured way.

OAuth: An authorization framework that allows users to grant third-party applications access to their data without sharing their username and password.

Different types of API

Regarding APIs, there's more than what meets the eye. While most people are familiar with web APIs that allow for communication between websites, operating system APIs also allow a program to interact with the underlying operating system.

And beyond those, there are hardware APIs that allow a program to interact with connected hardware devices such as printers or scanners. Then there's a language-specific API, which provides functionality specific to a certain programming language.

Here are some of the different types of APIs you might encounter during your work as an API tester:


SOAP (Simple Object Access Protocol) is a standards-based web services access protocol that has been around for a long time. SOAP allows applications to communicate with each other over the internet in a platform-independent way.


REST (Representational State Transfer) is an increasingly popular web services access protocol designed to be lightweight and easy to use. REST is often used in mobile applications as it can reduce data usage and improve performance.


GraphQL is a data query language developed by Facebook and used for creating APIs. Unlike REST, GraphQL provides only one endpoint for data retrieval, reducing the number of API requests. This can improve performance as less time is spent fetching data.


Hypermedia APIs are designed to be self-documenting, including links to other resources and relations between data. This makes it easier for developers to create applications that can navigate through an API without prior knowledge of its structure, improving scalability and reducing the need for documentation.


XML-RPC (eXtensible Markup Language Remote Procedure Call) is a simple yet powerful web services access protocol that uses XML to encode data. XML-RPC is platform-independent and can be used with a variety of programming languages.


JSON-RPC (JavaScript Object Notation Remote Procedure Call) is a newer web services access protocol that uses JSON to encode data. JSON-RPC is also platform-independent and can be used with various programming languages.


WSDL (Web Services Description Language) is an XML-based language used to describe a web service's functionality. Web service providers typically publish WSDL files so potential consumers can learn about the service's capabilities.


UDDI (Universal Description, Discovery, and Integration) is a directory service for web services that allows businesses to find and connect online. UDDI directories are typically maintained by third-party organizations such as government agencies or industry consortia.


WSIL (Web Services Inspection Language) is an XML-based language that allows users to inspect the contents of a web service's WSDL file. WSIL files can be used to determine the capabilities of a web service without actually consuming the service itself.


BPEL (Business Process Execution Language) is an XML-based language that allows developers to orchestrate the steps involved in complex business processes. BPEL processes can span multiple web services and can be executed on various platforms.

.NET Remoting

Microsoft's .NET Remoting technology allows objects created in one programming language to be used in another. This makes it possible for developers to create distributed applications that span multiple platforms and programming languages.

How do you go about performing API tests effectively?

Understand the API

The first step to performing effective API tests is to understand the API. This means knowing how the API works, what it does, and what it is designed to do. With this understanding, it will be easy to know what to test and how to test it.

Choose the Right Testing Tool

There are various testing tools available, so it is important to choose the right one for your needs. Some factors to consider include the type of API you are testing, the language the API is written in, and the operating system you are using.

Set Up a Testing Environment

Once you have chosen a testing tool, you must set up a testing environment. This environment should be separate from your development or production environment so that you can safely make changes without affecting live data.

Write Test Cases

After setting up your testing environment, you will need to write test cases. Test cases should be specific and cover all aspects of the API you want to test. They should also be written in a way that makes them easy to understand and follow.

Run Tests

Once you have written your test cases, you can run them against the API. This will help you find any errors or bugs in the API so they can be fixed before going live.

Analyze Results

After running your tests, you will need to analyze the results in order to determine whether or not the API is functioning correctly. This analysis should include looking at successful and failed tests to identify any areas that need improvement.

Fix Errors and Bugs

If any errors or bugs are found during testing, they will need to be fixed before the API can go live. This may involve changing code or adding new features to the API.

Retest After Fixes Are Made

Once all errors and bugs have been fixed, the API should be retested to ensure it functions correctly. This may require running all of the original test cases again or creating new ones specifically for the changed functionality.

Go Live!

And, of course, the API is ready to go live after successfully testing and fixing any errors or bugs! This means that real users in production environments can use it. Congratulations!

Common Issues During API Testing and How To Handle Them

API testing can present a unique set of issues due to the inherent complexity of the platform. Some of the most common challenges that occur during API testing include the following:

Incorrect URL

An incorrect URL is one of the most common issues that can occur during API testing. This can happen for some reasons, such as a typo in the URL or a change in the API endpoint. If this happens, you should first check the documentation to see if you have the correct URL. If not, make the necessary corrections and try again.

Authentication Issues

This can happen if you are using the wrong credentials or if your credentials have expired. You will need to contact the API provider to resolve the issue if this happens.

Rate Limiting

Some APIs may have rate limits in place, meaning you can only make a certain number of requests within a certain time period. If you exceed the rate limit, you will receive an error message. To avoid this, you will need to check the documentation to see if there are any rate limits and ensure that you stay within them.

Invalid Data

This can happen for many reasons, such as incorrect data types or missing required fields. If this happens, you will need to check the documentation to see the requirements for each field and ensure that you are providing valid data.

Missing Data

In some cases, data may need to be included in responses. This can happen for several reasons, such as an error on the server side or incomplete data being returned from the API. If this happens, you will need to check the documentation to see what data should be included in responses and contact the API provider if it needs to be returned correctly.

Unexpected Data

Unexpected data returns can happen for many reasons, such as new fields being added to responses without notice. If it does occur, you will first need to check the documentation for changes that might have been made. From there, update your tests accordingly.

Security Issues

In addition to the technical issues that can occur during API testing, security concerns may be related to the API. This could include unauthorized access, data breaches, or malicious activity. If you encounter any security testing issues while testing an API, you should notify the API provider immediately and follow their protocols for reporting these issues.

By being aware of the most common issues that can occur during API testing, you can take measures to address them and ensure a successful testing process.

How do you ensure that your API testing process is efficient and reliable?

Being efficient and reliable in your testing process will require careful planning and forethought. Some key strategies that can help to ensure efficient and reliable API testing include:

Use automated testing tools wherever possible.

Automated tests can be run more quickly and with less effort than manual tests, which means that they can be run more often. This helps ensure that your API always works as expected and that any new changes do not break existing functionality.

Create comprehensive test cases.

To ensure that your API testing process is efficient and reliable, it is important to create comprehensive test API cases that cover all essential use cases. This can help you promptly identify issues and bugs and prevent them from affecting users in production environments.

Make use of unit tests.

This can help you test smaller API components, such as individual functions or methods. This can be particularly useful when making changes to the codebase, as it allows you to identify and fix any issues that may arise quickly.

Run tests frequently.

It is essential to run your API tests regularly, as this can help you to identify and resolve problems quickly. This may mean running automated tests on a daily or weekly basis or manually testing the API after each code change or new release.

Best Practices For API Testing

To optimize API testing processes, it's important to first have a well-defined test strategy in place. This should outline the test's specific goals and desired outcomes as well as any constraints or limitations.

It's also helpful to prioritize which APIs should be tested first based on factors such as the business impact and level of complexity.

With these best practices in mind, you can focus on building a robust and efficient testing APIs process that will ensure your API's high quality and reliability.

Define your test cases upfront.

One of the key elements to optimizing your API testing process is having a clear plan and set of test cases in place. This will help you define your goals, identify potential issues, and determine what steps are needed to optimize the process moving forward.

Consider using a mocking tool.

With API testing, it's crucial to have access to API mock data that can be used to test the functionality of your API without actually calling an external API endpoint. This can help you identify and resolve any issues more quickly and allow you to run tests in various scenarios and environments.

Make use of real-time log analysis.

Another important way to optimize your API testing process is using tools that provide real-time log analysis. This can help you to identify any issues that may occur and quickly determine the source of the problem and whether it requires immediate attention. Ultimately, this can save you time and ensure a more reliable API testing process overall.

Perform load testing.

And for an extra layer of assurance, it's also important to perform load testing on your API. This will help you to identify any potential bottlenecks or areas that may not be able to handle a high volume of requests, which can help you to proactively address these issues and improve the overall performance of the API.

Preflight offers fast, accurate, and reliable API testing.

Are you tired of manually testing every API and struggling to keep up with the changing technologies? Look no further than Preflight, the best solution for all your API automation testing needs.

This advanced tool utilizes AI-based technologies like context awareness to make your testing process more efficient and effective. Preflight offers cross-browser and cross-platform support, making it easy to run tests on various devices. Creating, running, and managing tests is a breeze with its user-friendly drag & drop interface.

And if that wasn't enough, Preflight also has auto-recording and playback capabilities, as well as integrations with popular CI/CD tools. Say goodbye to manual testing headaches and hello to Preflight's unique solution.