API Testing – The Most Comprehensive Guide Ever

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APIs are becoming increasingly crucial in today’s society, and as technology advances, so will our dependence on APIs. Everything you communicate on the internet these days transmits using an API (Application Programming Interface).

When integrating them into our technologies, we must also consider API testing. Because, if we think about it, our APIs, like anything else, need to be rigorously evaluated on a variety of quality attributes.

We should not only pay strict attention to functional requirements, but we should also pay close attention to non-functional requirements.

What is API?

API stands for Application Programming Interface. It is a computer interface that allows two separate software systems to connect and share data.

For example, a software system with an API has many functions that another software system can perform.

An API explains the types of requests that can be made, how to make that request, the data formats that we need to use, and so on between two software systems.

In short, secure.

In place of using standard user inputs and outputs, API Testing employs software to send calls to the API, receive production, and record the system’s response.

API tests are distinct from GUI tests, as API testing does not focus on an application’s appearance and feel. Instead, it focuses primarily on the software architecture’s business logic layer.

Read also: Software Testing and AI round up.

Importance or Advantages of API testing?

#01 Language Ensuredence

Because data is exchanged using XML and JSON, any language can be used for automation, regardless of the language used to develop the application.

Because XML and JSON are typically structured data, verification is quick and stable. There are also built-in libraries to help with data comparison using these data formats.

#02 GUI Independent

Accessing the application without a user interface or interacting with the system is a significant advantage of API testing. In other words, QA testers can execute API tests without prior knowledge of the software application.

This is a substantial benefit because it gives QA engineers early visibility into faults and errors, allowing developers to solve issues before harming the GUI.

#03 Improves Test Coverage

Because most APIs and services have specifications, we can create automated tests with high coverage, including functional (happy cases) and non-functional testing.

#04 Enables Faster Releases

It is common for a UI regression test suite to take 8-10 hours to execute. In contrast, the same scenario with API testing takes only 1 to 2 hours. API testing also has lower flakiness than UI testing. All of this enables you to release API testing more quickly.

#05 Reduce Testing Cost

API testing allows us to find bugs and defects early, and whenever you detect a fault at an early stage, you save money and time.

#06 Easy Integration With GUI

API testing enables highly integrable tests. This is more useful if you intend to run functional GUI tests after your API testing. Easy integration, for example, would allow introducing new users within the programme before starting a GUI test.

#07 Test Core Functionality

Before executing GUI tests, testing an application’s code-level functionality provides an early assessment of its overall build strength. This helps expose minor flaws during GUI testing that can grow and become larger ones.

Core access allows testing to run concurrently with development, promoting communication and collaboration between the two teams. This is especially useful if an offshore QA team does your API testing.

#08 API Testing Enables Automation

There are numerous toolkits and frameworks available that we can use to automate API requests, allowing you to build an API testing automation suite.

For example, in the age of CI/CD, that suite might be hooked when performing deployments or integrations, saving the human labour of testing the entire set of APIs that work behind the application.

Types of API Testing:

Let’s look at the different types of API testings for a better understanding:

#01 Validation Testing

Validation testing is one of the final stages of the development process, yet it is among the most crucial tests that we need to perform.

We often perform validation testing at the very end of the primary development process, specifically after the constituent elements and functionality of the API have been verified. While many other tests focus on specific codebases or functionalities, validation testing is much more considered.

Validation testing is just a series of simple queries we apply to the entire product. Among these are the following:

Product: Did we create the right product? Is the API itself the right product for the problem at hand, and has there been any substantial code bloat or feature creep that has pushed an otherwise lean and focused implementation into an unacceptable direction?

Behaviour: Is the API accessing the correct data the way it is supposed to? Given the dataset’s confidentiality and integrity requirements, is the API obtaining too much information and storing it correctly?

Efficiency: Is the API the most precise, optimal, and efficient way of accomplishing the task at hand? Is it possible to remove or modify any codebase to improve the overall service?

These questions serve to validate the API as a comprehensive solution.

They are performed after the API has been developed against an established set of criteria to ensure correct environment integration, adherence to standards, and delivery of specific end goals and results.

Finally, this test can simply be described as a guarantee of accurate development about the specified user demands and criteria.

#02 UI Testing

While validation and functional testing methodologies are relatively universal, UI testing is more particular. UI testing is precisely what it sounds like: a test of the user interface for your API and its components.

This test is concerned with the functionality of the UI, regardless of whether it is graphical or relies on command-line endpoint calls.

In many respects, this is less of a test of the API and more of an examination of the interface that connects to the API and the developer experience of using that interface.

However, though not a direct codebase test of the API, this provides a highly generic perspective of the usability and efficiency of both the front-end and the back-end.

This is why we employ UI testing frequently in place of functional testing – in many ways, it performs the same job, albeit less comprehensively and more generically.

However, this is a terrible technique in modern testing, and UI testing should be completely limited to validating that the UI itself operates as intended.

Web UI testing is a part of this type of test that focuses on end-to-end integrations between web instances and the APIs they represent. Though online UI testing is a unique subset of UI testing, it is worth mentioning and including in this category.

#03 Functional Testing

Functional testing is a broad testing approach, but it is narrower than validation testing. Functional testing is merely a test of certain codebase functions.

These functions, in turn, describe specific situations to guarantee that the API functions within expected parameters and check that errors are handled appropriately when results are outside of desired parameters.

A scenario makes functional testing much easier to convey. For example, assume our API handles music ordering through an internet gateway.

When a person looks for a song, they look for it by Track Name and Artist Name. Functional testing employs a layered approach and addresses a few specific cases in this situation.

First, we validate API’s functions with appropriate inputs. The API validates the request and returns the anticipated results.

We should expect a few stated responses due to the test structure. However, we should anticipate either an error or a repaired response containing the requested material.

Functional testing should address all of these concerns – not only should the standard test case be included, but real and edge-case scenarios also should be included in the testing routine.

#04 Load Testing

Load testing is a test fascinated with realism – it deliberately avoids the theoretical and errors on the practical.

We often perform this test after completing a specific unit or the entire codebase to determine whether the theoretical solution works as a practical solution under a certain load.

Load testing simulates a variety of scenarios to assure peak performance. The first of these scenarios is termed as “baseline,” It compares the API to the typical theoretical traffic that the API anticipates in normal, day-to-day usage.

This comprises regular-sized tests interspersed with some incredibly considerable requests to see any difference between the two request types in practice.

Typically, we perform a second load test with the maximum traffic. This is done to ensure that, even at peak loads, procedures for properly throttling requests are in place. While the API may never reach this theoretical maximum, it is good to ensure that.

Finally, we perform an overload test which involves testing to the theoretical maximum and adding 10–20 per cent more traffic on top.

While this type of testing almost guarantees a failure as it is a test of the API function as well as the error code generation, handling and integrate into the API, thus it nearly becomes a hybrid test

#05 Security testing

This practice ensures that the API implementation is safe from outside attacks. Additional steps in security testing include validation of encryption technologies and the architecture of the API access control. It also manages user privileges and validates authorisations.

#06 Penetration Testing

The second test in the auditing process is penetration testing. Users with minimal API understanding will attempt to analyse the threat vector from the outside, which is about functions, resources, processes, or the whole API and its components.

#07 Fuzz Testing

Another phase in the security auditing process is fuzz testing. A large amount of random data (referred to as “noise” or “fuzz”) enters the system during fuzz testing to detect any crashes or bad behaviours. This technique puts the API to the test to prepare for “worst-case scenarios.”

#08 Performance Testing

We are attempting to shift the Load testing paradigm to the left with each commit. Previously, load testing was restricted to a few people and was difficult to implement in a CI/CD setup. Ready API is a performance testing solution for RESTful, SOAP, and other web services that allows almost any team member to integrate performance testing into their CI/CD workflow.

#09 End to End testing

We use End-to-end testing in the software development lifecycle (SDLC) to evaluate an application’s functionality and performance under product-like conditions and data to simulate live settings.

The purpose is to recreate a real-world user scenario from start to finish. This testing aims to validate the system under test and assure that its subsystems function and act as predicted.

#10 Component Testing

Component testing is a sort of software testing in which we check each component separately without integration with other elements.

When we consider it from an architectural standpoint, it is also known as Module Testing. Unit testing, programme testing, and module testing describe component testing.

#11 Scenario Tests

Scenario testing is the software testing technique that employs scenarios, or speculative stories, to assist the tester in working through a complex problem or test system. The ideal scenario test is a dependable, challenging, convincing, or motivating story with a clear outcome.

Typically, these tests differ from test cases in that test cases are single stages, whereas scenarios span multiple processes.

Scenario testing guarantees that the software’s end-to-end functionality and process flows are working effectively. In scenario testing, testers pretend to be end-users and look for real-world scenarios or use cases that the end-user can perform on the product.

Scenario testing involves testers working with clients, stakeholders, and developers to design test scenarios.

#12 Omni-channel Testing

Omni-Channel Testing tests an application or product across several devices and platforms. The goal is to guarantee that the user experience is uniform and seamless across all platforms.

This guarantee of a seamless user experience is an essential component of the broader subject of Digital Assurance.

Read also: The Right Automation Tool for Your Software QA

Where is iPerformed?

Now that we have a clear overview of what is API testing and the different classifications attached. So, let us look at where this can be performed. Typically, software or application has three vital layers, namely, the user interface (UI) layer, the database layer, and the business layer. Of these, the business layer is unequivocally the most crucial layer. Through API testings, you check the logic processing in the business layer, covering all the undertakings between the UI and database layers in the application.

Pre-requisites of API Testing

API testing comes with a set of preconditions for optimum performance and the best results. Let us quickly glance at the checklist now.

  • Evaluation of the team’s proficiency – Check how well-versed your team is when it comes to this testing and the related architecture. You should understand how well they comprehend the API and automation tools involved. Consider involving an external QA team in case your in-house team falls short of knowledge in the specific areas.
  • Setting up an API Testing environment – Configure the server and database to fit the testing requirements. 
  • Define the test plan – API testing needs a predefined test plan. It helps to get a clear vision of what should be tested and what outcome should be expected.
  • Choose the right API and automation tools – Selecting the testing tools is very important when it comes to testing. Choose the most suitable API tools, preferably automation tools, for a faster and more accurate testing experience.
  • Strategize the bug tracking and execution process – Create a well-defined strategy to execute the testing, detection, monitoring, reporting, and fixing the errors in the API framework.
  • Stipulate the API output status – Define the output results you are looking for as an outcome of the API test. This helps to verify the end results with the same.

Test Cases for API Testing:

  1. Validate the keys using the Min. and Max. API ranges (e.g. maximum and minimum length). If we have JSON or XML APIs, we should double-check that all keys arrive.
  2. Create a test case to validate XML and JSON Schema.
  3. Check the data that has been parsed from the response.
  4. Check the JSON Schema validity, check the Field Type and the Mandatory Fields.
  5. Negative Test Cases Response & Valid Response Headers.
  6. We need to check how to handle the API error codes
  7. You should check the HTTP status code of the response.
  8. The payload for a valid response.
  9. Request verification in a chain.
  10. API validation using data parameters.
  11. CRUD flows from beginning to end.
  12. Test Cases for Database Integrity.
  13. Test Cases for File Upload.

How To Use Postman For API Testing?

  1. New: To make a new request, collection, or environment.
  2. Import: This is used to bring in a collection or an environment. Other possibilities include importing a file, folder, link, or pasting a raw test.
  3. Runner: We can use Collection Runner to run automated tests.
  4. New: We can open the Postman or Runner windows.
  5. My Workspace: This is your workspace. You can use it to create a new workspace. A workspace is a collaborative environment for developing and consuming APIs. It enables real-time collaboration within and between teams.
  6. Invite: This is where you invite your team members to work together on a workspace.
  7. History: Automatically saves requests and responses in history, allowing you to monitor your previous actions readily.
  8. Collections: Its purpose is to arrange and track similar requests.
  9. Request tab: The title of the request you are working on, entitled ‘Untitled Request’ by default.
  10. HTTP Request: You may see HTTP requests such as GET, POST, COPY, DELETE, and so on.
  11. Request URL: The URL with which the API will communicate; also referred to as an endpoint.
  12. Save: To save the requestor to change an existing one.
  13. Params: These are the parameters (critical values) required for a request.
  14. Authorization: APIs employ authorisation to ensure that client requests gain secure access to data. This section discusses authorisation details such as login, password, bearer token, and essential authorisation.
  15. Headers: Some APIs require you to submit specific headers, such as JSON, JavaScript, and so on, along with requests, to offer more metadata about the operation you are conducting. These can be established up here.
  16. Body: This field allows you to specify the data sent with a request. To fit your API, you can transmit multiple sorts of body data.
  17. Pre-request Script: We need to execute pre-request scripts written in JavaScript before receiving the request. This is ideal for adding a timestamp in request headers or delivering a random alphanumeric string in URL parameters.
  18. Tests: Tests are scripts that are run during the request. Tests assist you to confirm that your API is performing as expected, that interfaces between services are reliable, and that new advancements have not disrupted any current functionality.

Best Practises of API Testing:

  • We need to organise API Test cases into test categories.
  • Include the declarations of the APIs called at the top of each test.
  • The parameters should be given correctly in the test case.
  • Prioritise API function calls so that testers can quickly test them.
  • A test case should be as self-contained and dependant-free as feasible.
  • In your development, avoid “test chaining.”
  • We must give special attention when dealing with one-time call functions such as Delete, CloseWindow, and so on.
  • You must perform call sequencing and plan meticulously
  • Create API test cases for all conceivable input combinations of the API to ensure thorough test coverage.

Challenges of API Testing:

Every type of software testing comes with its own share of challenges, and API testing no exception. Here are a few common challenges to note:

  • Setting up API testing – The prepping of this framework is of utmost importance. Often the first knot coming in the way of the testers and developers. You can overcome this by taking time to understand the prerequisites (given above for the benefit of the readers) and designing the framework accordingly.
  • Updating the schema of API testing – Update data formatting or the schema frequently. It helps to maintain relativity throughout the testing process. The challenge is that with every iteration in the program, additional parameters come into play.
  • Testing the parameters – It is necessary to test all the related parameters passing through the database and data requests. It is done across all possible combinations and specific configurations. 
  • Validating the parameters – Validating a number of parameters is a daunting task. Because it requires verifying each numerical data and sequence.
  • Sequencing API calls – API calls often need to follow a specific route or sequence. Creating the sequence map can be a hurdle to the testers and developers.
  • Tracking system integrations – Keeping a track of all the system integrations is a tedious task. It can often be intimidating to the team.

Types of Bugs that API testing detects

  • Unused flags.
  • It do not handle error circumstances are not gracefully.
  • Some functionality is missing or duplicated.
  • Concerns About Security.
  • Problems with multi-threading.
  • Incorrect errors/warnings to a caller.
  • Mishandling of valid argument values.
  • Problems with performance The API response time is high-speed.
  • It do not organise the response data is not correctly (JSON or XML). Concerns about dependability. Difficulty connecting to and receiving a response from the API.

Read also: The Game of Software Development LIFE Cycle

Why TestGrid for API Testing?

#01 Easy to Use

The automation tool offered by TestGrid.io provides an accurate automation tool and framework for developers and testers for API testing, UI testing and other essential authorisation testings.

#02 Cost Savings

TestGrid.io saves your money as you can use the tool instead of hiring an expert, and it is more accurate at the same time, so it provides better ROI and is very easy to use.

#03 Faster Debugging

Run automated tests in a continuous pipeline to provide your team with faster feedback while reducing troubleshooting and resolution time.

#04 Language Support

Don’t waste time learning proprietary languages as the TestGrid.io tool work with your favourite languages like Python, JavaScript, and others right out of the box.

#05 Time-saving

Save your time for the search for specialists, and also, our TestGrid.io tool is 70% faster than the traditional API automation process.

#06 Detailed report

Detailed report of the number of Source Type, Request Data, Response Data, Duration, Speed, Request Size, and Response Size

Features of TestGrid.io API automation tool

  • Easy to Use API Automation for SOAP and REST calls.
  • Deeper Assertions – can write test cases that validate against API calls. Supports MITM.
  • API Integration Automation Testing.
  • Transaction Monitoring.
  • No additional test cases are essential to discuss.
  • Plug n Play real-time UI value assertions with API data fetched.


The API comprises a collection of classes/functions/procedures that form the business logic layer. If we do not test API adequately, it may cause issues in the API application and the calling application. It is a must-have test in software testing.

As you read in the blog API Test Automation is a very tedious process and hiring someone will take a good amount of time and money and might not provide you best ROI

So going for automation tools like TestGrid.io which will automate this whole process is advisable