API Testing Tutorial

In our daily life, we use API a lot. For example, when we use the Facebook application, chat with friends or family via messages, or check for weather using mobile phones, we use API. In such circumstances, the particular application you are using connects to the internet and transmits data to the server. Upon receiving data, the server analyzes and interprets the data and carries out necessary actions. Later, it shares the data back to your mobile phone. And the application that you were using fetches the data from received by the server, converts it into the desired format, and provides you in an understandable form.

The entire process works through an Application Program Interface (API). But, you might like to know what exactly API means? In this article, we will learn a complete guide about API testing and other parameters, like

What is API?

API stands for Application Program Interface. An application program interface is a platform that interconnects or carries out communication between two or more software systems. Whenever the user copies any application from one system and pastes it on the other system, it uses API customs that function between the two systems. There are three primary elements of API as follows:

  1. Procedures: It implies the way in which any program or application performs a particular function. 
  2. Protocols: This API element means the format or arrangement used to communicate two more software systems. 
  3. Tools: Several tools are used for creating new programs. 

Let us clarify what exactly the API does from one example. 

The best example for elaborating on the meaning of API is the restaurant and waiter. You go to the restaurant and choose a table to sit on. You get a list of menus available. Additionally, a kitchen in the restaurant acts as a system to provide you with your desired food. 

However, the primary issue is how to connect to the kitchen, i.e., the system for placing your order. The waiter acts as an interface or intermediary between you and the kitchen. So, the waiter is the Application Program Interface in this example. The waiter controls all information between you and the kitchen. The waiter conveys your order to the kitchen, and the response you get is the food. 

Examples of API 

We have made a clear definition of the API using the example of a restaurant and waiter. In this section, we shall dive into the API examples in terms of technologies that we employ in daily life. 

  1. Many times, we use Google Maps to find a particular unknown location. Google Maps API enables developers to use JavaScript interface to incorporate geo-location knowledge on webpages. 
  2. Another example of API is Twitter API. It incorporates two different APIs. One API is utilized for content or information, and the other is meant for advertising. The first API regulates activities on Twitter, like direct messages, Twitter users, etc. And the second API monitors advertisement, campaigns, creative content, etc. 
  3. One of the commonly used examples of API is the YouTube API. It includes multiple APIs, like YouTube Data API, YouTube player API, YouTube Analytics API, etc. Many websites embed the YouTube video into their web pages for audiences to get a clear idea about the content.
  4. Amazon Product Advertising API is another example of API. Like many websites embed YouTube videos, some of them also include the Amazon product for advertisement purposes. They embed the link of the products from Amazon’s website. 

Need for API

We have seen some famous examples of API used commonly in daily life. But, what is the need for API? Why do we need API? Is it necessary to use an API? 

One of the significant purposes of using API is sharing a massive amount of data with several people instantly. Today, many government offices use the Application Program interface to share essential data with residents and other government bodies. 

Another purpose of using the Application Program, Interface (API) is the security. It allows one program of the system to connect with any other programs without knowing the entire code. When you access the server’s data, your device’s complete data is not exposed to the server. Only the required information is transmitted sequentially to the server in the form of small data packets. It ensures high-level security. 

Many renowned companies, like Google, Amazon, etc. offer their APIs and make money through it. Earlier, we have seen API examples of API, like YouTube API, Google Map API, etc. Each of the applications has its own API. 

Types of API

There are majorly four types of APIs. They are Open APIs, Partner APIs, Internal APIs, and Composite APIs. We shall discuss each of these APIs in brief.

  1. Open APIs or Public APIs: Open APIs or Public APIs are available to all users and developers with fewer limits. Generally, they are free to use. But sometimes, you need to register or use the API key. For example, Oauth APIs from Google is an open API.
  2. Partner APIs: Partner APIs are not available openly. Everyone does not use them as it requires access rights for using it. Partner APIs are utilized by several businesses to communicate with each other. 
  3. Internal APIs: Internal APIs are not commonly used APIs by people. They are mostly used in specific companies and industries. They use the internal API to improve the development process. The development team of a particular project can access any service of another project using API. 
  4. Composite APIs: The developer generally utilizes composite APIs for developing a microservice architecture that requires collecting a vast amount of data from multiple resources. 
API Testing

What is API Testing? 

API Testing is one of the kinds of software testing that verifies whether a particular Application Program Interface (API) meets all business or general requirements, includes all functionalities, ensures high performance and reliability, and security. APIs are used to ensure effective communication between the client and the server. The primary objective of API testing is to check whether a specific API carries out communication or responds correctly to the user.

API testing is best suitable for test automation or continuous testing. It tests the functionalities and back-end of any multi-tier architecture. Also, the release cycles are made short, and for every release cycle, the feedback is provided. So, many companies today have increased the rate of API testing, instead of GUI testing. The primary focus of API is on the business logic layer of the software architecture. 

Any application that we use has three different layers. The first layer is a data layer, the second is the service layer or application program interface layer (API), and the last is the presentation layer. The service layer defines the logic of the software application, guidelines, or validations about users’ interaction with the application’s services, and its functionalities. Most of the testings focus on the presentation layer, but the API testing is specially meant for testing the service layer. 

Types of API Testing

API testing incorporates a multiple set of testings as listed below:

  1. Unit Testing: This type of testing generally tests each function of every single operation of the application separately. Software developers usually carry out unit testing. You can refer to a unit as a particular part of the application. 
  2. Functional Testing: Another type of testing in API is functional testing. As its name suggests, it tests a variety of software application’s functions by designing test cases. It is generally black-box testing, where software developers or testers are unable to determine what is inside the system. It also includes regression testing. 
  3. Load Testing: This type of testing verifies whether the system responds correctly despite multiple users accessing the system. The system’s function should execute adequately when accessed by multiple users. 
  4. Runtime Error Detection: This testing is a software verification method that determines any bug or defect while the software is being executed. Several bugs can be reported during the software product’s execution, like race conditions, resource leaks, null pointers, uninitialized memory, etc. 
  5. Security Testing: This kind of testing involves ensuring data protection from unauthorized users. It also provides data integrity, confidentiality, authenticity, and non-repudiation, which are the primary objective of network security. 
  6. Penetration Testing: When the system undergoes penetration testing, testers can identify vulnerable cyberattacks on the network.  This testing finds weaknesses of the particular software application or website.
  7. Fuzzy Testing: Fuzzy Testing is performed to verify whether the particular software product takes structured and organized input. If the system accepts unstructured input, it may result in system crashes, memory leaks, etc. 
  8. Web UI testing: As its name indicates, this type of testing checks the functionality of every component of the user interface. 
Types of API Testing

Difference Between Application Program Interface and Graphical User Interface Testing

The Graphical User Interface Testing and the Application Program Interface Testing are not the same. API testing does not focus on the appearance of the software product; instead, it focuses on the parameters, like performance, stability, reliability, and security. Let us discuss the difference between GUI testing and API testing. 

Application Program Interface (API) TestingGraphical User Interface (GUI) Testing
The team of Quality Assurance (QA) performs API testing.Software developers perform GUI testing.
It usually involves black-box testing.It includes white-box testing. 
API testing is mostly performed to check the functionality, performance, reliability, and security of the software product. Unit testing does not involve the execution of the functional code. Preferably, it verifies the look of the software product. 
It covers software product’s all functional issues. Only limited and fundamental functionalities are testing. 
The Quality Assurance team executes API testing after the entire build is ready. Software developers perform GUI testing before the build of the product gets started. 
API Testing

How to Perform API Testing? – A Quick API Testing Tutorial

We have discussed API Testing, its types, and the difference between AI and GUI testing. Now, we shall dive into our core topic, API testing tutorial. 

Setup Requirement for API Testing

Before you carry out API testing, there are particular prerequisites for setup. They are as follows:

  1. As API testing is more complicated and in-depth than GUI testing, an initial environment setup is required to perform API testing to verify several parameters, like performance, functionalities, reliability, security, etc. Once the testing is finished, it produces test results.
  2. Another requirement for API testing is to configure the database and server to be compatible with the software product. 
  3. After installing your software on a specific system, a call to API testing is given to perform API testing. 

Test Cases for API Testing

Test cases are essential to verify the behavior of the system under certain circumstances. It must behave consistently with all kinds of inputs. The Quality Assurance team needs to consider all possible test cases to be performed on the system. This team design test cases depending upon the various parameters as listed below:

  1. The first parameter is the return value for a particular input. For a specific input type, the system should generate correct results. 
  2. Another parameter to consider while generating test cases is not returning anything. What if the system does not produce any value for a particular input. In such situations, testers test the behavior of the system. 
  3. If the particular input’s outcome triggers other functions or events, the system should trace those events. 
  4. Your test case should include a test based on the database. If any function of the system updates the data present in the database, it should be verified. 

How to execute API Testing ?

API Testing involves multiple phases. Here are some API Testing stages mentioned. The other steps included are the same as the SDLC stages. 

  1. Usability Testing: This phase of API testing performs usability testing. It verifies whether the API of a specific software product is easy to use or user-friendly. Additionally, usability testing also ensures whether the API can work hassle-free with other platforms. 
  2. Security Testing: The security of every software product is a crucial element. Another phase in API testing is security testing. It ensures the authentication types. Additionally, it checks whether the data is sent by the authenticated person and is encrypted over HTTP. 
  3. Automated Testing: API testing also involves an automated testing phase, where API testing should produce accurate data scripts or tools to carry out API testing frequently. 
  4. Documentation: Documentation is one of the significant elements in any type of software testing. It contains all the essential data required to perform testing and functions. Testers use documentation to gain information about the software product. 

What parameters to be checked during API testing?

When the API testing is performed, testers send an API call to the application program interface and the response it sends back is interpreted or analyzed. The Quality Assurance team verifies the response based on the following elements:

  1. The accuracy of the response
  2. HTTP Status codes
  3. The time is taken by the API to send the response, i.e., response time. 
  4. Verifies authenticity
  5. Performance and security testing. 
  6. If the response has an error, API should find the error code. 

Challenges in API Testing

The Quality Assurance team faces several issues while carrying out API testing. Let us discuss some challenges faced in API testing.

  1. We know that there is no GUI involved in API testing. It focuses on performance, security, and reliability. One of the significant challenges in API Testing is testers are not known about the GUI. So, it becomes difficult for them to provide input to the system. 
  2. Whenever testers provide input to the system, it produces respective output. It is necessary to validate and verify the produced result. But, in API testing, the validation and verification process is quite tricky. 
  3. The functional code of the system may include an exception-handling function. The testing of this function is mandatory. But, testers find it difficult to test exception-handling functions. 
  4. In general, there is no requirement for the coding skills for testers to perform testing. But in API testing, testers should possess a little bit of coding knowledge.
  5. Another challenge in API testing is call sequencing. Accurate and sequential calls are to be performed for the correct execution of the system. 
  6. Testers in the API testing need to select the proper parameters and categorize them appropriately. 

Best Practices of API Testing

  1. Create the test cases in such a way that it covers all possible combinations of the API. 
  2. The most significant point to take into account is to group the test cases based on their categories. 
  3. Before you write any test, you must add the declaration os the particular API on which you are working. 
  4. API testing requires parameter selection. You should include all parameters while writing test cases. 
  5. You have to prioritize the API call. Doing this will help testers to execute the API call function more comfortably. 
  6. Every test case you develop should be independent and should contain all information. Keep all test cases self-sufficient. 
  7. Do not include chain testing in your software product. Chain testing implies extracting test data from the system’s output of the system that is currently under the test process. 
  8. As call sequencing is one of the significant challenges of API testing, you must take great care of it. 

What defects are identified in API Testing?

API testing checks the functionally, performance, reliability, or security and identifies bugs and defects in the software product. The following is the of bugs or flaws that API testing identifies:

  1. Flags that are unused
  2. Failure to handle error situations. 
  3. The use of duplicate functions
  4. Missing functions
  5. Unreliability, i.e., not getting a faster response from API
  6. Multi-threading complications
  7. Using valid argument values incorrectly
  8. Unstructured response data (JSON or XML)
  9. Inaccurate messaging
  10. Security, performance, and stress problems. 

Tools used for API Testing

API Testing is usually carried out instantly using API Testing tools. Below are some standard testing tools used for API as well as Unit testing:

  1. API Fortress: API Fortress is one of the popularly used API testing tools. It is specially designed to carry out REST and SOAP API testing. Let us watch some features of API Fortress.
  • Using the API Fortress tool, testers can produce multiple tests within no time.
  • API Fortress is a web-based API testing tool. It works within the browser and does not require the installation of any external or desktop application. 
  •  This tool has a straightforward and easy to use interface. 
  1. Test Mace: Another high-featured API testing tool is Test Mace. It is a cross-platform tool that supports all major operating systems. Like API Fortress, Test MAce also generates automated API tests with a single click. 
  • One of the significant advantages of Test Mace is it generates tests for more complicated scenarios rapidly and quickly. 
  • Using Test Mace does not require knowledge of coding or programming languages. 
  • It contains human-understandable file formats, which becomes more accessible for testers to read. 
  • Test Mace is a cloud-based tool that allows users to use anywhere and from any desktop.
  1. Ping API: Ping API is another API testing tool that is very sturdy and robust. It enables testers to write tests in JavaScript and CoffeeScript language. If any function fails, this tool notifies testers about the failure. 
  • Using Ping API, testers can schedule tests at a particular time. 
  • As it supports JavaScript and CoffeeScript language, testers feel comfortable in writing tests. 

There are many other API Testing tools, like Runscope, Postman, Curl, Cfix, dotDESK, etc. 


API Testing is one of the significant aspects of the software testing process. It verifies whether a particular software or website responds accurately and quickly. Additionally, API testing confirms the performance, security, reliability, and response time of the software product. 

After you read this post, you get a precise idea about API testing. We have discussed API, the need for API, types of API, API Testing, types of, and API Testing. Later, we have seen how to perform API testing, its phases, setup requirement, and test cases. You might have got a clear difference between unit testing and API testing. Also, we have discussed defects identified by API testing, challenges in API Testing, and tools used to carry out API Testing. 

Recommended Articles