API Testing Interview Questions and Answers

API stands for Application Programming Interface and is a way of connecting two or more applications. It allows developers to access data and services from other applications, allowing them to create new systems with enhanced features. API interview questions can be an intimidating topic for many, but it is critical to understand them in the context of a technical job. This course covers a variety of API interview questions, from basic beginner concepts, such as authentication and authorization to more complex advanced topics, such as data modeling, caching strategies, scalability, and services architecture. You will learn about the different types of APIs available and the best practices for designing and developing them. With this knowledge, you can confidently approach any API-related job interview, prepared to answer difficult questions articulately and effectively. Ultimately, you should leave this course feeling prepared and confident for your next interview.

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API stands for an application programming interface. It is a software interface. The main purpose of API is to connect two or more applications that share data with each other. It is responsible for improving an organization's productivity and keeps the application's data in sync. API testing is the process that validates API testing and verifies the application's usability, conduct, security, and efficiency. It guarantees that the created APIs live up to the expectations of the organization. An application programming interface (API) links computers or pieces of software to one another as opposed to a user interface, which connects a computer to a human. The only person (the end user) who is supposed to utilize it is a computer programmer who is integrating it into the software.

A programming language interface (API) frequently consists of various components that serve as tools or services for programmers. A program or programmer is said to invoke a particular section of the API when they use it. The calls that make up the API are sometimes referred to as endpoints, subroutines, methods, or requests. These calls are defined by an API specification, which means that it describes how to use or implement them. An API makes programming simpler when creating apps by abstracting the underlying functionality and exposing the objects or actions the developer requires. An API for file input/output could give the developer a function that copies a file from one location to another without necessitating that the developer understands the file system operations taking place behind the scenes, in contrast to a graphical interface for an email client that might give the user a button that completes all the steps for fetching and highlighting new emails. 

API Testing:  

Application programming interfaces (APIs) are tested both directly and as part of integration testing during API testing to see if they live up to expectations for functionality, dependability, performance, and security. API testing is carried out at the message layer because APIs are devoid of a GUI. Because APIs increasingly serve as the principal interface to application logic and because GUI tests are challenging to maintain given the short release cycles and rapid changes often employed with Agile software development and DevOps, API testing is now considered essential for automated testing.

APIs are tested to see if they respond correctly (in a particular intended format) to a wide range of realistic queries, respond appropriately to edge circumstances like failures and unexpected/extreme inputs, deliver results quickly enough, and protect against security threats. By mimicking APIs and services that are not available for testing, service virtualization is used in conjunction with API testing to isolate the services being tested and increase access to the test environment. 

Test API is a library that is a collection of test and utility APIs that enable engineers and developers to make testing tools and computerized tests. Test API is an essential tool for developers. An API testing approach should start with a precisely defined programme scope and a thorough comprehension of how the API is intended to function. To make sure the application programming interface is functioning properly, various API tests can be run. They span general to specialised software analysis. API testing ensures that platform connections that are provided by API are dependable, safe, and easily scalable. automation of GUI tests, which speeds up testing and lowers testing costs. By allowing developers to access the app without using the user interface (UI), API testing enables developers to find bugs earlier in the development process rather than waiting for them to escalate into larger problems. Additionally, this saves money because faults can be fixed more effectively when they are discovered early.

REST API stands for Representational State Transfer Application Programming Interface. It is a set of architectural constraints. This architectural style REST API is used extensively in web designing. Doing Web Designing and Development courses alongside REST API can be beneficial for any developer. REST API automation interview questions are very important from an interview point of view. Four interface limitations are specified by REST: 

1) Resource identification 

2) Mismanagement of resources 

3) Self-descriptive messages 

4) and hypermedia as the application state's driving force 

In general, it describes a machine-to-machine interface, and more specifically, it permits the substitution or addition of server-side rendering in web development to help client-side rendering in client-server models, where the web browser serves as the client for web applications. Performance, scalability, simplicity, modifiability, visibility, portability, and dependability are all things that REST aims to improve. Following REST concepts, such as a client-server design, statelessness, cacheability, usage of a layered system, support for code-on-demand, and use of a standard interface, allows for this. To qualify as RESTful, a system must adhere to these standards. 

The browsers we use today come with built-in APIs, known as browser APIs. They give programmers the ability to carry out complicated operations without having to manage the intricate lower-level code. Browser APIs are used by developers and designers extensively, for anyone who is involved particularly with browser APIs should enroll in various web development courses to gain knowledge, Java Full Stack Developer Training is a good course, to begin with. Browser APIs are APIs that offer native functionalities that may also be utilized in a web app and are integrated into the browser.

These are also referred to as web APIs. The fact that most of the Web APIs have not yet been standardized is a significant drawback of using a browser API. This means that a web API's support may vary from one web browser to the next. For instance, an API that is not currently supported by Firefox or Edge may function using the Chrome browser. 

We can quickly implement some functionality with web APIs and fewer lines of code, like: 

  1. executing network requests 
  2. controlling client-side storage 
  3.  receiving device media streams 

Expect to come across this popular question in API Testing interview questions for freshers.

API is an interface, not software, that enables data interchange and functioning between various software applications or processes. Although an API offers functionality and access to data, it may also be seen as software that meets our demands. One kind of API is the interface to a software library. The library is a "real implementation" of this set of guidelines, whereas the API specifies and explains the "anticipated behavior" (a specification).

A single API might be implemented in many libraries using the same programming interface, or it can be abstract and have no implementations at all. Programs developed in one language may be able to utilize libraries written in another due to the separation of the API from its implementation. For instance, Scala developers can utilize any Java API because Scala and Java compile to compatible bytecode.

The five principles of API testing are:- 

  1. Setup: The first principle of API testing is to create objects, initialize data and start services. Adopting this approach would ensure that there are no hurdles to API adoption caused by language. 
  2. Execution: The second principle of API testing is execution, it applies steps to API and includes actions like logging. Adopting this principle aims to make the generated APIs as useful and problem-solving as possible. Implementers are more likely to devote time to practical solutions than theoretical or abstract ones. Interoperability is required if different enterprises share the same use case. 
  3. Verification: The third principle of API testing is verification, it evaluates the result of execution. The goal of implementing this principle is to maximise the adoption of the specifications. The creation and publication of data on the server side, as well as its consumption and processing on the client side, must be considered when determining how simple a system should be.
  4. Reporting: The fourth principle of API testing is reporting, it reports pass, fail, or blocked status. Adopting this principle makes it possible for the specifications to be executed in a range of languages and design paradigms. It is the outcome of combining the two earlier principles. 
  5. Clean up: The fifth and last principle of API testing is clean up, it gives a pretest state. Adopting this principle will make sure that the requirements are cacheable while also being simple to comprehend and use. 

A common question in basic API Testing interview questions, don't miss this one.

There are many different types of API testing, which are: 

  1. UI testing: Testing of the user interface for the API and other crucial components is referred to as UI testing. Instead of evaluating the API itself, UI testing concentrates primarily on the interface that connects to it. Although UI testing does not specifically examine API in terms of coding, it nevertheless offers a general assessment of the functionality, usability, and effectiveness of the front and back ends of the app. 
  2. Functional testing: Functional testing includes testing certain codebase functions. These features serve as a depiction of particular circumstances to ensure that the API functions are used effectively and within the intended bounds. 
  3. Security testing: By following this procedure, the API application is protected from outside dangers. Additional phases in security testing include verifying encryption techniques and the architecture of the API access control. User rights administration and authorization verification are also included. 
  4.  Load testing: Load testing often takes place after a particular unit or the entire codebase has been finished. Using this method verifies that the theoretical solutions function as intended. The performance of the app is observed during load testing under both average and heavy loads. 
  5. Penetration testing: The second trial in the audit procedure is penetration testing. Users who are unfamiliar with APIs will attempt to evaluate the threat vector from a distance, focusing on functions, resources, procedures, or the complete API and its constituent parts. 
  6. Fuzz testing: Another phase in security auditing is fuzz testing. To find any forced failures or unfavorable behaviors, the system will be exposed to a large amount of random data, sometimes known as "noise" or "fuzz." This method puts the API's restrictions to the test to be ready for worst-case scenarios. 
  7. Runtime testing and error detection: This sort of testing is concerned with how the API actually operates, in particular with the overall outcomes of applying the API codebase. This method focuses on one of the following: error detection, monitoring, execution errors, or resource leakage. 
  8. Validation testing: The final stage of the development process is validation testing, which is crucial. It checks efficiency, behavior, and product features. To put it another way, validation testing can be thought of as a guarantee of proper development. 

There is a certain procedure followed for performing API, in addition to the standard SDLC process, API automation testing should at least include the following testing approaches: 

  1. Testing for discovery: The test team should manually carry out the sequence of calls specified in the API, such as confirming that a particular resource accessible by the API may be listed, created, and deleted as necessary. 
  2. Usability testing: This testing determines how user-friendly and functional the API is. And how effectively does API integration with other platforms 
  3. Security testing: Testing for security includes determining the sort of authentication needed and whether or not sensitive data is encrypted over HTTP. 
  4. Automated testing: The goal of API testing should be to produce a tool or a set of scripts that can be used to perform the API on a regular basis. 
  5. Documentation: The test team must ensure that the documentation is sufficient and offers sufficient details to interact with the API. Including documentation in the final release is recommended. 

One must keep the following things in mind while performing API testing. Here are a few things to think about to make sure you get the most out of your tests throughout the product's life cycle if you want to get into API testing to gain the long-term rewards with less of a short-term burden. 

  1. Verify and update the documentation already in place: You should initially spend some time looking for existing documentation and getting acquainted with how things are meant to work before starting any API testing for your project. You'll require information on the endpoints that are available, the request and return formats, the expected payload structures, etc. Building a trustworthy, maintainable test suite requires an understanding of how your APIs operate. 
  2. Decide what you want to test first: The most frequent error any testing team may make when beginning their testing operations is to dive in headfirst without having a solid plan in place. Well before you make your initial API call, your efforts could be doomed if you don't know what needs testing first and, more significantly, what does not need any testing right now. Determining what to work on now and what to put off until later is crucial. Since you can't test anything, you must decide where to focus your efforts when you first start. 
  3. Choose what to verify and keep an eye out for contradictions: There are numerous similarities between what has to be tested for APIs in most contemporary applications. You'll probably use XML or JSON to communicate with a REST or GraphQL API. Verifying various request types, the response body, the received status codes, any modifications in the resource's status, etc. are typical claims made for these settings. There isn't much variance in what you need to verify between various APIs unless such application has special requirements. 
  4. Track down any potential security holes you may encounter: 

Whenever it comes to information, APIs may be powerful tools. An API endpoint can be created by a developer that returns a ton of useful data for use by another program. The developer may also give users control over the data that is returned. But just like with any software development, it's important to take extra precautions with APIs to make sure they're only creating the data that's required at the moment—neither more nor less. 

Testing the graphical user interface is known as UI (User Interface) testing. UI testing is focused on how the program looks and feels, including how the user interacts with its components like graphics, typeface, layout, etc. UI testing is different from API testing. API testing makes communication possible between two software systems. API satisfies the requirements for functionality, dependability, performance, and security. It does backend operations and is familiar with backend testing. Unlike UI testing, API testing focuses on the operation of the business intelligence. The business logic layer of the software architecture is the main focus. The appearance and functionality of an application will not be a focus of this testing.

A staple in API Testing interview questions and answers, be prepared to answer this one.

A bug is a mistake in software or hardware. While bugs are typically the product of programmer error or oversight, they can also enter a programme through compiler errors, false assumptions about how hardware or software interfaces work, or they can be purposefully incorporated into software to produce disguised interfaces or back doors. While vulnerabilities frequently result in software crashing or other unexpected effects, some bugs can be used to break into an otherwise protected computer system.There are many bugs we find in API testing, which are:- 

  • Faulty error conditions
  • Missing functionality
  • Duplicate functionality
  • Security
  • Inconsistent error handling
  • Performance
  • Reliability
  • Inconsistency
  • Multi-threading issues 
  • Flags

HTTP methods are an important concept for REST API testing interview questions. The following are the HTTP methods supported by REST API. 

  • GET: This HTTP method is used to request a resource at the request URL. It usually gets discarded if it contains a request body. It is usually cached locally or on the server.
  • POST: This HTTP method is used to submit information for processing and returns a new or modified resource. 
  • PUT: This HTTP method is used for updating the resource at the request URL. 
  • DELETE: This HTTP method is used to remove the resource at the request URL. 
  • OPTIONS: This HTTP method tells us about the techniques that are supported. 
  • HEAD: This HTTP method returns the meta-information about the request URL. 

A type of memory called a cache is used to speed up data access. Normally, the main memory contains all the data needed for any process. However, if it is utilized frequently enough, it gets momentarily moved to the cache memory. Caching is the technique of storing and retrieving data from a cache. Caching is an important concept in REST assured API testing interview questions. Caching helps networks run more efficiently by easing the burden on servers. It is the ability to keep copies of data that is accessed frequently. The POST HTTP mode has Post requests that can be made cacheable. However, Get requests of the GET HTTP mode are cacheable by default.  

The act of damaging digital systems and networks by gaining unauthorized access to a computer system or account is a popular definition of hacking. Although hacking is not always harmful, it is most frequently linked to online crime and data loss by cybercriminals. 

Hacking is the term for using technology improperly to harm or destroy systems, obtain user data, steal data and documents, or obstruct data-related activity. Examples of such technology include desktops, cell phones, tablets, and networks. The stereotypical image of a hacker is a lone, miscreant programmer with advanced coding and system modification skills. However, this limited perspective does not account for hacking's underlying technological nature. 

Yes, it is possible. This is due to the fact that we are making requests via the internet, which mostly uses the HTTP protocol. This protocol is easy to read because it is text-based. Therefore, security testing of the APIs is necessary to create safer systems.  

After the initial level of testing each individual API, security testing typically begins. You make test cases with faulty requests in addition to legitimate inputs. All the so negative tests enable you to assess the effectiveness of your API error handling. Through the clever design of incorrect inputs that could crash your API or expose data, you can also utilize these negative tests to confirm the security of your API. Discovering API utilization and potential edge situations that are worth testing can be accomplished by gaining insight from tracing data using a variety of methods. APIs can be hacked easily while testing an API so we test the security of an API. When evaluating the API, we must validate two elements in order to check its security: 

  • Authentication: Verifying that the end user's identification is accurate. 
  • Authorization: Whether or not a person is authorized to access a resource. 

API tools are widely available and help developers accomplish their tasks more quickly. They contain solutions for programming, testing, debugging, writing documentation, and other related tasks. The use of API testing tools enables effective API testing and monitoring. Any kind of API is supported by it. The design of complicated tests for API verification, managing, and analyzing API testing, etc. is made possible by the API Testing tools solution. API Testing indeed is a difficult job, various tools are available for API testing a few of them are:- 

  1. Postman 
  2. SoapUI Pro 
  3. Apigee 
  4. Jmeter 
  5. Karate DSL 
  6. Airborne 
  7. Swagger.io 
  8. ReadyAPI 
  9. ACCELQ 
  10. Pyresttest 
  11. REST-assured 
  12. Katalon studio 

API is a powerful web component, and businesses of all kinds are embracing it because of the following advantages:- 

  1.  Many businesses are understanding the value of API testing for delivering a positive customer experience. 
  2.  API testing verifies that the built API satisfies the business's requirements.
  3. Language-free: Any language can be used for test robotization because data is exchanged through XML and JSON positions. Since XML and JSON typically contain organized data, the check runs quickly and consistently. Additionally, similar information designs are used in libraries to assist in looking at the material. 
  4. GUI-free: Prior to GUI testing, GUI-free programming interface testing can be conducted in the application. Early input and improved group profitability are implied by early testing. To find minor errors and evaluate the quality, try out the application's core capabilities. 
  5.  Better test inclusion: The majority of API web administrations give information that enables you to create automated tests with high inclusion, including utilitarian testing and non-useful testing. 
  6.  Rapid Release: Executing API testing typically takes eight hours compared to UI testing, allowing programming development teams to release products more quickly. 
  1. Validation testing: One of the final and most crucial tasks in the development process is validation testing. It basically consists of a series of straightforward questions that are applied to the entire project, including product validation, API behavior on accessing the right data in the right defined manner, and the effective opportunity for any codebase to be removed or changed to get rid of deteriorations in the overall service. 
  2. Security testing: By locating potential application flaws, security testing reveals vulnerabilities, threats, and hazards. The outcomes of security tests stop hostile attacks from outsiders as well as the loss of data, money, and reputation.
  3. Runtime error testing: Critical flaws that become apparent during application execution are exposed through runtime error detection. An accurate detection report offers the chance to quickly pinpoint the underlying causes of application failure, unpredictable behavior, and inferior performance. 
  4.  Performance testing: Applications that function poorly risk developing a negative reputation and falling short of business needs. Performance testing establishes measures for API scalability, stability, and performance under anticipated workloads. 
  5. Functional testing: Functional testing employs the Black Box approach with no regard for the application's source code. The goal of functional testing is to ensure that the application is operating faultlessly. 

It's no surprise that this one pops up often in API Testing real time interview questions.

API documentation is like a deliverable. Technical content deliverables such as API documentation provide guidance on how to use and integrate APIs. It is a brief reference guide that includes all the details needed to use the API, including information on the functions, classes, return types, arguments, and more, all backed up with tutorials and examples. Traditionally, text editors and common content creation and maintenance tools have been used to create and maintain API documentation. Technical writing that describes the API is also known as API documentation. It contains guidelines for properly utilizing and integrating the API. The lifecycle of the API is also updated, including any new versions or retirements. Using Swagger or other documents, some portions of API documentation can be automatically generated.

The test cases that are run repeatedly during each sprint, such as regression cases, frequently need to be automated. Similar to this, there are several API testing scenarios that we must run before each release; these scenarios should be automated, and this is what we call API automation. We use various tools like SoapUI, apigee, etc., for API automation. The automatic creation of an API connecting two previously unconnected applications or services is the first application for API automation. If you can even find a competent API developer in the first place, manually developing an API can be a highly specialized and time-consuming project depending on the use case and technological complexity. With a project timetable of a few weeks or months, estimations for the cost of developing an API can rapidly reach the thousands and thousands of dollars range. Additionally, if the underpinning services change, these estimates do not take ongoing maintenance and support into consideration.

Web services and APIs are frequently confused with one another. They may carry out most of the same tasks, but they are not exactly the same. Let's examine both of these before moving on to their distinctions. 

Serial NO. 


Web services 

API needs a network to function sometimes 

Web services always need a network to function.

SOAP, REST, CURL calls, and XML is used for communication in API 

SOAP, REST, and RPC is used for communication by web services 

Not all APIs are web services 

All web services are APIs 

Example: amazon API 

Example: .NET SDK 

The first digit of the Status-Code element in a server response specifies the response class, and the last two digits play no part in classification. The Status-Code element are combination of 3 integers. A server responds to a request from a client by returning a status code. It includes codes from various specifications, IETF Request for Comments (RFCs), and some extra codes used in some widespread HTTP applications. One of five standard response classes is indicated by the status code's first digit. The normal message sentences that are displayed can be replaced with any human-readable alternative. The status code is a component of the HTTP/1.1 standard unless otherwise stated (RFC 7231).

Example: The error message "Page not found" will appear if the firm ABC changes its name to "ABD" and the user inputs the same old URL that has not yet been changed to the new company name. The code that pops up on the screen is the status code. Like error “404”.

There are a total of 63 status codes, but they are divided into 5 categories, and codes from each category are used for a different domain. The five categories are:- 

  • 1xx informational response: This status code is used to provide information like a request was received or the action is in process. 
  • 2xx successful: This status code is used to tell about the success or completion of an action or a process.
  •  3xx redirection: This status code is used to give direction again, this status code is shown in cases where more steps need to be taken to complete the action.
  • 4xx client error: This status code is shown when there is a client-side error. It usually pops up when there is a syntax error. 
  • 5xx server error: This status code is shown when there is an error from the server side. It usually pops up when there is something wrong with the server, even though the request is valid. 

The capacity factor of a package or other data transmission unit is referred to as a payload in computing. The phrase, which has military roots, is frequently used in relation to malicious code that can be executed and cause harm. Data payload and malware payload are two different types of payloads that are used to attack and corrupt IT networks and systems, respectively. Data payload refers to the movement of data through a network.

Programmers use the term "payload" to distinguish between the information that is necessary for a piece of data and the information that is used to support it. Secured input data is supplied to the API as the payload/body in order to process the request. In the REST API, the payload is often built in JSON format.

API stands for application programming interface, it acts as a connecting link between a computer and its programs. It is a type of software interface that allows applications to access the data and interact with operating systems, software components and microservices. In simple terms, an API is responsible for sending the user's response to the system and bringing back the system's response to the user. APIs are used in web development both backend and frontend.

From this picture, we can understand how an API works. We can see how the API is accessing data from the database and making the communication between operating systems, computer programs and software components possible.  

API has been used in various technical projects by very prominent technological organizations. Some examples are Google Maps API, YouTube API, Amazon advertising API, etc. In the Google maps API project, navigation data from satellites was stored in the Google maps database and it was sent to the application with the help of an API. In the YouTube API, data from YouTube surveys and history was stored in the YouTube database and was then shared with the application with the help of API and was later used to curate YouTube videos and advertisements recommendations. Similarly in Amazon advertising API, data from customer history and orders were stored in the Amazon database and was provided to the software via API and was later used to curate tailor-made advertisements and product recommendations for each customer. In each project, the customer data collected by various surveys and research were stored in the database and were then provided to the software. The smooth communication that we witnessed in all the programs was due to the API.

Indeed, people often get confused between API and web services, these two terms even overlap many times, but API and web services are two very different terms. An API acts as an interface between computer and computer programs, it is responsible for accessing data on various databases and sending it as a response to the computer applications, software, operating systems, and microservices. Whereas, a web service is a software system and is responsible for the machine-to-machine interaction that takes place on a network. In simple terms, web service is a resource that is available on the internet. People get confused between an API and a web service because all web services are an API, but not all APIs are web services.

The major point of differences between an API and a web service are: -

  • A web service always needs a network to operate whereas an API does not have any such requirements.
  • A web service does not contain all the specifications so it can not perform all the tasks performed by an API 
  • A web service only used three styles of communication which are SOAP, REST, and XML-RPC. An API uses all these styles of communications but uses other styles of communications as well

Note: We will study in detail REST in the advanced interview questions and will cover REST assured API testing interview questions and REST API automation interview questions. 


No, it is not true that there are limited options for API testing. API testing methods are wide and varying. A few methods to test an API are: -

  • Validation testing: Validation testing is a set of simple questions that are applied to the entire project. It is one of the last steps in the development process. It is done even after the verification of an API. The Validation testing includes some general questions about the product, behaviour, and efficiency.
    • Product: When we talk about products, we take into the count a few questions like was the correct product made? Was there an issue resolved within the limits of API building?, and did the API experience any code bloat or feature leak.
    • Behaviour: When we talk about behaviour we usually observe API and how is it dealing with the data, we ask questions like, Is API accessing the data in a proper defined method? Is the API accessing too much data? and is the data storage meets the confidentiality and integrity requirements of the dataset?
    • Efficiency: When we talk about Efficiency, we usually ask questions like, Is the API accurate, optimized, and efficient method of performing the required task? And do we need to add or alter any code base to enhance the efficiency and remove the impairments? 
  • Functional testing: Functional testing is a test of specific functions within the codebase. The functions represent the specific scenarios that ensure that the API is functioning within the expected parameters. And even if the API is outside the expected parameters, function testing ensures that the errors are handled well. 
  • UI testing: UI testing is the test of the user interface of the API and its constituent parts. It does not matter if the interface is graphical or it depends on command line endpoint calls, this test is only concerned with the functioning of UI.
  • Load Testing: Load testing is done after the completion of a specific unit or the codebase as a whole, usually the test of the theoretical solution will work as a practical solution if the load varies. For example, if a software requests a certain data from an API, we have a solution for it, load testing will find what will happen if 1000 such requests were made.
  • Runtime Error Detection: The runtime error detection test deals with the actual running of API. The other test is mostly concerned with the results of API, but this test is concerned with the universal results of utilizing the codebase of the API.
  • Security Testing: Security testing is designed to ensure that when the API is implemented it is secure from external threats. It includes the Validation of encryption methodologies and validates access control.
  • Penetration Testing: Penetration Testing is a step ahead of security Testing, in penetration testing the API is attacked by someone who has limited knowledge of the API. These attacks can be limited to functions, resources, processes or the constituent parts of the API.
  • Fuzz Testing: In fuzz testing, massive amounts of random data often referred to as "fuzz" or "noise", are forcibly sent into the system to attempt forced crash, overflow, or negative behaviour. It prepares the API for the worst-case scenario.


Protocols indeed are an integral part of every digital system, Protocols are the set of rules that define how applications interact with each other. Protocols are a significant part of any API system since Protocols are responsible for interaction and communication. Protocols play a significant role in API testing as well. 

The various protocols used in API are: -

  • HTTP 
  • REST 
  • SOAP 
  • JMS 
  • UDI 
  • XML 

It is indeed true that API testing is a lengthy and rigorous process, there are many ways by which we test an API. We test an API because it is essential to do so and there are many advantages of testing an API. 

A few advantages of API testing are: - 

  • Core functionality testing: It eliminates the need for a user interface, and provides access to the whole system. 
  • Time and cost-effectiveness: API testing is less time consuming as compared to GUI (Graphical user interface) testing. Due to the time and cost-effective advantage that API provides it is widely used in web designing and development, to know more about it, click on the link below 

web designing and development course. 

  • Language-independent: The data transfer that is done in API between the test platforms and computer applications is done using various protocols like XML and JSON and is completely independent of any language.
  • Ease of integration with GUI: API testing is highly flexible and it helps in easier integration with GUI testing.

There are 5 significant principles involved in the API test design and they are as follows:-

  • Setup: Setup involved basic initial steps like creating objects, starting services, initializing data, etc.
  • Execution: Execution involves steps that we apply to API or the scenario which includes steps like logging.
  • Verification: Verification involves the evaluation of the result of the execution.
  • Reporting: Reporting involves the task of updating whether the execution has been passed, failed or blocked.
  • Clean up: Clean up involves everything that comes under a pretest state.

Yes, there is a procedure that we need to follow when we are performing API testing. It involves some fundamental steps, which are:-

  • Choosing the suite that we need to add to the API test case.
  • Choosing the mode for test development
  • Demanding the development of test cases that are required for the API methods.
  • Configuring the control parameters of the application and then for the test conditions.
  • Configuring the method validation
  • Executing the API test
  • Checking the test reports and filtering the API test cases
  • Arranging all the API test cases

When we are performing API testing, there are a couple of things that we check. We check those things by raising a request to the API with the known data, and we analyze the validation response in this manner.

The basic things that we check during testing an API are: -

  • Schema validation
  • Status code of HTTP
  • The accuracy of data
  • Data type
  • Data validation
  • Data order
  • Data completeness
  • Response timeout implementation
  • Authorization checks
  • Error codes
  • Security

API testing indeed has major advantages, but this does not mean it is bereft of any challenges. We face many challenges while API testing, few of them are: -

  • Selection of parameter 
  • Parameter combination 
  • Call sequencing
  • Providing input values
  • Output validation
  • Output verification

Various testing methods come under API testing, a few of them are: -

  • Unit testing and Functional testing: It is used to test specific functions within the codebase
  • Load testing: It is used to test the performance under load 
  • Discovery testing: It is used to list, create and delete the number of calls that are documented in API 
  • Usability and Reliability testing: It is used to get consistent results 
  • Security and Penetration Testing: It is used to validate all types of authentication 
  • Automation testing: It is used to create and run scripts that require regular API calls 
  • End to end Integration and Web UI testing: It is used to test any software from start to finish. 
  • API documentation testing: It is used to determine the efficiency and the effectiveness 


To understand the difference between the two, we first need to understand what these two terms are individually. Structured data can be shared in a decentralized, distributed environment thanks to the broad messaging infrastructure defined by the SOAP standard communication protocol system, which employs XML technologies. In other words, SOAP enables the use of different technologies and computer languages for communication between applications running on various operating systems. On the other hand, REST is an architectural approach rather than a protocol. It stands for Representational State Transfer, as was already mentioned. This means that when a client uses a REST API to access a resource, the server sends back the resource's current state in a common format. In other terms, REST APIs respond to requests for resources by returning all pertinent data in a way that customers can easily understand. 




SOAP stands for simple object access protocol 

REST stands for representational state transfer 


SOAP is a protocol 

REST is an architectural pattern 


SOAP uses WS and SSL (Secure socket layer) for security purposes.

REST uses SSL(Secure socket layer) and HTTPS for security purposes. 


SOAP uses and supports only HTTP and XML 

REST uses and supports multiple data formats like, HTML, XML, JSON, plain text, etc., but JSON is the most preferred data format. 

This is a frequently asked question in API Testing interview questions for experienced.

POSTMAN API testing interview questions should be done with utmost care and priority since questions from this topic are asked very frequently in various interviews.

An HTTP Client-based program called Postman is mostly used to test APIs. The GUI (Graphical User Interface) can be used to test HTTP queries, and the outcomes can be examined. Providing a platform for designing, implementing, testing, and documenting APIs, also makes it easier for team members to collaborate on API development.

For testing purposes, using Postman eliminates the need to create any HTTP client network code. Instead, we create collections of test cases and let Postman communicate with the API.

A collection is a set of data objects that must be worked on collectively in a controlled manner and have some shared value to the problem being solved. In computer programming, a collection is a gathering of a variable number of data items (potentially zero). The data items will typically be of the same type or, in languages that permit inheritance, derived from a type that has a common ancestor. Although there is frequently a traditional option, a collection is a concept that applies to abstract data types and does not dictate a specific implementation as a physical data structure. The term “collection” in API is related to POSTMAN. In Postman, a collection is a bunch of API requests that have previously been saved and can be organized into folders. A collection can have any number of folders. The client's demands are better organized and documented when they are placed in folders and collections.

Each API request can be saved and organized into a collection, which can then be shared among the team members in the Postman workspace.

Such questions are very common in API testing POSTMAN interview questions, practicing them would be favourable for users appearing for interviews.  

Monitoring is a method for maintaining tabs on the functionality and health of APIs. We can keep track of API performance and development thanks to Postman's built-in monitoring features. The operation of the collection runners is the main focus of Postman's monitors. They execute each request in the collection and examine the test script values. The test scripts are used by the monitors to verify and keep track of the responses. Depending on our setting options, the reports produced are sent to the developers via emails or alerts in Slack, Hipchat, and other platforms.

An implicit or explicit caching mechanism is a technique for temporarily storing data and retrieving it from a high-performance data store. By making a copy of the requested asset and retrieving the cached copy later rather than the original, caching mechanisms enhance performance. Cache memory is allocated its own partition in RAM. Cache memory determines whether the requested data is already available in it before requesting it from storage on behalf of a software program. The application will read this data from the cache if it is present there. The software application will ask for the source if the necessary data isn't present in the cache memory. The same information is kept in cache memory for later use after reading it.

A framework is exactly what it sounds like. A framework,as the name suggests contains code, structure, processes, classes, and functions that have previously been established in the API. Any programming language, including PHP and Python, can make use of the framework in API. Any software created with the aid of a framework must adhere to its restrictions it lays down. An operating system may support a framework. The Android framework will be used if you're creating an Android application. As an alternative, some operating systems need particular frameworks to function, such as a Microsoft.Net framework for the Windows system.

A common question in advanced API Testing interview questions, don't miss this one.

A framework may include code libraries and APIs from which an app can draw data. A library is a collection of codes that are used to streamline or organize tasks. A particular language's constants, classes, functions, and procedures help programmers write less code. You will search the library for information. The framework serves as the backbone on which the library or libraries are supported. They both specify an API that programmers can utilize. We might conceive of a library as a specific application function, a framework as the foundation of the program, and an API as the connector to connect things in order to put them all together. A normal developmental process often begins with a framework and uses the API to complete functions described in libraries.

The act of stimulating user input is referred to as input injection. Windows applications can route input anywhere, including beyond the client area, by using input injection to simulate input from a number of input devices. Even to the applications running with Administrator privileges. For Windows apps and solutions that need to offer functionality such as accessibility, testing, ad hoc, automated, and remote access and support capabilities, input injection is helpful. There are several approaches to imitate user input, including: 

  • Invoking Direct Methods 
  • Use of an accessibility interface during invocation 
  • Using low-level input to perform simulation 
  • Using a device driver to perform simulation 
  • Using a robot to assist with simulation 

We work without relying on Postman servers due to the workspace Scratch Pad, which is supplied by Postman. It enables us to use some of Postman's features even while you are not online. The capabilities include adding requests, adding collections, and adding requests. These are kept locally, and once you log in, the work is saved in the workspace. It is a tool that has made API testing and automatio easier than ever. Very few tools can be used offline for API testing and API automation. The offline working feature helps the users to get away with the biggest problem of server crashing. Even when postman server crash, we can easily use the ScratchPad API tool.

  1. Health & wellness: With an efficient caching design, you can deliver speedy speed, reduce overall spending, and expand as your usage increases. 
  2. Technology used in advertising: When creating real-time bidding software, a millisecond can make the difference between placing a bid on time and it becoming irrelevant. This calls for a super-quick retrieval of bidding data from the database. To attain that high level of speed, database caching, which can fetch bidding data in milliseconds or less, is a fantastic option. 
  3. Mobile: With the right caching technologies, your mobile apps can provide the performance your users demand, expand rapidly, and lower their overall cost. 
  4. Gaming and media: By providing sub-millisecond query replies for frequently requested data, caching keeps the game running smoothly. 

One of the fundamental tenets of REST design is statelessness. The server does not save any records of prior communications in stateless communication. In other words, neither the client nor the server are aware of the other's condition. Each interaction between a request and a response is unique, and each request contains all the information a server needs to deliver a successful answer.

Because the server does not need to expend space or resources to store data from previous requests in order to handle requests in the future, statelessness makes client-server interactions simpler. Every HTTP request occurs in total isolation due to statelessness. When a client sends an HTTP GET request, it contains all the data the server needs to process it. The information from a client's prior queries is never used by the server. If any of this information is crucial, the client will include it in the current request.

A staple in interview questions API Testing, be prepared to answer this one.

POST and PUT are comparable but not identical. PUT replaces a resource at a specified URI with another resource, whereas POST creates a resource on the server. When you use PUT at a URI when another resource already exists, PUT will replace that resource. PUT creates a resource if there is none at the given URI. PUT is also idempotent, which implies that using it more once will only produce one resource. This is so because every call substitutes the current resource (or creates a new one if there is nothing to replace). POST lacks idempotence. Additionally, this indicates that PUT responses cannot be cached, in contrast to POST responses.

"Create, Read, Update, Delete" is the acronym for CRUD. These are the four fundamental operations that a REST API can execute on databases. Each action is matched with a specific HTTP request method: 

  1.  In CRUD, the C stands for creating, which has numerous subgroups. Before a doctor may check a patient's vitals or change their prescriptions, we need to add them to Shmealth's database. Let's first talk about this activity. 
  2.  R in CRUD stands for reading, which merely retrieves and displays data from the database. This is the most straightforward and secure CRUD operation from the standpoint of a programmer. We can choose which routes enable the access of all, some, or just one record as database architects. 
  3. "U" stands for "update." This entails replacing the original value in the database with a new one that is of the identical data type as the existing data. 
  4. The D stands for delete, and that's it. It's not difficult to set up and carry out this action. The removal of resources from a relational database, however, may have unintended repercussions. It's recommended practice to treat deleting a record from a database as permanent and to proceed with extreme caution while doing this operation. Without a backup, there is no command in SQL to retrieve deleted records. 

Every piece of server-side content that may be accessed is known as a resource in REST architecture in API. An object with a type, associated data, a connection to other resources on the server, and a set of methods that may be used with it is referred to as a resource. An executable code file, a picture or video, a data file, an HTML or text file, or other types of  API files are examples of resources. A uniform resource identifier, or URI, is used to identify resources. Clients use the URIs of resources in HTTP requests to access them.

A unified resource identifier is referred to as a URI. A URI in REST is a string that designates a web server's resource. Each resource has a distinct URI that, when used in an HTTP request, enables clients to target it and do actions on it. Addressing is the process of directing traffic to a resource using its URI. People often get confused between URI, URL, and URN. How can we distinguish between URIs, URLs, and URNs? 

  1. A resource on the internet, such as a particular article, is identified by a string identification called a URI (Universal Resource Identifier). 
  2.  The URL (Uniform Resource Locator) identifies a resource's location on the internet and provides access information (such as "https://"). 
  3.  Even when the resources it identifies have vanished, the URN (Uniform Resource Name) remains a permanent identification for a single entity. 

Web applications use a set of web development techniques known as AJAX or asynchronous JavaScript. AJAX's fundamental feature is the ability for a web page to send queries to a server and update the page interface without needing to be refreshed. Although an AJAX client may use REST APIs in its requests, AJAX is not required to just use REST APIs. Regardless of whether a client supports AJAX or not, REST APIs can communicate with any client. Additionally, AJAX communicates its requests to the server using the XMLHttpRequest object that is included in JavaScript, as opposed to REST, which uses HTTP requests and responses for communicating.

  • Comprehending the business application 

The usage rules for APIs typically include things like copyright, storage, and display standards. APIs are only created using business logic and this varies from business to business. 

  • Change's effect 

The entire application will probably suffer from the new API version. Implementing a change is frequently elevated to risk due to the numerous dependent components and uncertain executions. Hence changes are brought in very carefully. 

  • Controlling test results 

APIs are all about data, and since constructive testing includes many factors, it requires additional data. For API testers, it can be difficult to manage such a large amount of data and make sure that it can still be used. 

Statelessness can be both an advantage as well as a disadvantage of REST API,it mostly depends on the function or process that a user has to carry out. But if we talk generally, Statelessness can be a benefit of REST API for some, but it can also be a drawback. The state of an API is not preserved through REST API. In other words, the server doesn't save a history of previous communications. If maintaining a state is required, the client is responsible for doing so. Developers must exercise caution and only use APIs from reliable, authentic providers as REST has less stringent security controls than SOAP. REST is therefore a bad choice for transmitting private data between servers and clients.

Engineers and analysts can create testing tools and automated tests for.NET and Win32 programs using the Test API, a suite of test and utility APIs. The test building blocks kinds, information constructs, and calculations are provided by the test API in a simple, layered, componentized, and archived stack. Test API is often used because:

  • Highlights minor issues before they develop into larger ones Before running GUI tests, one can examine the durability of the apps and evaluate the usability of the code at the API level. 
  • It can help in spotting smaller mistakes early on before they develop into larger problems later on. 
  • The exchange mechanism is language-free when guiding API testing. JSON or XML is used to exchange the data. 
  • Testing for improved coordination is possible because to GUI API testing's more straightforward integration. Later on, you want to take control of GUI testing. 

There are three layers in Test API: 

  1. Business layer 
  2. Presentation layer 
  3. Data set layer 

Before sending the modifications to the production environment, shrewd businesses test API for convenience. Testing in both staging and development environments is important to verify that the software program complies with requirements. The following are some rewards of testing APIs: 

  • Accessing the application without a user interface allows for early detection of code-level errors, which facilitates quicker test maintenance. This is a significant advantage of API testing. 
  • Since it needs to transmit data via XML or JSON, it is totally agnostic of any specific programming language. 
  • Since API testing doesn't require a readily available GUI, it can be done more quickly and simply. 
  • The essential functionality is well covered by API testing, which also lowers testing expenses. 

Since REST APIs don't use as rigorous security safeguards as SOAP APIs, sensitive data shouldn't be sent or retrieved using them. However, trustworthy REST APIs continue to integrate security controls for safe and dependable data transmissions. 

  1. Authentication and authorization: Every API request must be both authenticated and authorized. Verifying the client's identity through authentication and confirming that they have authority to access the requested resources through authorization are two different processes. 
  2. Validation: Requests still need to be checked for possibly dangerous code after authentication and authorization before the API grants access to its resources. A server would then be open to an injection attack. 
  3. Encryption: TLS/SSL encryption safeguards client-server connections and deters attackers. 

Digestion Authorization is related to POSTMAN API, this concept is a part of API testing using POSTMAN interview questions. As a choice for authorization, Postman offers the Digest Authorization technique. The server replies with a number that can only be used once, a realm value, and a 401 unauthorized response code once the client sends the request to the API. The login, password, and the information we previously obtained from the server are all included in an encrypted data array that we get. To authenticate the request, the server creates an encrypted data string from this data and compares it to what was sent. By navigating to the Authorization tab and choosing "Digest Auth" from the drop-down menu, you can accomplish this. The Postman window shows the fields for both phases of the authentication request.

API requests should be made in chronological order that matches the functionality. For instance, before placing an order, the product must first be placed in the shopping basket. Logging onto the website, searching for the product, choosing it, adding it to the cart, and placing an order are the steps in the call chain. Any inconsistency in the order of API calls causes the system's functionality to change because the output of one API is the input for the next one. When multithreaded programs are involved, this complexity increases. Each time a feature changes, validate these sequences. The set of instructions needed to transfer a command to a subroutine or process includes supplying parameters and storing the return address. If it is necessary to call processes written in a variety of languages from the calling program, consistency of calling sequences is essential.

The data that is transferred between these systems serves as the medium for communication between API & systems. The parameters have been given this data, and it is essential to verify the parameters using the data. The operation of API systems depends critically on the evaluation of the parameters. You must build every configuration with conceivable parameter combinations and evaluate various scenarios. Consider the type of data the parameter uses, if it fits within the assigned length/range, whether it passes another validation, etc. while validating these parameters. A good test case coverage can be achieved using boundary value combinations, equivalent partitions, pair-wise testing tools, and constraint-based elimination strategies. Additionally, you can use continuous API Monitoring Systems, which will provide speedy confirmation of system faults.

The test documents' API syntax and grammar are defined by the schema, which acts as a template for these elements. So we can check to see if a document is valid when a schema is appropriately applied to an input document. When using a mock server, documentation, tests, or monitors to validate the response parameters, you must set up the appropriate schema. We must update the schema if there is a change to the API document, such as the addition of new features or the upgrading of current ones. Throughout the testing phase, keep doing this.

Both API testing and Unit testing are testing methods, but both of them are strikingly different from each other.

API testing 

Unit testing 

API testing is concerned with the final output of the system which is under test.

Unit testing is concerned with the functionality of the module of the system which is under test. 

API testing is executed solely after the build of the system is fully ready.

Unit testing is accomplished when the unit test code or test script is written by the developers.

API testing portrays the system as a whole as it is the user interface and it interacts with the end-user.

Unit testing tries to portray each module in isolation and checks if each module is functioning properly as a single entity. 

It is factual that both API testing and UI (User Interface ) testing deal with multiple interfaces but these two are strictly distinct from each other.  

API testing enables the communication between two or more separate software systems. Whereas, in UI testing we test the graphical interface and see how users interact with it.  

If we were to explain it to a person from a non-technical background we can explain API testing as the communication or exchange of data that takes place between two different software. And UI testing is when a user interacts with the graphical part of any interface, like when we see various fonts, graphics, etc. whenever we open a website.

In today's time, the most suitable form of automation testing is API testing, and it has replaced GUI testing a long time ago. Developers all across the world prefer API testing over any other form of automation testing. It is considered to be the most suitable form of automation testing because:-

  • All the functional paths that come under the system are verified very efficiently and effectively by API.
  • API provides the most stable interface.
  • API is easy to maintain and it is known to provide faster feedback. 

REST stands for Representational State Transfer. It is an architectural style that is used for developing web services, it is known to exploit the all-presence or ubiquity of HTTP. The main focus of REST is on the resource, it can be accessed through any shared interface. REST server provides access to resources and the REST client makes them obtainable.

REST architecture refers to any sort of content as a " resource". The content comprises anything, it can be text, graphics, dynamic information, etc. REST uses distinct representations to define any resource. XML and JSON are the most popular ones.

There are 5gettingy components in any HTTP request: -

  • An action which defines the methods in HTTP like GET, PUT, POST, DELETE. 
  • The presence of URI ( Uniform Resource Identifier), the resource identifier on the server.
  • The HTTP version checker, tells the version of the HTTP being used. 
  • Request Header carries the metadata for the HTTP Request message. \
  • The presence of the Request body indicates the resource representation.

Some of the most commonly used methods of HTTP requests that are supported by REST are: -

  • GET: It is used to request data from a specified resource.
  • POST: It is used to send data to a server to create or update a resource.
  • PUT: It replaces all representations of the target resource with the request payload.
  • DELETE: It is used to remove a specified resource.
  • OPTIONS: It is used to define the communication options for the specified resource.
  • HEAD: It asks for a response but without a response body.

SOAP stands for Simple Object Access Protocol. It is defined as an XML based protocol. It is used in designing and developing web services and it is known to enable the communication between applications. Since it is both platform and language independent, it can communicate between applications designed and developed on different platforms using different programming languages.

Various facilities are provided by SOAP protocol like:-

  • PutAddress(): To enter an address on any web page we use PutAddress() and it also has an address instance on address call. 
  • PutListing(): To allow the insertion of a complete XML document into the web page we use PutListing(). 
  • GetAddress(): To generate a query name and get a result that best suits the query we use GetAddress(). 
  • GetAllListing(): To return the whole list in the XML format we use GetAllListing().

Postman is an HTTP client-based software and is widely used in API testing. It is known to support the testing of HTTP requests by simultaneously utilizing the GUI ( Graphical user interface). It provides a platform for designing, developing, testing, and documentation APIs and it is used by developers for the development of APIs.
Note:- API testing postman interview questions are asked very frequently by job interviewers.


An application program interface (API) is examined during an API test to ensure that it satisfies functionality, security, performance, and dependability requirements. The tests are run on the API directly or as a component of integration testing. If you want to learn more about API, enroll in our REST API course and expand your knowledge. 

In API testing, you use software to make calls to the API, retrieve output, and record the system's response rather than the typical user inputs (keyboard) and outputs. API tests differ considerably from GUI tests in that they don't focus on how an application looks and feels. The business logic layer of the software architecture is the main focus. 

The business logic layer is represented by a set of classes, functions, and procedures that make up the API. If an API is not properly tested, both the caller and the API applications may have issues. This test is crucial in software engineering.

Once the logic is developed, API testing may create tests to verify that the data and replies are accurate. The test cases are isolated and prepared to be built right away, so we don't have to wait for separate teams to finish their work or for complete apps to be constructed. 

When API testing fails, we are able to pinpoint exactly where our system malfunctioned and the flaw was present. As a result, it takes less time to prioritize defects across builds, integrations, and even team members. Faster MTTR data, a crucial KPI for DevOps teams, are ideal for an API test's small, isolated footprint. 

Anybody can gain from learning about API testing, but programmers should focus on fluency. We have discussed some crucial API testing interview inquiries that should demonstrate a candidate's comprehension of the topic. Although it's crucial to have some general knowledge about API testing, the majority of this knowledge is learned through actual practice. Use this article as a template and conversation starter for your upcoming interview. 

Regarding quantity and areas covered, API testing interview questions are virtually limitless. Although it's possible that you won't utilize all of the aforementioned responses in your interview, we do hope that they still provide you with a thorough understanding of Web API testing and prepare you for upcoming jobs and projects involving API testing. 

Well, it is known that practical experience with such technology is the only way to obtain application expertise. Although interviewers are more interested in checking your knowledge and experience, the fundamentals are still crucial. Along with REST API, if you want to learn more about programming courses, check out KnowledgeHut's courses. 

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