Monday, April 11, 2016

An "Ideal" Interview with the Tester for Performance Engineering Position Part 1

Interviewer: Hello Mr. Tester. How are you?

Candidate: Hello Sir, Thank you for asking. I am really good. How are you?

Interviewer: I am fine. Thanks. Did you find the office conveniently?

Candidate: Yes sir, address was elaborated very clearly and the map helped a lot.

Interviewer: Great. What would you like to drink? Coffee, Tea?

Candidate: Sir a glass of water will be fine.

Interviewer: Sure. (Rang the bell and asked peon to deliver a glass of water. Peon delivered it and candidate drank it)

Interviewer: So Mr. Tester, Let me introduce myself. I am Mr. QA Manager. I am working here since last 5 years. I have a team of 6 Manual testers and 2 automation engineers. We are now looking to expand our team and lately we are trying to hire a performance engineer. Your resume showed you are well versed with performance testing and you have listed different tools that you are using in your current organization. We will come to that later, but first I really want to hear your introduction from you. So can you please tell me briefly about yourself?

Candidate: Sure sir (nervously folding the hands and leaning forward a little bit). My name is Mr. Tester and I am working in the software quality assurance field since last 3 years. I graduated in 2012. I was fortunate to be given performance testing chance since very early in my career. I have done performance testing on around 10 Web applications. The user load varies from 25 users to 1 million users. I got a chance to work on difference performance testing tools in my tenure. Now I am looking for better opportunity and to expand my learning and growth.

Interviewer: That's great. You mentioned user load in your answer. That is interesting. I always get confused between performance testing and load testing. Can you help me a little and highlight the difference between performance and load testing.

Candidate: (With a confident voice). That is very easy sir. In performance testing, we test about non-functional aspects like speed and like like ... scalability and ....stability. We test about…mmm response times, throughput and resource utilization levels that should meet the performance objectives of our application. You can say that performance testing is the superset of all other subcategories of performance-related testing.

Load testing is the subcategory of performance testing. In Load testing, we validate that when the application is subjected to expected user load, how it will behave in terms of performance.

Interviewer: And what is stress testing?

Candidate: Stress testing is also a subcategory of performance testing. In Stress testing, we test the application under user load which is beyond our normal and peak load expectation.

Interviewer: Can you tell the difference between load testing and stress testing by giving an example?

Candidate: Sure sir. For example, we want to test the performance of an e-commerce application. We were expecting that when we launch that e-commerce store, maximum around 500 users will access that website in 1 hour. We designed the website with that maximum number in mind.

500 users is the peak load condition for our website and 100 to 300 users is the normal load condition of our website. So we should test the application for at least what we are expecting. So we will use a tool and simulate 100 users, then 200 users, then 300 users up to 500 virtual users and we will validate that the response time of web pages, CPU and Memory utilization of server should be in acceptable limits at each user range. In simple words, we verify the behavior of our application under normal and peak load conditions. This is Load Testing.

When we want to stress test our application, we will increase the user load to more than 500 users and we will see how our application reacts. For stress, it is not necessary to increase just the user load. We can limit the memory of server or we can make the disk space insufficient just to see that how our application reacts. Does it crash? And does it crash gracefully or not? What will be the response times of web page in these stressful conditions? Etc.

Interviewer: Good example and nice answer. It seems that your concepts are clear in these terminologies. Let me ask you about some more terminologies of performance testing. Tell me about endurance testing and spike testing.

Candidate: Endurance testing is the subset of Load Testing. When we put our application under normal and peak conditions over an extended period of time it becomes endurance testing.

Spike testing is the subset of Stress Testing. When we repeatedly put stress on our application for short period of time, it becomes spike testing.

Interviewer: Great. What about capacity testing?

Candidate: Capacity testing is done in conjunction with capacity planning. With capacity testing, we determine the ultimate breaking point of the application. With capacity planning we plan by knowing how much additional resources (such as memory, processing power) are necessary to support that much load. Capacity testing helps us to determine a scaling strategy as to whether we should scale up or scale out.

Interviewer: What is the difference between scale-up and scale-out?

Candidate: Scale up is also knows as vertical scaling in which we add more resources to the single server such as more RAM and more CPU power etc. When we scale out, it means we add another server and we make our environment distributed. Now the load will be distributed to 2 machines. Scaling out is also called horizontal scaling. be continued......
For Part 2, Click here

Note: Please give your feedback and other interview questions for which you wish to see the answers here.


  1. Very informative knowledge in the form of in the form of interview questions. Really like the idea.

    If you please include following questions and provide their answers too. It will be very helpful

    1. Before starting load testing of a website, what are the important checks that you need to perform?
    2. Is there any formula through which you can check before executing load test that if the current resources are good enough to execute performance test? (like webpage size, processor, ram etc).

    1. Thanks bilal. I will sure include these types of questions in the next part.

  2. Very unique and effective approach of sharing information. Really enjoyed reading your blog, as always. Will look forward for next episode of interview :)

  3. Very well crafted interview. Keep up the good work!

  4. Very informative saad, can you explain how can we determine the resource(memory,processor etc) required for an application and on which parameters it is dependent.

  5. Really nice way of explaining the concept.. Tester is really very intelligent.

  6. Cool. I like the idea of preparing myself for an interview and this one give an insight that what may be facing and it is a step ahead in preparing for it. Even if is not for performance. Thanks for sharing.


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