Transaction Response Time in Load runner

Transaction Response Time :Time taken for the application to complete a transaction or business process.

Objective of measuring the Transaction Response Time – to ensure that the application is working perfectly under heavy load. However, the definition of heavy load may vary from system to system. In Loadrunner by defining an initial acceptable response time, we can benchmark the application if it is performing as anticipated.

Measuring transaction response time is important as it gives an idea of how the aplication is performing in terms of time. This performance parameter can be related to the end users at the time of processing request. It helps in understanding how their application performed.

Transaction Response Time is the sum of

1. Time taken for the request made to the web server
2. Time taken by the web server to process
3. Time taken by the application server to process
4. Time taken by the database server to process
5. Time taken for the procesed request to come back to the client / user through various routes
6. Time taken for the request or data in the network transmission is also considered in this – Network latency between the servers and the client

Measuring Transaction Response Time in LoadRuner: 

It begins when the defined transaction makes a request to the application. From here, till the transaction completes before proceeding with the next subsequent request (in terms of transaction), the time is been measured and will stop when the transaction completes.

What is Little's law?

 Little's Law:

Little's law is quite simple and intuitively appealing.

The law states that the average number of customers in a system (over some time interval), N, is equal to their average arrival rate, X, multiplied by their average time in the system, R.

N = X . R (or) for easy remembrance use L = A . W

This law is very important to check whether the load testing tool is not a bottleneck.
For Example, in a shop , if there are always 2 customers available at the counter queue , wherein the customers are entering the shop at the rate of 4 per second , then time taken for the customers to leave from the shop can be calculated as

N = X. R
R = 2/4 = 0.5 seconds

A Simple example of how to use this law to know how many virtual users licenses are required:

Web system that has peak user load per hour = 2000 users
Expected Response time per transaction = 4 seconds
The peak page hits/sec = 80 hits/sec

For carrying out Performance tests for the above web system, we need to calculate how many number of Virtual user licenses we need to purchase.

N = X . R
N = 80 . 4 = 320

Therefore 320 virtual user licenses are enough to carry out the Load Test.