Load Testing

Introduction

Load testing generally refers to the practice of modeling the expected usage of a software program by simulating multiple users accessing the program's services concurrently. As such, this testing is most relevant for multi-user systems, often one built using a client/server model, such as web servers. However, other types of software systems can be load-tested also.

For example, a word processor or graphics editor can be forced to read an extremely large document; or a financial package can be forced to generate a report based on several years' worth of data. The most accurate load testing occurs with actual, rather than theoretical, results.

When the load placed on the system is raised beyond normal usage patterns, in order to test the system's response at unusually high or peak loads, it is known as stress testing. The load is usually so great that error conditions are the expected result, although no clear boundary exists when an activity ceases to be a load test and becomes a stress test.

There is little agreement on what the specific goals of load testing are. The term is often used synonymously with performance testing, reliability testing, and volume testing.

Testing

Load and Performance testing is to test software intended for a multi-user audience for the desired performance by subjecting it with an equal amount of virtual users and then monitoring the performance under the specified load, usually in a test enviromnent identical to the production, before going live.

For example if a web site with a shopping cart is intended for 100 concurrent users who are doing the following functions:

* 25 VUsers are browsing through the items and logging off
* 25 Vusers are adding items to the shopping cart and checking out
and logging off
* 25 VUsers are returning items previously purchased and logging off
* 25 VUsers are just logged in without any activity

Using various tools available to generate these VUsers the application is subjected to a 100 VUser load as shown above and its performance is monitored. The pass fail criteria is different for each individual organization and there are no standards on what an acceptable criteria should be, across the board.

It is a common misconception that these are record and playback tools like regression testing tools, however it must be clarified that the similarity ends just there. The Load testing tools work at the protocol level where as the regression testing tools work at the GUI object level. To give an example a regression testing tool will simulate a mouse click on an OK button on the browser, but a load testing tool will send out the hypertext that the browser will send after the user clicks the OK button, and again it will send out the hypertext for multiple users each having a unique login ID and password.

Tools

Various tools are also available to find out the causes for slow performance which could be in the following areas:

* Application
* Database
* Network
* Client side processing
* Load Balancer


Input

The following are useful inputs for load-testing a Web application:

* Performance-critical usage scenarios
* Workload models
* Performance acceptance criteria
* Performance metrics associated with the acceptance criteria
* Interview feedback from the designer or developer of the Web application
* Interview feedback from end users of the application
* Interview feedback from the operations personnel who will maintain and manage the application

Output

The main outcomes that load testing helps you to accomplish are:

Updated test plans and test designs for load and performance testing
Various performance measures such as throughput, response time, and resource utilization
Potential bottlenecks that need to be analyzed in the white-box testing phase
The behavior of the application at various load level

Approach for Load Testing

The following steps are involved in load-testing a Web application:

Step 1 - Identify performance acceptance criteria
Step 2 - Identify key scenarios
Step 3 - Create a workload model
Step 4 - Identify the target load levels
Step 5 - Identify metrics
Step 6 - Design specific tests
Step 7 - Run tests
Step 8 - Analyze the results

Summary

Load testing helps to identify the maximum operating capacity of the application and any bottlenecks that might be degrading performance. The basic methodology for performing load testing on a Web application is to identify the performance-critical key scenarios; identify the workload profile for distributing all the load among the key scenarios; identify metrics that you want to collect in order to verify them against your performance objectives; create test cases that will be used to simulate
the load test; use tools to simulate the load according to the test cases and capture the metrics; and finally, analyze the metrics data captured during the tests.