Xml and More: JMeter—Use Extractors (Post-Processor Elements) for Correlation

What Is Correlation

Correlation is the process of capturing and storing the dynamic response (e.g., "Session ID" in the above diagram) from the server and passing it on to subsequent requests. A response is considered dynamic when it can return different data for each iterating request, occasionally affecting successive requests. Correlation is a critical process during performance load test scripting, because if it isn’t handled correctly, your script will become useless.

Correlation is 2-step process:

Parse and extract the dynamic value from the response of a step using a Post Processor element such as:

Refer the extracted value in the request of a subsequent step

http://.../.../...?sessionID=${sessionId}#....

How to Use Regular Expression Extractor

Watching above video, you can learn how to create Regular Expression Extractors in JMeter in the following steps:^[2]

Create a Test Plan where you want to do dynamic referencing in JMeter
Add Regular Expression Extractor in the Step from where response value(s) needs to be extracted

You can use RegExr—an online tool—to learn, build, and test Regular Expressions

Refer the extracted value (referred by Reference Name) in subsequent step(s)
Run and validate it

You can use RegExp Tester in View Results Tree, to test regular expressions directly on response data

How to Use CSS/JQuery Extractor

Watching above video, you can learn how to create CSS/JQuery Extractors in JMeter in similar steps:^[8]

Create a Test Plan where you want to do dynamic referencing in JMeter
Add CSS/JQuery Extractor^[9]in the Step from where response value(s) needs to be extracted

You can find a detailed explanation of CSS syntax here. jQuery's selector engine uses most of the same syntax as CSS with some exceptions. For selecting an arbitrary locator, you can use field Match No. with the ‘0’ value, which returns a random value from all found results.
It is also worth mentioning there is a list of very convenient browser plugins to test CSS locators right into your browser. For Firefox, you can use the ‘Firebug’ plugin, while for Chrome ‘XPath Helper’ is the most convenient tool.