Controllers

JMeter has two types of Controllers: Samplers and Logical Controllers. These drive the processing of a test.

Samplers tell JMeter to send requests to a server. For example, add an HTTP Request Sampler if you want JMeter to send an HTTP request. You can also customize a request by adding one or more Configuration Elements to a Sampler. For more information, see Samplers.

Logical Controllers let you customize the logic that JMeter uses to decide when to send requests. For example, you can add an Interleave Logic Controller to alternate between two HTTP Request Samplers. For more information, see Logical Controllers.

Samplers

Samplers tell JMeter to send requests to a server and wait for a response. They are processed in the order they appear in the tree. Controllers can be used to modify the number of repetitions of a sampler.

JMeter samplers include:

• FTP Request
• HTTP Request
• JDBC Request
• Java object request
• LDAP Request
• SOAP/XML-RPC Request
• WebService (SOAP) Request

Each sampler has several properties you can set. You can further customize a sampler by adding one or more Configuration Elements to the Test Plan.

If you are going to send multiple requests of the same type (for example, HTTP Request) to the same server, consider using a Defaults Configuration Element. Each controller has one or more Default elements (see below).

Remember to add a Listener to your test plan to view and/or store the results of your requests to disk.

If you are interested in having JMeter perform basic validation on the response of your request, add an Assertion to the sampler. For example, in stress testing a web application, the server may return a successful “HTTP Response” code, but the page may have errors on it or maybe missing sections. You could add assertions to check for certain HTML tags, common error strings, and so on. JMeter lets you create these assertions using regular expressions.

Logic Controllers

Logic Controllers let you customize the logic that JMeter uses to decide when to send requests. Logic Controllers can change the order of requests coming from their child elements. They can modify the requests themselves, cause JMeter to repeat requests, etc.

To understand the effect of Logic Controllers on a test plan, consider the following test tree:

• Test Plan
• Thread Group
• Once Only Controller
• Login Request (an HTTP Request )
• Load Search Page (HTTP Sampler)
• Interleave Controller
• Search “A” (HTTP Sampler)
• Search “B” (HTTP Sampler)
• HTTP default request (Configuration Element)
• HTTP default request (Configuration Element)
• Cookie Manager (Configuration Element)

The first thing about this test is that the login request will be executed only the first time through. Subsequent iterations will skip it. This is due to the effects of the Once Only Controller.

After the login, the next Sampler loads the search page (imagine a web application where the user logs in, and then goes to a search page to do a search). This is just a simple request, not filtered through any Logic Controller.

After loading the search page, we want to do a search. Actually, we want to do two different searches. However, we want to re-load the search page itself between each search. We could do this by having 4 simple HTTP request elements (load search, search “A”, load search, search “B”). Instead, we use the Interleave Controller which passes on one child request each time through the test. It keeps the ordering (ie – it doesn’t pass one on at random, but “remembers” its place) of its child elements. Interleaving 2 child requests may be overkill, but there could easily have been 8, or 20 child requests.

Note the HTTP Request Defaults that belongs to the Interleave Controller. Imagine that “Search A” and “Search B” share the same PATH info (an HTTP request specification includes domain, port, method, protocol, path, and arguments, plus other optional items). This makes sense – both are search requests, hitting the same back-end search engine (a servlet or cgi-script, let’s say). Rather than configure both HTTP Samplers with the same information in their PATH field, we can abstract that information out to a single Configuration Element. When the Interleave Controller “passes on” requests from “Search A” or “Search B”, it will fill in the blanks with values from the HTTP default request Configuration Element. So, we leave the PATH field blank for those requests, and put that information into the Configuration Element. In this case, this is a minor benefit at best, but it demonstrates the feature.

The next element in the tree is another HTTP default request, this time added to the Thread Group itself. The Thread Group has a built-in Logic Controller, and thus, it uses this Configuration Element exactly as described above. It fills in the blanks of any Request that passes through. It is extremely useful in web testing to leave the DOMAIN field blank in all your HTTP Sampler elements, and instead, put that information into an HTTP default request element, added to the Thread Group. By doing so, you can test your application on a different server simply by changing one field in your Test Plan. Otherwise, you’d have to edit each and every Sampler.

The last element is a HTTP Cookie Manager. A Cookie Manager should be added to all web tests – otherwise, JMeter will ignore cookies. By adding it at the Thread Group level, we ensure that all HTTP requests will share the same cookies. 

Logic Controllers can be combined to achieve various results. See the list of built-in Logic Controllers.

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