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JMeter Series

This is a short series of howtos for and a critique of JMeter v2.4.

The following articles have been done:

Tomáš Pospíšek, 4.1.2011

Working with big files/calling external scripts in JMeter

(This article is part of the JMeter Series)

JMeter’s “HTTP Request Sampler” will read the whole input (the downloaded page, pdf, movie etc.) into its main memory. That means that working with larger (f.ex. movies) files will bring JMeter, the JVM and/or many machines to their memory limits.

If you do not not need to do more than making sure that something gets downloaded, then you can “offload” the download work to an outside process, f.ex. to curl or wget:

A few things to note:

  • I used the “BeanShell Sampler” module, since both the “BeanShell Pre-” and the “PostProcessors” are only executed when there’s something to process after another sampler produced some data.

  • As you can see, we are passing a JMeter variable to the script

  • Aparently JMeter or the BeanShell do not care much about the output of the exec’ed script, thus you won’t see anything inside the “Response data” in the “View Results Tree Sampler”. I don’t know whether that behaveour is a missing feature, a bug or for some unknown reason intended as such.

You can download this JMeter test from here

Tomáš Pospíšek, 30.12.2010

Waiting for a page change in JMeter

(This article is part of the JMeter Series)

While testing a Rails application there was a situation where a background worker (DJB or background task) would get an order to execute and eventually the completed order would appear on a page.

Thus we needed to wait for the page to be updated and continue the test after.

User Defined Variables

I’m predefining used variables here - see “We set up our variables” in the Extracting text from a page and using it somewhere else in JMeter article for an exaplanation why.

Clear Loop Variable

This step is not strictly necessary in our example here, however if you want to reuse this sequence of steps more than one time - that is call it from different places, then you need to clear the loop variable first.

Debug

Since creating these steps was not easy, I’ve put a debug statement in so that I see, when JMeter calls into these steps and to see how the variables are set.

The While Controler

Here we really start the loop.

I’m not sure whether if would be better to use BeanShell commands here instead - using JavaScript to test the loop condition works in any case…

Getting the page

We get the page we want to check.

Extract the data we want to check from the page and assign it to our loop variable

Make sure our request is not being cached

It’s better - as far as we can - to be sure we force the web application to recreate the page we’re waiting for to change. I’m not sure I’ve done it right here, however it works:

Wait a bit before re-looping

We wait two seconds here before retrying.

You can download this JMeter test from here

Tomáš Pospíšek, 4.1.2011

Extracting text from a page and using it somewhere else in JMeter

(This article is part of the JMeter Series)

In the following we’ll do these things:

  1. we go to a form submit page
  2. we order something
  3. we get that something we ordered

In more detail:

We use a “Cookie Manager” to carry forward cookies between calls

We set up our variables

The idea here being that we predefine the variables, so they would show up in the “Debug Sampler” which makes debugging easier, because you see at each step, whether the variable has the correct value or not. I do predefine the variables with a “non-value” so that I can see immediately whether the variable has been already assigned something during the execution of the test or not.

We go to a form submit page

We extract the AUTHTOKEN from that form

(The auth token is being used by the web application to prevent cross-site-scripting)

Notice that the extractor is set to extract the string from the HTTP reply-body.

We also extract the session cookie

This is because we want to pass the cookie to an outside application, so that it can call the web app from within the same session.

Notice that this time we told the extractor to extract the string from the HTTP reply-headers.

We now use all our extracted parameters to submit our order

Note that:

  • we are passing the AUTHTOKEN along with the HTTP POST
  • since we are not sure whether possibly the AUTHTOKEN contains some problematic character (in my case it was an equal sign ‘=’ that was interferring with the parameter encoding) we tell JMeter to URL-encode the string.
  • we also pass along another parameter telling the web app what we’re interested in getting back from our order
  • we are using HTTPS

Submitting the order will redirect us to the page that’ll show us the resulting order.

We extract the ORDER_ID from HTTP headers which would otherwise redirect us to the resulting page

The headers contain the location of the resulting order page where we would be redirected to.

Finally we download the resulting artefact with curl

Note that:

  • we are passing the SESSION_COOKIE to curl to be able to download the artifact in the same (potentially authenticated) session.

  • we also construct the download URL from the ORDER_ID

You can download this JMeter test from here

Tomáš Pospíšek, 4.1.2011

The JMeter "Workbench", a trapdoor for the newbie

(This article is part of the JMeter Series)

Upon starting JMeter you’ll see two branches: “Test Plan” and “WorkBench”.

“Test Plan” is the place where your tests will live.

What the purpose of “WorkBench” is, is not really clear. It seems to be meant to be a place to do your throw-away experimentation.

The really crucial “trap” of the “WorkBench” is however, that JMeter will throw away whatever you put into the “WorkBench” upon exit. JMeter will not save the contents of the “WorkBench” if you tell it to “Save” your work and you’ll loose whatever is in there. It will only save the contents of the “Test Plan” branch.

So be ware of putting anything in the “Work Bench”. You’re bound to get burned.

Tomáš Pospíšek, 27.12.2010

Making your JMeter Test modular

(This article is part of the JMeter Series)

As tests get larger, or as steps need to be repeated you’ll want to structure your tests into distinct entities - these seem to be called “Modules” in JMeter.

However, there is no “Module” element JMeter. As a “Module” you can however use the “Simple Controller”. It allows you to drop other elements into it and to name them as a whole. I don’t know whether it has additional features such as providing scoping of any kind.

Thus if you need to “call” the same set of steps from different places, then you can place them in a “Simple Controller”. However you’ll need to place that controller somewhere and as such it will get executed in that place and order. You can prevent it being executed by disabling it:

even though being disabled, components inside a Controller still can be called and executed. Therefore, you can use a disabled “Simple Controller” as a repository for reusable components or modules:

On the picture you can see a “module repository” and a call to a specific “Module” inside it.

Tomáš Pospíšek, 30.12.2010

Debugging JMeter Tests

(This article is part of the JMeter Series)

Useful ways to debug JMeter as far as I know:

  • insert a “Debug Sampler”

the “Debug Sampler” will emit “everything that’s known” to JMeter. That output can be displayed in the “View Results Tree Listener”:

  • have a look at the JMeter log file (which is usually dumped from where you’ve started JMeter)

  • check the output of one of the listeners such as the “View Results Tree Listener”.

  • if you need to debug regexes of a “Regular Expression Extractor” then you can have a look at the source code of the page you want to extract a value from inside the “View Results Tree Llistener” and enter the same regular expression in the “Search” field (searching is only implemented in JMeter > v2.4).

In that picture you can see JMeter matching and displaying the regex entered in the search field inside an HTML page retrieved though the “HTTP Client Sampler”.

Tomáš Pospíšek, 1.1.2011

Some words on overall usefulnes of JMeter

(This article is part of the JMeter Series)

Purpose of JMeter

JMeter is a testing tool. It comes accross as a graphical tool, where you can half visually half through text define your test. Its roots seem to lie in web testing - that means testing a website on how long it takes to return pages, how well it does under stress, how well it scales with increasing numbers of parallel requests etc.

However JMeter can be both used non-visually and has extended far beyond web testing.

Stability of JMeter

While working on a test with JMeter 2.3.4 it has had a multitude of stability problems:

  • running out of heap space and crashing
  • running out of heap space while testing and thus not being able to correctly execute and finish tests
  • hanging and not being responsive any more
  • not being able to terminate its internal threads
  • not terminating launched outside processes
  • while saving the work damaging the saved file itself and rendering it unreadable and thus loosing the work

That means that you can expect JMeter to hang or crash at any point while developing, executing or saving your tests.

That means that you should really be saving (CTRL-S or Apple-S) very often. On the other hand saving often will increase the likeness of your save file itself being destroyed and your work lost (see last point above). Thus saving is not enough - you need to version your work too, in order to be able to access older versions, that are not damaged.

Running something like this from a Unix command line should be saving revisions of your current work file - you’ll need to be saving your work continuously though:

  $ revision=99
  $ save_interval=300 # seconds
  $ work_file="Load Test.jmx"
  $ while true; do
  >    cp "$work_file" "$work_file.$revision"
  >    echo "Saved "$work_file" under "$work_file.$revision"
  >    sleep $save_interval
  >    if [ "$?" != 0 ]; then break; fi
  > done

One problem that does not seem to be resolved within JMeter at all is “big” requests. The various Samplers seem allways to put whatever the get from the test target into memory. Thus doing tests on multimedia content with gigabyte sized files will make JMeter run out of memory. There does not seem to be a way to tell JMeter to throw away downloaded content and/or to treat it in flight and not to save it in its entirety.

Documentation

The online JMeter documentation is brief and is rather just mentioning features than describing them in depth. Also, it’s not easy at all to find ready made examples that demonstrate syntax and finer points of how to use JMeter. When accessing the help coming with JMeter itself the JMeter instance would just hang.

I did a lot of searching the web, without much success and a lot of trying.

Debugging

What do you do when tests don’t run the way you want or do unexpected things? Then you need a way to debug them. There are three debugging facilities of JMeter, that are not hard to access:

  • the Debug Sampler
  • the JMeter log file
  • the various Listeners

Here’s a snapshot of the Debug Sampler:

And a snapshot of the Debug Sampler displaying JMeter properties inside the “View Results Tree” Listener.

These facilities are very useful, however the various JMeter elements themselves are blackboxes - there seems to be no way to introspect them or “step through” them at runtime. When they “don’t work”, then either they might display something useful inside the JMeter Log or possibly you’ll be able to collect hints of why they might not work after they’ve run through the Debug Sampler results. If both approaches fail, then it’s not clear what else can be done other than pluging into the source code of JMeter itself.

Complexity

I’d describe working JMeter as “to do easy things is non-trivial and doing hard things is extremely demanding”

The various testing elements, such as “Logic Controllers”, “Listeners” etc. seem to have different scopes and different orders of execution.

“Listeners” for example seem to be global in scope, in that they “see” and can react on whatever happens in the whole of JMeter. They might be possibly limited to a “Thread Group” though, depending on where they are placed - I did not try to find out.

Doing slightly more complex things - such as a loop to repeat a row of steps multiple times, or executing external scripts does not seem to be supported within JMeter itself - one needs to ressort to nearby tools such as the Bean Shell and accordingly to learn the syntax and the working of that tool as well.

Same goes for “Extractors” which refer to a nearby tools that do Perl regexes or XPath matching.

In short to do even rather easy tests one needs to get to know a vast array of tools and syntaxes and how those various tools interact with each other.

The pros

All those negative points seem to weight quite heavily and they do. However after quite a lengthy and steep learning curve JMeter as a tool starts showing its strength. Once one has resolved a larger number of problems, adding more steps and functionality to JMeter starts getting easier and one productivity starts to increase.

It’s also fun working with such a complex, versatile and powerful tool and it’s fun to work with a tool that starts from a fresh and different perspective how to solve (visually) problems.

Conclusion

One thing that one gets for “free” with JMeter is the nice reports generated by the various included Listeners. It’s something that will certainly impress management and is useful for gaining a high level understanding of the performance of a web-site.

Other things - like traversing web sites, extracting and matching information might be just as well done with specialised tools coming along with Python, Ruby or command line tools available under Unix.

Ease of automation when using proper scripting languages and their effectiveness and efficiency will be hard to beat given that one allready has a good working knowledge of them.

To me it’s not really clear whether using a scripting language for the task of testing and profiling is not more efficient and more flexible than using JMeter.

However I do reccomend JMeter: it’s fun to work with and it does give a new perspective on how things can be done.

Tomáš Pospíšek, 28.12.2010

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