(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

(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

(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

(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

For data collection, it is a common situation to work with a laptop or a phone offline in the field. Upon returning to the network, the changes need to be synchronized with the master data source, e.g. a PostGIS database. If several persons are working simultaneously on the same datasets, it is difficult to merge the edits by hand, even if people don’t change the same features.

Therefore, Mathias Walker implemented an offline plugin for QGIS. This plugin automates the synchronisation by copying the content of a datasource (usually PostGIS or WFS-T) to a spatialite database and storing the offline edits to dedicated tables. After being connected to the network again, it is possible to apply the offline edits to the master dataset.

To give the plugin a try, unpack the sources, apply the patch ‘qgissvn.diff’ to a current svn version of QGIS. Then copy the offlineediting folder to $PREFIX/src/plugins and recompile QGIS.

The usage of the plugin is straightforward:

• Open some vector layers, e.g. from a PostGIS or WFS-T datasource
• Save the project
• Press the ‘Convert to offline project’ button and select the layers to save. The content of the layers is saved to spatialite tables.
• Edit the layers offline
• After being connected again, upload the changes with the ‘Synchronize’ button

Screenshot

Presumably, the offline editing plugin will be part of the next QGIS version (1.6)

In cartography, it is a frequent operation to set labels to fixed positions, together with the position of the fix point (left/middle/right, Top, Half, Bottom) that is kept constant in case of font change, rotation or zoom. Therefore, three new editing tools to manipulate text labels are now in the QGIS developer version:

1. the move label tool drags text labels to a new position

1. the rotate label tool is for interactive rotation of labels
2. the label property tools opens a dialog that lets the user manipulate the data defined properties of a label (and also the text of the label attribute)

All three tools work on the new labeling engine and data defined labeling needs to be enabled for the layer (e.g. x coordinate attribute / y coordinate attribute for the move tool, rotation for the rotate tool). Additionally, the layer needs to be in edit mode. The new tools are well suited to mix fixed label positions and automated label opsitioning in the same or among several layers. If the x- or y attribute value is NULL, the position is set automatically by the pal library (http://pal.heig-vd.ch/). As soon as a position is manipulated by the move label tool, the position is written into the attribute field and the label position for this feature is fixed. So if a layer does not yet have attribute fields for x, y, you could create two new fields of type double (using the buttons in the vector properties dialog or in the attribute table). Initially, all values will be ‘NULL’ and all the label positions set automatically.

There are further plans to improve the user interface. It could be handy to have the properties dialog always open (non-modal), which would allow faster edits of a large number of labels. And a live text rotation preview is planned too. And yes, if someone likes making real icons, this would be highly appreciated (my graphical skills are somewhat limited…).

Finally I’d like to thank the city of Thun (Switzerland) for funding these tools and sharing it with the rest of the FOSSGIS world.

The Sunday night session of the QGIS hackfest resulted in a new release of the Mapfile Tools plugin.

This QGIS plugin allows you to display an UMN Mapserver mapfile in QGIS without running a Mapserver instance. It depends only on Mapscript (apt-get install python-mapscript on Debian/Ubuntu) and allows you to zoom and pan on the mapfile layer.

In release 0.6, an output window has been added, which shows error messages and detailed layer information. This makes it a convenient tool to test your mapfiles.

Marco, Matthias and me spent three days at the QGIS hackfest in Wroclaw (pictures). There I got the time to work on the QGIS globe plugin and made a presentation of the current state.

As soon as the threading branch (Martin Dobias’ Google of Summer project) is merged into trunk, the globe should make its way into trunk as well. In the meantime you can compile the QGIS branch from guthub to test the globe. Thanks Vincent for writing step by step build instructions.

Recently I had to pre-select a Node inside a TreePanel ExtJS widget. I tried many ways but failed because most of the time when I tried to:

  node.select();

that node would not yet be rendered into the browser's DOM and so the select would fail somewhere inside the extjs.js blob with something like "this ... undefined ...". What I needed was a "rendered" event, but there doesn't seem to be such an event for neither TreePanel not TreeNode or any of their superclasses. Diving into ExtJS code was not really very helpful because it's a framework that does things through layers and layers and as such is not trivial to understand quickly.

Thus, the same ole problem with JavaScript as ever: "show me all the events there are". However, surprisingly, in contrast to standard JavaScript, ExtJS has an easy standard way to accomplish this:

 
  Ext.util.Observable.capture( myTree, function(event) {                                                                                                        
                                         console.log("got an event in myTree");
                                         console.log(event); });

And quickly I discovered that there indeed is an obscure event that I can, out of alternatives, missuse to do what I want, which is expandnode.

Aparently, after a node is exapanded, all its children are put into the DOM and seem to be manipulable then. Thus:

        /*
         * select node with real_id - this is only called once
         * after the tree is rendered for the first time.
         * After that the listener itself is unregistered.
         */
        function select_node(node) {
          node.eachChild( function(child) { 
            if(child.attributes.real_id == real_id ) {
              child.select();
              categories_panel.un('expandnode', select_node);
            }
          });
        };
        myTree.on('expandnode', select_node);

Tomáš Pospíšek

Problem statement:

Which of our strings are not translated yet in our Ruby on Rails app?

Unfortunately there’s no easy way to know for sure. One solution is to log untranslated strings as soon as they appear - the following solution applies to the spree-i18n extension but should be easily adaptable to other contexts.

What we do here is monkey patching I18n#t, to check whether the original I18n#t told us that there’s no translation and log it in that case. Then we return whatever the original I18n#t gave us:

I18n.class_eval do                                                                                    

      class << self                                                                                   
        alias_method :alias_for_t, :t                                                                 
      end                                                                                             

      # make t log missing translations                                                               
      def self.t( keyz, options = {})                                                                 
        translation = self.alias_for_t( keyz, options )                                               
        if translation =~ /translation missing:/                                                      
          MISSING_TRANSLATION.info translation                                                        
        end                                                                                           
        return translation                                                                            
      end                                                                                             
    end                                                                                               

end

To initialize the logger:

class MissingTranslationLogger < Logger
  def format_message(severity, timestamp, progname, msg)
    "#{msg}\n" 
  end 
end

logfile = File.open('log/missing_translations.log', 'a')    
# optional: # logfile.sync = true
MISSING_TRANSLATION = MissingTranslationLogger.new(logfile)

You’ll need to place those two code snipplets in apropriate places. I’ve put the first one in vendor/extensions/site/site_extension.rb and the second one into config/initializers/missing_translation_logger.rb.

Hope somebody’ll find this snippet useful.

Tomáš Pospíšek

Here are the slides from our presentation at FOSS4G 2010 in Barcelona:

SpatiaLite, the Shapefile of the future?