Intern Concurrency Problem

By  on  

Update: I've found the root issue and have detailed it at the end of this post.

Over the past year I've created and implemented Selenium testing on the Mozilla Developer Network using the Intern interface created by SitePen.  Intern's been awesome;  sure there's a learning curve with async JavaScript coding but it's simple when you get the hang of it.

One problem I encountered with functional testing via service like BrowserStack and Sauce Labs is that we get failures we generally don't get  when testing locally.  When I tested with one browser everything went well but testing multiple browsers sent our tests into a spiral of transient failures.  When I tweaked one setting, however, everything went to plan:

// Maximum number of simultaneous integration tests that should be executed on the remote WebDriver service
maxConcurrency: 1,

Setting the maxConcurrency value down to 1 was all we needed to do.  Instead of all browsers spawning at once, each test runs within one browser and then within another.  Bingo!

As to what was causing the ultimate issue, I'm not quite sure.  The tests included authorization and login testing so it's possible there were overlaps in signing in and out, causing confusion on the server side.  Regardless, if you need to get things moving quickly, limit the maxConcurrency setting and you may start seeing loads more test passes.

Update: Firefox + Focus + Selenium Bug

After loads of testing and digging, I found the root issue for my problems with tests passing when one browser is run and not when run concurrently: when Firefox is not the focused/"on top" browser, focus events don't get passed up. And in my specific case, I was testing CSS animations, and those don't occur when a browser isn't focused. Hopefully this bug is fixed in Selenium too!

Recent Features

  • By
    5 More HTML5 APIs You Didn’t Know Existed

    The HTML5 revolution has provided us some awesome JavaScript and HTML APIs.  Some are APIs we knew we've needed for years, others are cutting edge mobile and desktop helpers.  Regardless of API strength or purpose, anything to help us better do our job is a...

  • By
    Create a CSS Flipping Animation

    CSS animations are a lot of fun; the beauty of them is that through many simple properties, you can create anything from an elegant fade in to a WTF-Pixar-would-be-proud effect. One CSS effect somewhere in between is the CSS flip effect, whereby there's...

Incredible Demos

  • By
    Implementing Basic and Fancy Show/Hide in MooTools 1.2

    One of the great parts of MooTools is that the library itself allows for maximum flexibility within its provided classes. You can see evidence of this in the "Class" class' implement method. Using the implement method, you can add your own methods to...

  • By
    Xbox Live Gamer API

    My sharpshooter status aside, I've always been surprised upset that Microsoft has never provided an API for the vast amount of information about users, the games they play, and statistics within the games. Namely, I'd like to publicly shame every n00b I've baptized with my...


  1. Marco

    Yes, but your test suite will eventually take forever to finish.

    You could try improving the isolation of the different scenario’s. Have you tried using seperate users for each scenario?

    Best case you have a webservice for creating test users which you call at the beginning of every scenario.

    • I do use a web service to get test credentials for one test but I don’t want to flood our environments with test-only users.

  2. Marco

    Have you tried using separate users for each scenario?

    Sacrificing ui test concurrency is usually a bad idea, something you will definitely regret later.

    • Separate users would be a great idea and something I was hoping I wouldn’t have to do. Going to give that a shot today! :)

    • Marco

      Oops I accidentally double posted!

      Create a web-service to clean up the test-only users and use a new user for every scenario.
      That’s the safest approach

  3. Jeff

    Um… are you sure that your test failures aren’t a symptom of real race conditions in your code?

    • I think the race condition is login-related since testing the browsers individually works fine.

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!