Track dead WebDriver instances during parallel tas

问题:

I am seeing some dead-instance weirdness running parallelized nested-loop web stress tests using Selenium WebDriver, simple example being, say, hit 300 unique pages with 100 impressions each.

I'm "successfully" getting 4 - 8 WebDriver instances going using a ThreadLocal<FirefoxWebDriver> to isolate them per task thread, and MaxDegreeOfParallelism on a ParallelOptions instance to limit the threads. I'm partitioning and parallelizing the outer loop only (the collection of pages), and checking .IsValueCreated on the ThreadLocal<> container inside the beginning of each partition's "long running task" method. To facilitate cleanup later, I add each new instance to a ConcurrentDictionary keyed by thread id.

No matter what parallelizing or partitioning strategy I use, the WebDriver instances will occasionally do one of the following:

• Launch but never show a URL or run an impression
• Launch, run any number of impressions fine, then just sit idle at some point

When either of these happen, the parallel loop eventually seems to notice that a thread isn't doing anything, and it spawns a new partition. If n is the number of threads allowed, this results in having n productive threads only about 50-60% of the time.

Cleanup still works fine at the end; there may be 2n open browsers or more, but the productive and unproductive ones alike get cleaned up.

Is there a way to monitor for these useless WebDriver instances and a) scavenge them right away, plus b) get the parallel loop to replace the task segment immediately, instead of lagging behind for several minutes as it often does now?

回答1:

Since there is no OnReady event nor an IsReady property, I worked around it by sleeping the thread for several seconds after creating each instance. Doing that seems to give me 100% durable, functioning WebDriver instances.

Thanks to your suggestion, I've implemented IsReady functionality in my open-source project Webinator. Use that if you want, or use the code outlined below.

I tried instantiating 25 instances, and all of them were functional, so I'm pretty confident in the algorithm at this point (I leverage HtmlAgilityPack to see if elements exist, but I'll skip it for the sake of simplicity here):

public void WaitForReady(IWebDriver driver)
{
    var js = @"{ var temp=document.createElement('div'); temp.id='browserReady';" +
             @"b=document.getElementsByTagName('body')[0]; b.appendChild(temp); }";
    ((IJavaScriptExecutor)driver).ExecuteScript(js);

    WaitForSuccess(() =>
    {
        IWebElement element = null;
        try
        {
            element = driver.FindElement(By.Id("browserReady"));
        }
        catch
        {
            // element not found
        }

        return element != null;
    },
    timeoutInMilliseconds: 10000);

    js = @"{var temp=document.getElementById('browserReady');" +
         @" temp.parentNode.removeChild(temp);}";
    ((IJavaScriptExecutor)driver).ExecuteScript(js);
}

private bool WaitForSuccess(Func<bool> action, int timeoutInMilliseconds)
{
    if (action == null) return false;

    bool success;
    const int PollRate = 250;
    var maxTries = timeoutInMilliseconds / PollRate;
    int tries = 0;
    do
    {
        success = action();
        tries++;
        if (!success && tries <= maxTries)
        {
            Thread.Sleep(PollRate);
        }
    }
    while (!success && tries < maxTries);
    return success;
}

The assumption is if the browser is responding to javascript functions and is finding elements, then it's probably a reliable instance and ready to be used.

回答2:

I was having a similar problem. It turns out that WebDriver doesn't have the best method for finding open ports. As described here it gets a system wide lock on ports, finds an open port, and then starts the instance. This can starve the other instances that you're trying to start of ports.

I got around this by specifying a random port number directly in the delegate for the ThreadLocal<IWebDriver> like this:

        var ports = new List<int>();
        var rand = new Random((int)DateTime.Now.Ticks & 0x0000FFFF);

        var driver = new ThreadLocal<IWebDriver>(() =>
        {
            var profile = new FirefoxProfile();
            var port = rand.Next(50) + 7050;
            while(ports.Contains(port) && ports.Count != 50) port = rand.Next(50) + 7050;
            profile.Port = port;
            ports.Add(port);
            return new FirefoxDriver(profile);
        });

This works pretty consistently for me, although there's the issue if you end up using all 50 in the list that is unresolved.