Selenium Grid: MaxSessions vs MaxInstances

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Nice question….i would say it’s bit confusing…. But will try to answer it in simple terms..

MaxInstances This says….how many instances of same version of browser can run over the Remote System.

For example, i have a FF12,IE and i declared the command as follows
-browser browserName=firefox,version=12,maxInstances=5,platform=LINUX
-browser browserName=InternetExplorer,version=9.0,maxInstances=5,platform=LINUX

So i can run 5 instances of Firefox 12 and as well as 5 instances of IE9 at the same time in remote machine. So total user can run 10 instances of different browsers (FF12 & IE9) in parallel.

MaxSession This says….how many browsers (Any Browser and any version) can run in parallel at a time in the remote system. So this overrides the Max Instances settings and can restrict the number of browser instances that can run in parallel.

For above example, when maxSession=1 forces that you never have more than 1 browser running. 

With maxSession=2 you can have 2 Firefox tests at the same time, or 1 Internet Explorer and 1 Firefox test).

Irrespective of what MaxInstances you have defined.

For more clear info do visit – https://seleniumhq.github.io/docs/grid.html

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