Selenium Grid: MaxSessions vs MaxInstances

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I was wondering if anybody could shed some light on a Selenium question that has been giving us a bit of head scratching.

We’re confused on meaning of MaxSession and MaxInstances of Selenium Grid. We think that the MaxSession is the total number of test sessions that can run on a single node. And we also think that the MaxInstances is the total number of browsers that a test can open.

Or is MaxInstances the total number of browsers available to the node?

The command that we are using is:

java -Xrs -jar selenium-server.jar -role node -port 44506 -hub http://localhost:44500
/grid/register -firefoxProfileTemplate SeleniumProfile -timeout 300000 -browser
"browserName=firefox,maxInstances=10,platform=ANY,seleniumProtocol=WebDriver" -browser
"browserName=chrome,maxInstances=10,platform=ANY,seleniumProtocol=WebDriver"

We think the way we are using our node (above) is 5 concurrent test sessions by default.

Does each test have 20 browsers available to it?

Or does each test session share the 20 browsers (10 chrome/10 FF) in a pool – with the other test sessions?

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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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