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From Amir Nashat <>
Subject CloudStack Allocator Pre-Condition Check
Date Wed, 08 Aug 2012 20:58:05 GMT

I would like to get feedback as to the best approach for performing a pre-condition check
before allocating a guest vm to a physical host. For my example, a pre-condition check is
to verify the guest vm does not use an existing DNS entry. If the pre-condition fails we do
not want to allocate the guest vm to the physical host and we want to record this as an error

As far as I can tell the only option I have is to extend the default FirstFitAllocator class
and override the allocateTo methods to first perform my pre-condition verification and if
it passes then to call the the overridden allocateTo() method. If it fails then I would throw
a RuntimeException of some sort or possibly set the returnUpTo value equal to 0.

Can you please advise as what would be the best approach? If anyone else is performing pre-conditions
checks in their allocation logic before provisioning a guest VM please let me know of your

Thank you,


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