Techniques are described for placing virtual machines (VM) on computer hosts. In one embodiment, a user may compose a constraint specification document which includes VM and host properties and how they are retrieved, as well as constraint predicates that define valid VM placements on hosts. Use of the constraint specification document permits new constraints, including constraints that involve new properties, to be handled without requiring changing the underlying code for collecting required input data and processing said data to determine whether placement constraints are satisfied. Instead, based on the constraint specification document, a resource scheduler or high availability module may program a programmable data collector to fetch the needed properties from the appropriate places. Then, the resource scheduler or high availability module may parse the constraint predicates, evaluate potential placements to determine whether the constraint predicates are satisfied, and place VMs according to placements that satisfy the constraint predicates.