Several constraints must be fulfilled for a device filter to be valid. This includes constraints that apply to the structure of the device filter's expression tree as well as constraints defined by the individual device filter term types used in a device filter.

Hard & soft validity constraints

The Client Service will check some of the validity constraints before each execution of a device filter (hard validity constraints) and will break the execution if one of these constraints is violated. Other validity constraints will, by default, not be checked before execution of the device filter (soft validity constraints). The result of an execution of a device filter that violates a soft validity constraint will however yield results that are probably different than the user intended.

A full check of all validity constraints (hard & soft) is offered by the Client Service as a separate endpoint. The separation between hard and soft validity constraints is implemented to provide the fastest possible execution of device filters.

Expression tree constraints

•Mandatory root node (hard validity constraint): The expression tree must have a root node.

•No missing leaf nodes (hard validity constraint): The expression tree of a device filter must have at least one term beneath each logical operation node.

Constraints on filter terms

Each device filter term type defines its own validity constraints. See the individual device filter term type documentation for more details.