device properties and change events

I'm using udev-147 in CentOS 6.3.

I have two rules that import properties into the udev db. I'll use a
simplified example.

    IMPORT{program}="program1 %k"
    ACTION=="add", IMPORT{program}="program2 %N"

program1 exports MY_FOO, and program2 exports MY_BAR. I've noticed that
when a change event is triggered, MY_BAR is no longer available in the
udev database. So it appears change events clear the existing environment.
The work done in program2 is relatively expensive so I'd rather only run
it when the device is first added.

It looks like I can re-import MY_BAR if I use IMPORT{db} in a separate
rule:

    IMPORT{program}="program1 %k"
    ACTION=="add", IMPORT{program}="program2 %N"
    ACTION=="change", IMPORT{db}="MY_BAR"


Is this the correct approach? Or is there another way to persist the
properties that were imported during add, besides re-importing program2?

Also, is it possible to determine *what* causes a change event to occur?
With 'udevadm monitor' I can see change events being triggered whenever my
application exits (for the devices it has opened), and would like to
determine why. I was hoping udev could give me a hint.

-Keith

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