Testing with pywbem

Recently a colleague pointed out that our VirtualSystemManagementService’s RemoveResourceSettings method (in libvirt-cim) was marked with the wrong type.  This was done early on, and probably was just a mistake, given that AddResourceSettings and ModifyResourceSettings both took the same type: an EmbeddedObject.  RemoveResourceSettings takes a reference, since anything you could possibly delete would be representable as a reference.  It’s the right way, but annoying, given that I had already built a common resource management infrastructure around the idea of having a list of resource objects to (Add,Modify,Remove).

Anyway, I coded up the change to the provider itself and set out to test it.  I quickly realized that I didn’t know what the CIMXML for a method call with an array of references looked like.  Up to this point, I have been unit testing the method providers by just hacking up a CIMXML template and stuffing it into wbemexec or wbemcat.  It sucks, but it works.

However, this time I was stuck.  I didn’t have an example, and some guessing didn’t work in a short amount of time.  I decided to try to work up something in pywbem to see if I could get not only the method call made for testing, but also a glimpse at what the CIMXML for it looked like.  I ended up with this very small bit of python code that was extremely helpful:

from pywbem import WBEMConnection, CIMInstanceName

c = WBEMConnection(“http://localhost”,
(“root”, “password”Smilie: ;),
“root/virt”Smilie: ;)
c.debug = True

r = CIMInstanceName(“Xen_DiskResourceAllocationSettingData”,

res = c.InvokeMethod(“RemoveResourceSettings”,

print c.last_request

This connects to localhost, creates a reference, and makes the method call with that reference as a single-element array.  After the operation, the “last_request” field contains the CIMXML it sent to the server, but only if the “debug” property is set to True (something the documentation conveniently leaves out).  The result (with the boiler-plate cut out) looks like this:

<PARAMVALUE NAME=”ResourceSettings” PARAMTYPE=”reference”>
<INSTANCENAME CLASSNAME=”Xen_DiskResourceAllocationSettingData”>

Not that I really need it at this point, given that the pywbem case was so easy to write.  However, it provides a good example of how to not only use pywbem to figure out the proper CIMXML for an operation, but also debug it if it’s wrong.

Category(s): Codemonkeying
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