Open Stack

On Wed, Feb 20, 2013 at 02:14:31PM +0100, Julien Danjou wrote:
> Hi,
> 
> Context: Ceilometer provides a special notifier driver for Nova,
> publishing the last counters before an instance get destroyed.
> 
> That means this code is run from `nova-compute' and uses some Ceilometer
> code. The problem is that when importing Ceilometer modules from this
> notifier, it ends up importing `ceilometer.openstack.common.rpc', which
> tries to register the `rpc_backend' option inside
> `oslo.config.cfg.CONF', object which already has this option defined by
> nova.openstack.common.rpc. So `DuplicateOptError' is raised, and
> badaboum.
> 
> It's likely we're a unique case because most projects using their own
> copy of oslo don't mix, but we do, and we're facing a regression now
> that oslo.config has been released in the wild.

IMHO the core issue here is that Ceilometer should not be putting
its code into Nova. Any code that is intended to run inside Nova
processes should be part of the Nova GIT repo, not the ceilometer
GIT repo. By mixing code from two projects you are exposing both
projects to each others' internal implementation details. Any
integration between the projects should be exclusively via well
defined RPC / REST APIs interfaces, rather than dropping in chunks
of external code.

This similar kind of problem was seen with Quantum where the Quantum
GIT repo held a number of VIF driver plugins for Nova, and these
VIF drivers hardcoded specific knowledge of Quantum requirements.
Of course when Nova code was refactored, this broke all the VIF
drivers stored in Quantum. The fix (which is partially complete)
was to define a clear RPC interface between Quantum & Nova and so
de-couple Nova VIF driver implementations from needing to hardcode
any direct knwoledge of Quantum & vica-verca.

Regards,
Daniel
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