[openstack-dev] [Heat] Neutron resources and hidden dependencies

Zane Bitter zbitter at redhat.com
Wed Dec 18 22:26:52 UTC 2013

On 18/12/13 05:21, Thomas Herve wrote:
>> At the design summit we had a discussion[1] about redesigning Neutron
>> resources to avoid the need for hidden dependencies (that is to say,
>> dependencies which cannot be inferred from the template).
>> Since then we've got close to fixing one of those issues[2] (thanks
>> Bartosz!), but the patch is currently held up by a merge conflict
>> because... we managed to add two more[3][4].
> Sorry, I'm one of the reviewer of those, and [4] was a bit suspicious indeed, but it sounded like the best solution. [3] doesn't add dependencies, though.

Quite right, my mistake, and I can definitely see how it would not have 
looked suspicious at the time. I can easily imagine though that there 
might be a bug raised in the future saying we need to add some magic 
dependencies to [3] to make sure the DHCP agent is selected before any 
instances/ports get created. Actually there's already an inline comment 
from the author in patch set 1 of [4] that says "I will add dependency 
between subnet and NetDHCPAgent."

>> There's also another patch currently under review[5] that adds the most
>> impressively complex hidden dependencies we've yet seen (though,
>> fortunately, the worst of this is actually redundant and can be removed
>> without effect).
>> I know that due to the number of Neutron resources that currently
>> override add_dependencies(), this may look like a normal part of a
>> resource implementation. However, this is absolutely not the case. If
>> you feel the need to override add_dependencies() for any reason then
>> some part of the design is wrong. If you feel the need to do it for any
>> reason other than not breaking existing soon-to-be-deprecated wrong
>> designs (i.e. RouterGateway), then your part of the design is the part
>> that's wrong.
>> Core reviewers should treat any attempt to override add_dependencies()
>> as a red flag IMO.
>> It's unfortunate that many parts of the Neutron API are not great,
>> especially that some pretty core functionality is currently balkanised
>> into various 'extensions' that don't have a coherent interface.
>> Nevertheless, that just means that we need to work harder to come up
>> with resource designs that express the appropriate relationships between
>> resources *in the template*, not with hidden relationships in the code.
>> This means that orchestration will actually work regardless of whether
>> all of the related resources are defined in the same template, and in
>> fact regardless of whether they are defined in templates at all.
>> I'd like to propose that we revert NetDHCPAgent and RouterL3Agent (which
>> are somewhat misnamed, and serve only to connect a Net or Router to an
>> existing agent, not to create an agent) and replace them with properties
>> on the Net and Router classes, respectively.
> I'm not sure about NetDHCPAgent, as it doesn't add a dependency.

That's true, although I suspect it may only be working by coincidence 
now. It also seems to me that we should have a consistent approach for 
selecting agents. This resource does nothing but connect two other 
resources together in a way that, if it makes any difference at all, 
makes the dependency resolution worse not better. (Contrast this with 
VolumeAttachment or EIPAssociation, which exist to solve some specific 
dependency issues.) Conceptually, it is just a property of the Network IMO.

> Regarding RouterL3Agent, the only objection I have is if you want to use a router defined outside of the template, which if I remember correctly was the justification of this class. Do we have an answer for that?

I haven't seen that justification advanced anywhere in relation to this 
particular resource type, but maybe I missed it. In any event, if you 
want to use a Router defined outside of the template, you should also 
assign it to the correct agent outside of the template. It's difficult 
to get upset about that, especially given that it needs to be done 
before you use the Router for anything anyway. Basically there's no 
there's no safe way to do this other than at the time you create the 
router... that's the nice thing about not hacking the dependencies - 
when they're all explicit in the template then everything works the same 
way whether it's created in Heat or not, in the same template or not.


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