[openstack-dev] [trove] My thoughts on the Unified Guest Agent

Steven Dake sdake at redhat.com
Wed Dec 18 16:05:09 UTC 2013

On 12/18/2013 08:34 AM, Tim Simpson wrote:
> I've been following the Unified Agent mailing list thread for awhile 
> now and, as someone who has written a fair amount of code for both of 
> the two existing Trove agents, thought I should give my opinion about 
> it. I like the idea of a unified agent, but believe that forcing Trove 
> to adopt this agent for use as its by default will stifle innovation 
> and harm the project.
> There are reasons Trove has more than one agent currently. While 
> everyone knows about the "Reference Agent" written in Python, 
> Rackspace uses a different agent written in C++ because it takes up 
> less memory. The concerns which led to the C++ agent would not be 
> addressed by a unified agent, which if anything would be larger than 
> the Reference Agent is currently.
> I also believe a unified agent represents the wrong approach 
> philosophically. An agent by design needs to be lightweight, capable 
> of doing exactly what it needs to and no more. This is especially true 
> for a project like Trove whose goal is to not to provide overly 
> general PAAS capabilities but simply installation and maintenance of 
> different datastores. Currently, the Trove daemons handle most logic 
> and leave the agents themselves to do relatively little. This takes 
> some effort as many of the first iterations of Trove features have too 
> much logic put into the guest agents. However through perseverance the 
> subsequent designs are usually cleaner and simpler to follow. A 
> community approved, "do everything" agent would endorse the wrong 
> balance and lead to developers piling up logic on the guest side. Over 
> time, features would become dependent on the Unified Agent, making it 
> impossible to run or even contemplate light-weight agents.
> Trove's interface to agents today is fairly loose and could stand to 
> be made stricter. However, it is flexible and works well enough. 
> Essentially, the duck typed interface of the trove.guestagent.api.API 
> class is used to send messages, and Trove conductor is used to receive 
> them at which point it updates the database. Because both of these 
> components can be swapped out if necessary, the code could support the 
> Unified Agent when it appears as well as future agents.
> It would be a mistake however to alter Trove's standard method of 
> communication to please the new Unified Agent. In general, we should 
> try to keep Trove speaking to guest agents in Trove's terms alone to 
> prevent bloat.
> Thanks,
> Tim


You raise very valid points that I'll summarize into bullet points:
* memory footprint of a python-based agent
* guest-agent feature bloat with no clear path to refactoring
* an agent should do one thing and do it well

The competing viewpoint is from downstream:
* How do you get those various agents into the various linux 
distributions cloud images and maintain them

A unified agent addresses the downstream viewpoint well, which is "There 
is only one agent to package and maintain, and it supports all the 
integrated OpenStack Program projects".

Putting on my Fedora Hat for a moment, I'm not a big fan of an agent per 
OpenStack project going into the Fedora 21 cloud images.

Another option that we really haven't discussed on this long long thread 
is injecting the per-project agents into the vm on bootstrapping of the 
vm.  If we developed common code for this sort of operation and placed 
it into oslo, *and* agreed to use it as our common unifying mechanism of 
agent support, each project would be free to ship whatever agents they 
wanted in their packaging, use the proposed oslo.bootstrap code to 
bootstrap the VM via cloudinit with the appropriate agents installed in 
the proper locations, whamo, problem solved for everyone.


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