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

Tim Simpson tim.simpson at rackspace.com
Wed Dec 18 15:34:14 UTC 2013

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.


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