[openstack-dev] [all] [tc] [PTL] Cascading vs. Cells – summit recap and move forward

henry hly henry4hly at gmail.com
Fri Dec 12 08:55:35 UTC 2014

On Fri, Dec 12, 2014 at 4:10 PM, Steve Gordon <sgordon at redhat.com> wrote:
> ----- Original Message -----
>> From: "henry hly" <henry4hly at gmail.com>
>> To: "OpenStack Development Mailing List (not for usage questions)" <openstack-dev at lists.openstack.org>
>> On Fri, Dec 12, 2014 at 11:41 AM, Dan Smith <dms at danplanet.com>
>> wrote:
>> >> [joehuang] Could you pls. make it more clear for the deployment
>> >> mode
>> >> of cells when used for globally distributed DCs with single API.
>> >> Do
>> >> you mean cinder/neutron/glance/ceilometer will be shared by all
>> >> cells, and use RPC for inter-dc communication, and only support
>> >> one
>> >> vendor's OpenStack distribution? How to do the cross data center
>> >> integration and troubleshooting with RPC if the
>> >> driver/agent/backend(storage/network/sever) from different vendor.
>> >
>> > Correct, cells only applies to single-vendor distributed
>> > deployments. In
>> > both its current and future forms, it uses private APIs for
>> > communication between the components, and thus isn't suited for a
>> > multi-vendor environment.
>> >
>> > Just MHO, but building functionality into existing or new
>> > components to
>> > allow deployments from multiple vendors to appear as a single API
>> > endpoint isn't something I have much interest in.
>> >
>> > --Dan
>> >
>> Even with the same distribution, cell still face many challenges
>> across multiple DC connected with WAN. Considering OAM, it's easier
>> to
>> manage autonomous systems connected with external northband interface
>> across remote sites, than a single monolithic system connected with
>> internal RPC message.
> The key question here is this primarily the role of OpenStack or an external cloud management platform, and I don't profess to know the answer. What do people use (workaround or otherwise) for these use cases *today*? Another question I have is, one of the stated use cases is for managing OpenStack clouds from multiple vendors - is the implication here that some of these have additional divergent API extensions or is the concern solely the incompatibilities inherent in communicating using the RPC mechanisms? If there are divergent API extensions, how is that handled from a proxying point of view if not all underlying OpenStack clouds necessarily support it (I guess same applies when using distributions without additional extensions but of different versions - e.g. Icehouse vs Juno which I believe was also a targeted use case?)?

It's not about divergent northband API extension. Services between
Openstack projects are SOA based, this is a vertical splitting, so
when building large and distributed system (whatever it is) with
horizontal splitting, shouldn't we prefer clear and stable RESTful
interface between these building blocks?

>> Although Cell did some separation and modulation (not to say it's
>> still internal RPC across WAN), they leaves cinder, neutron,
>> ceilometer. Shall we wait for all these projects to re-factor with
>> Cell-like hierarchy structure, or adopt a more loose coupled way, to
>> distribute them into autonomous units at the basis of the whole
>> Openstack (except Keystone which can handle multiple region
>> naturally)?
> Similarly though, is the intent with Cascading that each new project would have to also implement and provide a proxy for use in these deployments? One of the challenges with maintaining/supporting the existing Cells implementation has been that it's effectively it's own thing and as a result it is often not considered when adding new functionality.

Yes we need a new proxy, but nova proxy is just a new type of virt
driver, neutron proxy a new type of agent, cinder proxy a new type of
volume store...They just utilize existing standard driver/agent
mechanism, no influence on other code in tree.

>> As we can see, compared with Cell, much less work is needed to build
>> a
>> Cascading solution, No patch is needed except Neutron (waiting some
>> upcoming features not landed in Juno), nearly all work lies in the
>> proxy, which is in fact another kind of driver/agent.
> Right, but the proxies still appear to be a not insignificant amount of code - is the intent not that the proxies would eventually reside within the relevant projects? I've been assuming yes but I am wondering if this was an incorrect assumption on my part based on your comment.
> Thanks,
> Steve
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