[openstack-dev] [Nova] bp proposal: discovery of peer instances through metadata service

John Garbutt john at johngarbutt.com
Tue Jan 28 09:38:01 UTC 2014

On 27 January 2014 14:52, Justin Santa Barbara <justin at fathomdb.com> wrote:
> Day, Phil wrote:
>> >> We already have a mechanism now where an instance can push metadata as
>> >> a way of Windows instances sharing their passwords - so maybe this
>> >> could
>> >> build on that somehow - for example each instance pushes the data its
>> >> willing to share with other instances owned by the same tenant ?
>> >
>> > I do like that and think it would be very cool, but it is much more
>> > complex to
>> > implement I think.
>> I don't think its that complicated - just needs one extra attribute stored
>> per instance (for example into instance_system_metadata) which allows the
>> instance to be included in the list
> Ah - OK, I think I better understand what you're proposing, and I do like
> it.  The hardest bit of having the metadata store be full read/write would
> be defining what is and is not allowed (rate-limits, size-limits, etc).  I
> worry that you end up with a new key-value store, and with per-instance
> credentials.  That would be a separate discussion: this blueprint is trying
> to provide a focused replacement for multicast discovery for the cloud.
> But: thank you for reminding me about the Windows password though...  It may
> provide a reasonable model:
> We would have a new endpoint, say 'discovery'.  An instance can POST a
> single string value to the endpoint.  A GET on the endpoint will return any
> values posted by all instances in the same project.
> One key only; name not publicly exposed ('discovery_datum'?); 255 bytes of
> value only.
> I expect most instances will just post their IPs, but I expect other uses
> will be found.
> If I provided a patch that worked in this way, would you/others be on-board?

I like that idea. Seems like a good compromise. I have added my review
comments to the blueprint.

We have this related blueprints going on, setting metadata on a
particular server, rather than a group:

It is limiting things using the existing Quota on metadata updates.

It would be good to agree a similar format between the two.


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