Open Stack

On Fri, Jan 24, 2014 at 12:55 PM, Day, Phil <philip.day at hp.com> wrote:

>  > I haven't actually found where metadata caching is implemented,
> although the constructor of InstanceMetadata documents restrictions that
> really only make sense if it is.  Anyone know where it is cached?
>
>  Here’s the code that does the caching:
>
>
> https://github.com/openstack/nova/blob/master/nova/api/metadata/handler.py#L84-L98
>
>
>
> Data is only cached for 15 seconds by default – the main reason for
> caching is that cloud-init makes a sequence of calls to get various items
> of metadata, and it saves a lot of DB access if we don’t have to go back
> for them multiple times.
>
>
>
> If your using the Openstack metadata calls instead then the caching
> doesn’t buy much as it returns a single json blob with all the values.
>

Thanks (not quite sure how I missed that, but I did!)  15 second
'micro-caching' is probably great for peer discovery.  Short enough that
we'll find any peer basically as soon as it boots if we're polling (e.g. we
haven't yet connected to a peer), long enough to prevent denial-of-service.
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