On Fri, Jan 24, 2014 at 12:55 PM, Day, Phil <philip....@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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