Azure's cloud provider VMSS VMs API accesses are mediated through
a cache holding and refreshing all VMSS together.
Due to that we hit VMSSVM.List API more often than we could: an
instance's cache miss or expiration should only require a single
VMSS re-list, while it's currently O(n) relative to the number of
attached Scale Sets.
Under hard pressure (clusters with many attached VMSS that can't all
be listed in one sequence of successive API calls) the controller
manager might be stuck trying to re-list everything from scratch,
then aborting the whole operation; then re-trying and re-triggering
API rate-limits, affecting the whole Subscription.
This patch replaces the global VMSS VMs cache by per-VMSS VMs caches.
Refreshes (VMSS VMs lists) are scoped to the single relevant VMSS; under
severe throttling the various caches can be incrementally refreshed.
Signed-off-by: Benjamin Pineau <benjamin.pineau@datadoghq.com>
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