By caching AWS refs for nodes/EC2 instances already known to be not in any of ASGs managed by cluster-autoscaler(CA).
Please beware of the edge case - this method is safe as long as users don't attach nodes by calling AttachInstances API after CA cached them. I believe, even if it was necessary, a warning in the documentation about the edge case is enough for now. If we really need to support the case, I will submit an another PR to invalidate the cache periodically so that CA can detect the formerly cached nodes are attached to ASG(s).
Also refactor AwsManager for less complexity by extracting types, accordingly to the discussion made [here](https://github.com/kubernetes/autoscaler/pull/46#discussion_r117912687)
This is an alternative implementation of https://github.com/kubernetes/contrib/pull/1982
Notable differences from the original PR are:
* A new flag named `--node-group-auto-discovery` is introduced for opting in to enable the auto-discovery feature.
* For example, specifying `--cloud-provider aws --node-group-auto-discovery asg:tag=k8s.io/cluster-autoscaler/enabled` instructs CA to auto-discover ASGs tagged with `k8s.io/cluster-autoscaler/enabled` to be used as target node groups
* The new code path introduced by this PR is executed only when `node-group-auto-discovery` is specified. There is relatively less chance to break existing features by introducing this change
Resolves https://github.com/kubernetes/contrib/issues/1956
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Other notes:
* We rely mainly on the `DescribeTags` API rather than `DescribeAutoScalingGroups` so that AWS can filter out unnecessary ASGs which doesn't belong to the k8s cluster, for us.
* If we relied on `DescribeAutoScalingGroups` here, as it doesn't support `Filter`ing, we'd need to iterate over ALL the ASGs available in an AWS account, which isn't desirable due to unnecessary excessive API calls and network usages
* Update cloudprovider/aws/README for the new configuration
* Warn abount invalid combination of flags
according to the review comment https://github.com/kubernetes/autoscaler/pull/11#discussion_r113713138
* Emit a validation error when both --nodes and --node-group-auto-discovery are specified
according to the review comment https://github.com/kubernetes/autoscaler/pull/11#discussion_r113958080
TODO/Possible future improvements before recommending this to everyone:
* Cache the result of an auto-discovery for a configurable period, so that we won't invoke DescribeTags and DescribeAutoScalingGroup APIs too many times
mmerrill3
Krzysztof Jastrzebski
Seth Pollack
Marcin Wielgus
Yusuke Kuoka
andrewsykim
Jan Chaloupka
osxi