Feature Request: enable user-managed Pod Migration

[erictune]

A user wants to extend Kubernetes to allow for application-specific migration in response to pod deletion events, whenever possible.

1. Normally, there should be 1 instance of a pod -- call it pod-1.
2. However, something (usually the system, e.g. rescheduler or node upgrades) wants to delete pod-1, then a replica, pod-2 should be created.
3. Before pod-1 is actually terminated, it will discover pod-2 and they will do an application-level handoff of state.
4. After handoff, scale down to just 1 pod, for economy.

This issue is created to suggest possible ways to implement this pattern.

[erictune]

Possible approach 1:

• Use statefulset for this pod. Initial size 1. Also a headless service.
• When pod-1 gets graceful termination notice, it should call back to the API and scale its statefulset up to size 2, then wait to discover pod-2 via DNS, then sync with it, then exit.
• When a new instance ofpod-1 is later created, sync the other way, and scale back down.

Advantages of this approach:

• Can be implemented today without Kubernetes changes, and basically using any script or program that can do a scale up, scale down, and discover the peer using DNS.

Drawbacks to this approach:

• Requires migrating twice, when in principle only 1 migration is needed, in order to get the stateful set down to size 1 again.
• Requires authorizing the pod to scale its own controller. Might be undesirable for some security-sensitive applications.

[erictune]

Possible approach 2:

• write an "operator", in the style of https://github.com/upmc-enterprises/elasticsearch-operator which implements a "migratingPod" concept, which has a pod Template.
• It makes one pod with a random name, say pod-32jdg
• When that pod gets a graceful termination, it waits for a peer to appear.
• When the operator sees a deletion timestamp on pod-32jdg, then it creates a replacement such as pod-m2k87. It uses the same labels.
• Both pods discover each other, either by watching DNS + using a headless service, or by direct message from the operator. They initiate migration.
• pod-32jdg exits gracefully when migration is done.
• A regular service in front of both of them can provide a stable name as the pods go through different random names. Operator could even manage endpoints if it wants to control the moment of handoff between the two instances (modulo pre-existing connections).

Advantages of this approach:

• Can be implemented today without Kubernetes changes.
• Only one migration needed. A regular service

Drawbacks to this approach:

• Writing operator is more complex than Approach 1, but not too bad.

[0xmichalis]

Approach 1 seems like a custom strategy: https://github.com/kubernetes/kubernetes/issues/14510

cc: @kubernetes/sig-apps-feature-requests

[dhilipkumars]

This proposal can simplify both the approaches. I believe future Operators can become lightweight if we allow more elaborate cleanup mechanism.

[bgrant0607]

Ref #3949

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[xtchenhui]

@erictune have you tried the approach 2? Actually i'm investigating on the similar method now to migrate runv containters from one node to another.

[ashish-billore]

Related: https://github.com/kubernetes/kubernetes/issues/3949