KEP: in-place update of pod resources

keps/sig-autoscaling/draft-20181106-in-place-update-of-pod-resources.md

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+---
+kep-number: draft-20181106
+title: In-place Update of Pod Resources
+authors:
+  - "@kgolab"
+  - "@bskiba"
+  - "@schylek"
+owning-sig: sig-autoscaling
+participating-sigs:
+  - sig-node
+  - sig-scheduling
+reviewers:
+  - "@bsalamat"
+  - "@derekwaynecarr"
+  - "@dchen1107"
+approvers:
+  - TBD
+editor: TBD
+creation-date: 2018-11-06
+last-updated: 2018-11-06
+status: provisional
+see-also:
+replaces:
+superseded-by:
+---
+
+# In-place Update of Pod Resources
+
+## Table of Contents
+
+   * [In-place Update of Pod Resources](#in-place-update-of-pod-resources)
+      * [Table of Contents](#table-of-contents)
+      * [Summary](#summary)
+      * [Motivation](#motivation)
+         * [Goals](#goals)
+         * [Non-Goals](#non-goals)
+      * [Proposal](#proposal)
+         * [API Changes](#api-changes)
+         * [Flow Control](#flow-control)
+            * [Transitions of InPlaceResize condition](#transitions-of-inplaceresize-condition)
+            * [Notes](#notes)
+         * [Affected Components](#affected-components)
+         * [Risks and Mitigations](#risks-and-mitigations)
+      * [Graduation Criteria](#graduation-criteria)
+      * [Implementation History](#implementation-history)
+      * [Alternatives](#alternatives)
+
+## Summary
+
+This proposal aims at allowing Pod resource requests & limits to be updated
+in-place, without a need to restart the Pod or its Containers.
+
+The **core idea** behind the proposal is to make PodSpec mutable with regards to
+Resources, denoting **desired** resources.
+Additionally PodStatus is extended to provide information about **actual**
+resource allocation.
+
+This document builds upon [proposal for live and in-place vertical scaling][] and
+[Vertical Resources Scaling in Kubernetes][].
+
+[proposal for live and in-place vertical scaling]: https://github.com/kubernetes/community/pull/1719
+[Vertical Resources Scaling in Kubernetes]: https://docs.google.com/document/d/18K-bl1EVsmJ04xeRq9o_vfY2GDgek6B6wmLjXw-kos4/edit?ts=5b96bf40
+
+## Motivation
+
+Resources allocated to a Pod's Container can require a change for various reasons:
+* load handled by the Pod has increased significantly and current resources are
+  not enough to handle it,
+* load has decreased significantly and currently allocated resources are unused
+  and thus wasted,
+* Resources have simply been set improperly.
+
+Currently changing Resources allocation requires the Pod to be recreated since
+the PodSpec is immutable.
+
+While many stateless workloads are designed to withstand such a disruption, some
+are more sensitive, especially when using low number of Pod replicas.
+
+Moreover, for stateful or batch workloads, a Pod restart is a serious
+disruption, resulting in lower availability or higher cost of running.
+
+Allowing Resources to be changed without recreating a Pod nor restarting a
+Container addresses this issue directly.
+
+### Goals
+
+* Primary: allow to change Pod resource requests & limits without restarting its
+  Containers.
+* Secondary: allow actors (users, VPA, StatefulSet, JobController) to decide
+  how to proceed if in-place resource update is not available.
+* Secondary: allow users to specify which Pods and Containers can be updated
+  without a restart.
+
+### Non-Goals
+
+The explicit non-goal of this KEP is to avoid controlling full life-cycle of a
+Pod which failed an in-place resource update. These cases should be handled by
+actors which initiated the update.
+
+Other identified non-goals are:
+* allow to change Pod QoS class without a restart,
+* to change resources of Init Containers without a restart.
+
+## Proposal
+
+### API Changes
+
+PodSpec becomes mutable with regards to resources and limits.
+Additionally, PodSpec becomes a Pod subresource to allow fine-grained access control.
+
+PodStatus is extended with information about actually allocated resources.
+
+Thanks to the above:
+* PodSpec.Container.ResourceRequirements becomes purely a declaration,
+  denoting **desired** state of the Pod,
+* PodStatus.ContainerStatus.ResourceAllocated (new object) denotes **actual**
+  state of the Pod resources.
+
+To distinguish between possible states of the Pod resources,
+a new PodCondition InPlaceResize is added, with the following states:
+* (empty) - the default value; resource update awaits reconciliation
+  (if ResourceRequirements differs from ResourceAllocated),
+* Awaiting - awaiting resources to be freed (e.g. via pre-emption),
+* Failed - resource update could not have been performed in-place
+  but might be possible if some conditions change,
+* Rejected - resource update was rejected by any of the components involved.
+
+To provide some fine-grained control to the user,
+PodSpec.Container.ResourceRequirements is extended with ResizingPolicy flag,
+available per each resource request (CPU, memory) :
+* InPlace - the default value; allow in-place resize of the Container,
+* RestartContainer - restart the Container to apply new resource values
+  (e.g. Java process needs to change its Xmx flag),
+* RestartPod - restart whole Pod to apply new resource values
+  (e.g. Pod requires its Init Containers to re-run).
+
+By using the ResizingPolicy flag the user can mark Containers or Pods as safe
+(or unsafe) for in-place resources update.
+
+This flag **may** be used by the actors starting the process to decide if
+the process should be started at all (for example VPA might decide to
+evict Pod with RestartPod policy).
+This flag **must** be used by Kubelet to verify the actions needed.
+
+Setting the flag to separately control CPU & memory is due to an observation
+that usually CPU can be added/removed without much problems whereas
+changes to available memory are more probable to require restarts.
+
+### Flow Control
+
+The following steps denote a positive flow of an in-place update,
+for a Pod having ResizingPolicy set to InPlace for all its Containers.
+Some alternative flows are given in indented steps,
+unless noted otherwise they abort the flow.
+
+1. The initiating actor updates ResourceRequirements using PATCH verb.
+1. API Server validates the new ResourceRequirements
+   (e.g. limits are not below requested resources, QoS class does not change).
+1. API Server calls all Admission Controllers to verify the Pod Update.
+   1. If any of the controllers rejects the update,
+      the InPlaceResize PodCondition is set to Rejected.
+1. API Server updates the PodSpec object and clears InPlaceResize condition.
+1. Scheduler observes that ResourceRequirements and ResourceAllocated differ.
+   It updates its resource cache to use max(ResourceRequirements, ResourceAllocated).
+   1. If required it pre-empts lower-priority Pods, setting
+      the InPlaceResize PodCondition to Awaiting.
+      Once the lower-priority Pods are evicted, Scheduler clears
+      the InPlaceResize PodCondition and the flow continues.
+1. Kubelet observes that ResourceRequirements and ResourceAllocated differ
+   and the InPlaceResize condition is clear.
+   This is done potentially prior to Scheduler pre-empting lower-priority Pods.
+   1. Kubelet checks that new ResourceRequirements do not fit Node’s
+      allocatable resources and sets the InPlaceResize condition to Failed.
+1. Kubelet applies new resource values to cgroups, updates values
+   in ResourceAllocated to match ResourceRequirements
+   and clears InPlaceResize condition.
+1. Scheduler observes that ResourceAllocated has changed.
+   It updates its resource cache to use new value of ResourceAllocated
+   for the given Pod.
+1. The initating actor observes that ResourceRequirements and
+   ResourceAllocated match again which signifies the completion of an update.
+
+#### Transitions of InPlaceResize condition
+
+The following diagram shows possible transitions of InPlaceResize condition.
+
+```text
+                   +---------+
+       +-----------+         +-----------+
+       |           | (empty) |           |
+       | +--------->         <---------+ |
+       | |         +----+----+         | |
+      1| |2            3|             4| |5
+ +-----v-+--+           |          +---+-v--+
+ |          |           |          |        |
+ | Awaiting |           |          | Failed |
+ |          |           |          |        |
+ +-------+--+           |          +---+----+
+        3|              |              |3
+         |         +----v-----+        |
+         |         |          |        |
+         +---------> Rejected <--------+
+                   |          |
+                   +----------+
+```
+
+1. Scheduler, on pre-emption.
+1. Scheduler, after pre-emption finishes.
+1. Any Controller, on permanent issue.
+1. Kubelet, on successful retry.
+1. Kubelet, if not enough space on Node.
+
+#### Notes
+
+* In case when there is no pre-emption required, Kubelet and Scheduler
+  will pick up the ResourceRequirements change in parallel.
+* In case when there is pre-emption required Kubelet and Scheduler might
+  pick up the ResourceRequirements change in parallel,
+  Kubelet will then set the InPlaceResize condition to Failed
+  and Scheduler will clear it once pre-emption is done.
+* Kubelet might try to apply new resources also if InPlaceResize
+  condition is set to Failed, as a normal retry mechanism.
+* To avoid races and possible gamification, all components should use
+  max(ResourceRequirements, ResourceAllocated) when computing resources
+  used by a Pod. TBD if this can be weakened when InPlaceResize condition
+  is set to Rejected, or should the initiating actor update
+  ResourceRequirements back to reclaim resources.
+
+### Affected Components
+
+Pod v1 core API:
+* extended model,
+* added validation.
+
+Admission Controllers: LimitRanger, ResourceQuota need to support Pod Updates:
+* for ResourceQuota it should be enough to change podEvaluator.Handler
+  implementation to allow Pod updates; max(ResourceRequirements, ResourceAllocated)
+  should be used to be in line with current ResourceQuota behaviour
+  which blocks resources before they are used (e.g. for Pending Pods),
+* for LimitRanger TBD.
+
+Kubelet
+* support in-place resource management,
+* set ResourceRequirements on placing the Pod on Node.
+
+Scheduler:
+* update its caches with proper resources, depending on InPlaceResize condition.
+
+Other components:
+* check how the change of meaning of resource requests influence other kubernetes components.
+
+### Possible Extensions
+
+1. Allow resource limits to be updated too.
+1. Allow ResizingPolicy to be set on Pod level, acting as default if
+   (some of) the Containers do not have it set on their own.
+1. Extend ResizingPolicy flag to separately control resource increase and decrease
+   (e.g. a container can be given more memory in-place but
+   decreasing memory requires container restart).
+
+### Risks and Mitigations
+
+TODO
+
+## Graduation Criteria
+
+TODO
+
+## Implementation History
+
+- 2018-11-06 - initial KEP draft created
+- 2019-01-18 - implementation proposal extended
+
+## Alternatives
+
+TODO
+