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# KEP-NNNN: Kubelet limit of Parallel Image Pulls

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<!-- toc -->
- [Release Signoff Checklist](#release-signoff-checklist)
- [Summary](#summary)
- [Motivation](#motivation)
- [Goals](#goals)
- [Non-Goals](#non-goals)
- [Proposal](#proposal)
- [User Stories (Optional)](#user-stories-optional)
- [Story 1](#story-1)
- [Story 2](#story-2)
- [Notes/Constraints/Caveats (Optional)](#notesconstraintscaveats-optional)
- [Risks and Mitigations](#risks-and-mitigations)
- [Design Details](#design-details)
- [Test Plan](#test-plan)
- [Prerequisite testing updates](#prerequisite-testing-updates)
- [Unit tests](#unit-tests)
- [Integration tests](#integration-tests)
- [e2e tests](#e2e-tests)
- [Graduation Criteria](#graduation-criteria)
- [Upgrade / Downgrade Strategy](#upgrade--downgrade-strategy)
- [Version Skew Strategy](#version-skew-strategy)
- [Production Readiness Review Questionnaire](#production-readiness-review-questionnaire)
- [Feature Enablement and Rollback](#feature-enablement-and-rollback)
- [Rollout, Upgrade and Rollback Planning](#rollout-upgrade-and-rollback-planning)
- [Monitoring Requirements](#monitoring-requirements)
- [Dependencies](#dependencies)
- [Scalability](#scalability)
- [Troubleshooting](#troubleshooting)
- [Implementation History](#implementation-history)
- [Drawbacks](#drawbacks)
- [Alternatives](#alternatives)
- [Infrastructure Needed (Optional)](#infrastructure-needed-optional)
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## Release Signoff Checklist

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## Summary

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This KEP proposes adding to kubelet a node-level limit on the number of parallel image pulls.

## Motivation

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### QPS/burst limits on kubelet are confusing
Currently kubelet limits image pulls with QPS and burst, but they are confusing as they only limit the number of requests sent to container runtime, and does not consider the number of inflight image pulls. In other words, even a small QPS is set, there still could be many image pulls in progress in parallel, if each pull takes a long time. See [issue #112044](https://github.com/kubernetes/kubernetes/issues/112044) as an example.

### No way to limit the number of inflight image pulls
Currently neither kubelet or containerd limits the number of inflight image pulls. On kubelet, as mentioned above, the QPS/burst limit does not take the in-progress pulls into account. On containerd, there is only [a per-image limit](https://github.com/containerd/containerd/blob/8e787543deede10f372b0e16ad5c07790fc680b6/pkg/cri/config/config.go#L299-L300), which only limits the number of parallel layer downloading for each image, but potentially allows unlimited number downloads overall.

### Goals

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Adding a node-level limit of parallel image pulls to kubelet. This limit will limits the maximum number of images being pulled in parallel. Any image pull request beyond the limit will be blocked until one image pull finishes.

### Non-Goals

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* Prioritizing image pulls in any way.
* Using the number of inflight image pulls as a signal to direct pod scheduling.

## Proposal

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* Add `maxParallelImagePulling` to `KubeletConfiguration`.
* If `serialize-image-pulls` is set to false, maxParallelImagePulling default to 0 for no limitation.
* Firstly, Kubelet pass `maxParallelImagePulling` setting to CRI as option of image pulling.
* If image already exists, succeed. (If lazy pulling is enable, the layers are treated as AlreadyExists.)
* Else
* If maxParallelImagePulling=0, keep it as before
* If maxParallelImagePulling=n, check current image pulling counts
* If current>=n, retry 3 times and wait 10s interval for 30s. (Not sure if this is needed as kubelet will retry. However, I think we can do something in container runtime.)
* If still current>=n, fail with a new Error like `TooManyPullingInParallelErr`.
* If current<n, keep it as before
* Fallback, if container runtime is not supporting `maxParallelImagePulling` option.
* Kubelet will keep a list of image pulling triggered in a cached list
* If there is new pulling, add it to the cached list
* When the image pull failed or completed, remove it from the cached list
* Keep the list size less than or equals `maxParallelImagePulling`.
* If list is full, hang the image pull process to wait for other image pulling completed.
* If the image is already in pulling list, return.

### User Stories (Optional)

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#### Story 1

#### Story 2

### Notes/Constraints/Caveats (Optional)

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### Risks and Mitigations

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## Design Details

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### Test Plan

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[ ] I/we understand the owners of the involved components may require updates to
existing tests to make this code solid enough prior to committing the changes necessary
to implement this enhancement.

##### Prerequisite testing updates

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##### Unit tests

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##### Integration tests

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### Graduation Criteria

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### Upgrade / Downgrade Strategy

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### Feature Enablement and Rollback

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###### How can this feature be enabled / disabled in a live cluster?

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###### What specific metrics should inform a rollback?

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### Monitoring Requirements

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###### How can an operator determine if the feature is in use by workloads?

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###### How can someone using this feature know that it is working for their instance?

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- [ ] Events
- Event Reason:
- [ ] API .status
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### Dependencies

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###### Does this feature depend on any specific services running in the cluster?

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### Scalability

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###### Will enabling / using this feature result in any new API calls?

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###### Will enabling / using this feature result in increasing size or count of the existing API objects?

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###### Will enabling / using this feature result in increasing time taken by any operations covered by existing SLIs/SLOs?

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###### Will enabling / using this feature result in non-negligible increase of resource usage (CPU, RAM, disk, IO, ...) in any components?

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### Troubleshooting

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###### How does this feature react if the API server and/or etcd is unavailable?

###### What are other known failure modes?

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- Detection: How can it be detected via metrics? Stated another way:
how can an operator troubleshoot without logging into a master or worker node?
- Mitigations: What can be done to stop the bleeding, especially for already
running user workloads?
- Diagnostics: What are the useful log messages and their required logging
levels that could help debug the issue?
Not required until feature graduated to beta.
- Testing: Are there any tests for failure mode? If not, describe why.
-->

###### What steps should be taken if SLOs are not being met to determine the problem?

## Implementation History

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Major milestones in the lifecycle of a KEP should be tracked in this section.
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- the date implementation started
- the first Kubernetes release where an initial version of the KEP was available
- the version of Kubernetes where the KEP graduated to general availability
- when the KEP was retired or superseded
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## Drawbacks

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Why should this KEP _not_ be implemented?
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## Alternatives

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What other approaches did you consider, and why did you rule them out? These do
not need to be as detailed as the proposal, but should include enough
information to express the idea and why it was not acceptable.
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## Infrastructure Needed (Optional)

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Use this section if you need things from the project/SIG. Examples include a
new subproject, repos requested, or GitHub details. Listing these here allows a
SIG to get the process for these resources started right away.
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