mirror of https://github.com/k3s-io/k3s
128 lines
5.7 KiB
Markdown
128 lines
5.7 KiB
Markdown
# CRI: the Container Runtime Interface
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## What is CRI?
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CRI (_Container Runtime Interface_) consists of a
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[protobuf API](../../pkg/kubelet/api/v1alpha1/runtime/api.proto),
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specifications/requirements (to-be-added),
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and [libraries] (https://github.com/kubernetes/kubernetes/tree/master/pkg/kubelet/server/streaming)
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for container runtimes to integrate with kubelet on a node. CRI is currently in Alpha.
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In the future, we plan to add more developer tools such as the CRI validation
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tests.
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## Why develop CRI?
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Prior to the existence of CRI, container runtimes (e.g., `docker`, `rkt`) were
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integrated with kubelet through implementing an internal, high-level interface
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in kubelet. The entrance barrier for runtimes was high because the integration
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required understanding the internals of kubelet and contributing to the main
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Kubernetes repository. More importantly, this would not scale because every new
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addition incurs a significant maintenance overhead in the main kubernetes
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repository.
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Kubernetes aims to be extensible. CRI is one small, yet important step to enable
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pluggable container runtimes and build a healthier ecosystem.
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## How to use CRI?
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1. Start the image and runtime services on your node. You can have a single
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service acting as both image and runtime services.
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2. Set the kubelet flags
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- Pass the unix socket(s) to which your services listen to kubelet:
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`--container-runtime-endpoint` and `--image-service-endpoint`.
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- Enable CRI in kubelet by`--experimental-cri=true`.
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- Use the "remote" runtime by `--container-runtime=remote`.
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Please see the [Status Update](#status-update) section for known issues for
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each release.
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Note that CRI is still in its early stages. We are actively incorporating
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feedback from early developers to improve the API. Developers should expect
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occasional API breaking changes.
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## Does Kubelet use CRI today?
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No, but we are working on it.
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The first step is to switch kubelet to integrate with Docker via CRI by
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default. The current [Docker CRI implementation](https://github.com/kubernetes/kubernetes/blob/release-1.5/pkg/kubelet/dockershim)
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already passes most end-to-end tests, and has mandatory PR builders to prevent
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regressions. While we are expanding the test coverage gradually, it is
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difficult to test on all combinations of OS distributions, platforms, and
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plugins. There are also many experimental or even undocumented features relied
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upon by some users. We would like to **encourage the community to help test
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this Docker-CRI integration and report bugs and/or missing features** to
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smooth the transition in the near future. Please file a Github issue and
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include @kubernetes/sig-node for any CRI problem.
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### How to test the new Docker CRI integration?
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Start kubelet with the following flags:
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- Use the Docker container runtime by `--container-runtime=docker`(the default).
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- Enable CRI in kubelet by`--experimental-cri=true`.
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Please also see the [known issues](#docker-cri-1.5-known-issues) before trying
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out.
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## Design docs and proposals
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We plan to add CRI specifications/requirements in the near future. For now,
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these proposals and design docs are the best sources to understand CRI
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besides discussions on Github issues.
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- [Original proposal](https://github.com/kubernetes/kubernetes/blob/release-1.5/docs/proposals/container-runtime-interface-v1.md)
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- [Exec/attach/port-forward streaming requests](https://docs.google.com/document/d/1OE_QoInPlVCK9rMAx9aybRmgFiVjHpJCHI9LrfdNM_s/edit?usp=sharing)
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- [Container stdout/stderr logs](https://github.com/kubernetes/kubernetes/blob/release-1.5/docs/proposals/kubelet-cri-logging.md)
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- Networking: The CRI runtime handles network plugins and the
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setup/teardown of the pod sandbox.
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## Work-In-Progress CRI runtimes
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- [cri-o](https://github.com/kubernetes-incubator/cri-o)
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- [rktlet](https://github.com/kubernetes-incubator/rktlet)
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- [frakti](https://github.com/kubernetes/frakti)
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## [Status update](#status-update)
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### Kubernetes v1.5 release (CRI v1alpha1)
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- [v1alpha1 version](https://github.com/kubernetes/kubernetes/blob/release-1.5/pkg/kubelet/api/v1alpha1/runtime/api.proto) of CRI is released.
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#### [CRI known issues](#cri-1.5-known-issues):
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- [#27097](https://github.com/kubernetes/kubernetes/issues/27097): Container
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metrics are not yet defined in CRI.
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- [#36401](https://github.com/kubernetes/kubernetes/issues/36401): The new
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container log path/format is not yet supported by the logging pipeline
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(e.g., fluentd, GCL).
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- CRI may not be compatible with other experimental features (e.g., Seccomp).
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- Streaming server needs to be hardened.
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- [#36666](https://github.com/kubernetes/kubernetes/issues/36666):
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Authentication.
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- [#36187](https://github.com/kubernetes/kubernetes/issues/36187): Avoid
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including user data in the redirect URL.
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#### [Docker CRI integration known issues](#docker-cri-1.5-known-issues)
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- Docker compatibility: Support only Docker v1.11 and v1.12.
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- Network:
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- [#35457](https://github.com/kubernetes/kubernetes/issues/35457): Does
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not support host ports.
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- [#37315](https://github.com/kubernetes/kubernetes/issues/37315): Does
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not support bandwidth shaping.
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- Exec/attach/port-forward (streaming requests):
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- [#35747](https://github.com/kubernetes/kubernetes/issues/35747): Does
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not support `nsenter` as the exec handler (`--exec-handler=nsenter`).
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- Also see (#cri-1.5-known-issues) for limitations on CRI streaming.
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## Contacts
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- Email: sig-node (kubernetes-sig-node@googlegroups.com)
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- Slack: https://kubernetes.slack.com/messages/sig-node
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