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blog: node: kubelet podresources API GA in 1.28
I'm intentionally covering multiple related enhancements with a single blog post. Enhancements: - kubernetes/enhancements#606 - kubernetes/enhancements#2403 - kubernetes/enhancements#3743 Signed-off-by: Francesco Romani <fromani@redhat.com>
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content/en/blog/_posts/2023-08-23-kubelet-podresources-api-ga.md
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| --- | ||
| layout: blog | ||
| title: 'Kubernetes 1.28: Node podresources API Graduates to GA' | ||
| date: 2023-08-23 | ||
| slug: kubelet-podresources-api-GA | ||
| --- | ||
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| **Author:** | ||
| Francesco Romani (Red Hat) | ||
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| The podresources API is an API served by the kubelet locally on the node, which exposes the compute resources exclusively | ||
| allocated to containers. With the release of Kubernetes 1.28, that API is now Generally Available. | ||
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| ## What problem does it solve? | ||
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| The kubelet can allocate exclusive resources to containers, like | ||
| [CPUs, granting exclusive access to full cores](https://kubernetes.io/docs/tasks/administer-cluster/cpu-management-policies/) | ||
| or [memory, either regions or hugepages](https://kubernetes.io/docs/tasks/administer-cluster/memory-manager/). | ||
| Workloads which require high performance, or low latency (or both) leverage these features. | ||
| The kubelet also can assign [devices to containers](https://kubernetes.io/docs/concepts/extend-kubernetes/compute-storage-net/device-plugins/) | ||
| Collectively, these features which enable exclusive assignments are known as "resource managers". | ||
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| Without an API like podresources, the only possible option to learn about resource assignment was to read the state files the | ||
| resource managers use. While done out of necessity, the problem with this approach is the path and the format of these file are | ||
| both internal implementation details. Albeit very stable, the project reserves the right to change them freely. | ||
| Consuming the content of the state files is thus fragile and unsupported, and projects doing this are recommended to consider | ||
| moving to podresources API or to other supported APIs. | ||
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| ## Overview of the API | ||
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| The podresources API was [initially proposed to enable device monitoring](https://kubernetes.io/docs/concepts/extend-kubernetes/compute-storage-net/device-plugins/#monitoring-device-plugin-resources). | ||
| In order to enable monitoring agents, a key prerequisite is to enable introspection of device assignment, which is performed by the kubelet. | ||
| Serving this purpose was the initial goal of the API. The first iteration of the API only had a single function implemented, `List`, | ||
| to return information about the assignment of devices to containers. | ||
| The API is used by [multus CNI](https://github.com/k8snetworkplumbingwg/multus-cni) and by | ||
| [monitoring tools](https://github.com/NVIDIA/dcgm-exporter). | ||
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| Since its inception, the podresources API increased its scope to cover other resource managers than device manager. | ||
| In Kubernetes 1.20, the `List` API reports also CPU cores and memory regions (including hugepages); the API also | ||
| reports the NUMA locality of the devices, while the locality of CPUs and memory can be inferred from the system. | ||
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| In Kubernetes 1.21, the API [gained](https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/2403-pod-resources-allocatable-resources/README.md) | ||
| the `GetAllocatableResources` function. | ||
| This newer API complements the existing `List` API and enables monitoring agents to determine the unallocated resources, | ||
| thus enabling new features built on top of the podresources API like a | ||
| [NUMA-aware scheduler plugin](https://github.com/kubernetes-sigs/scheduler-plugins/blob/master/pkg/noderesourcetopology/README.md). | ||
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| Finally, in Kubernetes 1.27, another function, `Get` was introduced to be more friendly with CNI meta-plugins, to make it simpler to access resources | ||
| allocated to a specific pod, rather than having to filter through resources for all pods on the node. The `Get` function is currently alpha level. | ||
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| ## Consuming the API | ||
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| The podresources API is served by the kubelet locally, on the same node on which is running. | ||
| On unix flavors, the endpoint is served over a unix domain socket; the default path is `/var/lib/kubelet/pod-resources/kubelet.sock`. | ||
| On windows, the endpoint is served over a named pipe; the default path is `npipe://\\.\pipe\kubelet-pod-resources`. | ||
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| In order for the containerized monitoring application consume the API, the socket should be mounted inside the container. | ||
| A good practice is to mount the directory on which the podresources socket endpoint sits rather than the socket directly. | ||
| This will ensure that after a kubelet restart, the containerized monitor application will be able to re-connect to the socket. | ||
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| An example manifest for a hypothetical monitoring agent consuming the podresources API and deployed as a DaemonSet could look like: | ||
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| ```yaml | ||
| apiVersion: apps/v1 | ||
| kind: DaemonSet | ||
| metadata: | ||
| name: podresources-monitoring-app | ||
| namespace: monitoring | ||
| spec: | ||
| selector: | ||
| matchLabels: | ||
| name: podresources-monitoring | ||
| template: | ||
| metadata: | ||
| labels: | ||
| name: podresources-monitoring | ||
| spec: | ||
| containers: | ||
| - args: | ||
| - --podresources-socket=unix:///host-podresources/kubelet.sock | ||
| command: | ||
| - /bin/podresources-monitor | ||
| image: podresources-monitor:latest # just for an example | ||
| volumeMounts: | ||
| - mountPath: /host-podresources | ||
| name: host-podresources | ||
| serviceAccountName: podresources-monitor | ||
| volumes: | ||
| - hostPath: | ||
| path: /var/lib/kubelet/pod-resources | ||
| type: Directory | ||
| name: host-podresources | ||
| ``` | ||
| I hope you find it straightforward to consume the podresources API programmatically. | ||
| The kubelet API package provides the protocol file and the go type definitions; however, a client package is not yet available from the project, | ||
| and the existing code should not be used directly. | ||
| The [recommended](https://github.com/kubernetes/kubernetes/blob/v1.28.0-rc.0/pkg/kubelet/apis/podresources/client.go#L32) | ||
| approach is to reimplement the client in your projects, copying and pasting the related functions like for example | ||
| the multus project is [doing](https://github.com/k8snetworkplumbingwg/multus-cni/blob/v4.0.2/pkg/kubeletclient/kubeletclient.go). | ||
| An [effort](https://issues.k8s.io/119817) is ongoing to improve the current state and to create a reference client package ready to be consumed. | ||
| When operating the containerized monitoring application consuming the podresources API, few points are worth highlighting to prevent "gotcha" moments: | ||
| - Even though the API only exposes data, and doesn't allow by design clients to mutate the kubelet state, the gRPC request/response model requires | ||
| read-write access to the podresources API socket. In other words, it is not possible to limit the container mount to `ReadOnly`. | ||
| - Multiple clients are allowed to connect to the podresources socket and consume the API, since it is stateless. | ||
| - The kubelet has [built-in rate limits](https://github.com/kubernetes/kubernetes/pull/116459) to mitigate local Denial of Service attacks from | ||
| misbehaving or malicious consumers. The consumers of the API must tolerate rate limit errors returned by the server. The rate limit is currently | ||
| hardcoded and global, so misbehaving clients can consume all the quota and potentially starve correctly behaving clients. | ||
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| ## Future enhancements | ||
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| For historical reasons, the podresources API has a less precise specification than typical kubernetes APIs (such as the Kubernetes HTTP API, or the container runtime interface). | ||
| This leads to unspecified behavior in corner cases. | ||
| An [effort](https://issues.k8s.io/119423) is ongoing to rectify this state and to have a more precise specification. | ||
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| The [Dynamic Resource Allocation (DRA)](https://github.com/kubernetes/enhancements/tree/master/keps/sig-node/3063-dynamic-resource-allocation) infrastructure | ||
| is a major overhaul of the resource management. | ||
| The [integration](https://github.com/kubernetes/enhancements/tree/master/keps/sig-node/3695-pod-resources-for-dra) with the podresources API | ||
| is already ongoing. | ||
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| ## Getting involved | ||
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| This feature is driven by [SIG Node](https://github.com/Kubernetes/community/blob/master/sig-node/README.md). | ||
| Please join us to connect with the community and share your ideas and feedback around the above feature and | ||
| beyond. We look forward to hearing from you! |