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Add a diagram shows how metrics-server handles kubectl top pod request #1654

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Add a diagram shows how metrics-server handles kubectl top pod request #1654

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Add a diagram shows how metrics-server handles a list pod request /ap…
CatherineF-dev May 7, 2025
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Update README.md
CatherineF-dev May 8, 2025
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Update README.md
CatherineF-dev May 8, 2025
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20 changes: 20 additions & 0 deletions README.md
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Expand Up @@ -166,6 +166,26 @@ For more information, see:
[Metrics API design]: https://github.com/kubernetes/design-proposals-archive/blob/main/instrumentation/resource-metrics-api.md
[Metrics Server design]: https://github.com/kubernetes/design-proposals-archive/blob/main/instrumentation/metrics-server.md

This diagram shows how metrics-server handles a `kubectl top pods` request:
```mermaid
sequenceDiagram
participant Client
participant APIServer
participant MS as api.podMetrics
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Would be good to be able to recognize participants that are part of Metrics Server and which are not. I also don't know if mentioning internal implementation details like structures storage.Storage brings any value or would just confuse people that don't know them.

Could maybe just simplify the diagram to have metrics server as one participant and maybe create a separate diagram for internal component architecture?

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Maybe we could squeeze in how metrics are collected from Kubelets

participant Stg as storage.Storage
participant PL as cache.GenericLister (Pods)

Client->>APIServer: GET /apis/metrics.k8s.io/v1beta1/namespaces/{ns}/pods[/{podName}]
APIServer->>MS: List(ctx, opts) or Get(ctx, name, opts)
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Could we simplify this diagram to just show List and skip mention of Get. At the top you mention kubectl top pods which will do List as pod name is not specified.

MS->>PL: List(selector) or Get(name) (via ByNamespace)
PL-->>MS: pods[] (PartialObjectMetadata)
MS->>Stg: GetPodMetrics(pods...)
Stg->>Stg: Read last and prev pod/container points, calculate usage
Stg-->>MS: podMetricsList[]
MS->>APIServer: metrics.PodMetricsList or metrics.PodMetrics
APIServer-->>Client: Response
```

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