53 lines
3.1 KiB
Markdown
53 lines
3.1 KiB
Markdown
# Cluster add-ons
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Cluster add-ons are resources like Services and Deployments (with pods) that are
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shipped with the Kubernetes binaries and are considered an inherent part of the
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Kubernetes clusters. The add-ons are visible through the API (they can be listed using
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`kubectl`), but direct manipulation of these objects through Apiserver is discouraged
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because the system will bring them back to the original state, in particular:
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- If an add-on is deleted, it will be recreated automatically.
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- If an add-on is updated through Apiserver, it will be reconfigured to the state given by
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the supplied fields in the initial config.
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On the cluster, the add-ons are kept in `/etc/kubernetes/addons` on the master node, in
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yaml / json files. The addon manager periodically `kubectl apply`s the contents of this
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directory. Any legit modification would be reflected on the API objects accordingly.
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Particularly, rolling-update for deployments is now supported.
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Each add-on must specify the following label: `kubernetes.io/cluster-service: true`.
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Config files that do not define this label will be ignored. For those resources
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exist in `kube-system` namespace but not in `/etc/kubernetes/addons`, addon manager
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will attempt to remove them if they are attached with this label. Currently the other
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usage of `kubernetes.io/cluster-service` is for `kubectl cluster-info` command to recognize
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these cluster services.
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The suggested naming for most types of resources is just `<basename>` (with no version
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number) because we do not expect the resource name to change. But resources like `Pod`
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, `ReplicationController` and `DaemonSet` are exceptional. As `Pod` updates may not change
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fields other than `containers[*].image` or `spec.activeDeadlineSeconds` and may not add or
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remove containers, it may not be sufficient during a major update. For `ReplicationController`,
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most of the modifications would be legit, but the underlying pods would not got re-created
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automatically. `DaemonSet` has similar problem as the `ReplicationController`. In these
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cases, the suggested naming is `<basename>-<version>`. When version changes, the system will
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delete the old one and create the new one (order not guaranteed).
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# Add-on update procedure
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To update add-ons, just update the contents of `/etc/kubernetes/addons`
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directory with the desired definition of add-ons. Then the system will take care
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of:
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- Removing objects from the API server whose manifest was removed.
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- Creating objects from new manifests
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- Updating objects whose fields are legally changed.
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# Cooperating with Horizontal / Vertical Auto-Scaling
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As all cluster add-ons will be reconciled to the original state given by the initial config.
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In order to make Horizontal / Vertical Auto-scaling functional, the related fields in config should
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be left unset. More specifically, leave `replicas` in `ReplicationController` / `Deployment`
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/ `ReplicaSet` unset for Horizontal Scaling, and leave `resources` for container unset for Vertical
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Scaling. The periodical update won't include these specs, which will be managed by Horizontal / Vertical
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Auto-scaler.
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[]()
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