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Reconfigure a Node's Kubelet in a Live Cluster

FEATURE STATE: Kubernetes v1.9 alpha

As of Kubernetes 1.8, the new Dynamic Kubelet Configuration feature is available in alpha. This allows you to change the configuration of Kubelets in a live Kubernetes cluster via first-class Kubernetes concepts. Specifically, this feature allows you to configure individual Nodes’ Kubelets via ConfigMaps.

Warning: All Kubelet configuration parameters may be changed dynamically, but not all parameters are safe to change dynamically. This feature is intended for system experts who have a strong understanding of how configuration changes will affect behavior. No documentation currently exists which plainly lists “safe to change” fields, but we plan to add it before this feature graduates from alpha.

• Before you begin
• Reconfiguring the Kubelet on a Live Node in your Cluster
  • Basic Workflow Overview
  • Node Authorizer Workarounds
  • Generating a file that contains the current configuration
  • Edit the configuration file
  • Push the configuration file to the control plane
  • Authorize your Node to read the new ConfigMap
  • Set the Node to use the new configuration
  • Observe that the Node begins using the new configuration
  • Edit the configuration file again
  • Push the newly edited configuration to the control plane
  • Authorize your Node to read the new ConfigMap
  • Configure the Node to use the new configuration
  • Observe that the Kubelet is using the new configuration
  • Deauthorize your Node fom reading the old ConfigMap
  • Reset the Node to use its local default configuration
  • Observe that the Node is using its local default configuration
  • Deauthorize your Node fom reading the old ConfigMap
• Kubectl Patch Example
• Understanding ConfigOK Conditions

Before you begin

• A live Kubernetes cluster with both Master and Node at v1.8 or higher must be running, with the DynamicKubeletConfig feature gate enabled and the Kubelet’s --dynamic-config-dir flag set to a writeable directory on the Node. This flag must be set to enable Dynamic Kubelet Configuration.
• The kubectl command-line tool must be also v1.8 or higher, and must be configured to communicate with the cluster.

Reconfiguring the Kubelet on a Live Node in your Cluster

Basic Workflow Overview

The basic workflow for configuring a Kubelet in a live cluster is as follows:

1. Write a YAML or JSON configuration file containing the Kubelet’s configuration.
2. Wrap this file in a ConfigMap and save it to the Kubernetes control plane.
3. Update the Kubelet’s correspoinding Node object to use this ConfigMap.

Each Kubelet watches a configuration reference on its respective Node object. When this reference changes, the Kubelet downloads the new configuration and exits. For the feature to work correctly, you must be running a process manager (like systemd) which will restart the Kubelet when it exits. When the Kubelet is restarted, it will begin using the new configuration.

The new configuration completely overrides the old configuration; unspecified fields in the new configuration will receive their canonical default values. Some CLI flags do not have an associated configuration field, and will not be affected by the new configuration. These fields are defined by the KubeletFlags structure, here.

The status of the Node’s Kubelet configuration is reported via the ConfigOK condition in the Node status. Once you have updated a Node to use the new ConfigMap, you can observe this condition to confirm that the Node is using the intended configuration. A table describing the possible conditions can be found at the end of this article.

This document describes editing Nodes using kubectl edit. There are other ways to modify a Node’s spec, including kubectl patch, for example, which facilitate scripted workflows.

This document only describes a single Node consuming each ConfigMap. Keep in mind that it is also valid for multiple Nodes to consume the same ConfigMap.

Node Authorizer Workarounds

The Node Authorizer does not yet pay attention to which ConfigMaps are assigned to which Nodes. If you currently use the Node authorizer, your Kubelets will not be automatically granted permission to download their respective ConfigMaps.

The temporary workaround used in this document is to manually create the RBAC Roles and RoleBindings for each ConfigMap. The Node Authorizer will be extended before the Dynamic Kubelet Configuration feature graduates from alpha, so doing this in production should never be necessary.

Generating a file that contains the current configuration

The Dynamic Kubelet Configuration feature allows you to provide an override for the entire configuration object, rather than a per-field overlay. This is a simpler model that makes it easier to trace the source of configuration values and debug issues. The compromise, however, is that you must start with knowledge of the existing configuration to ensure that you only change the fields you intend to change.

In the future, the Kubelet will be bootstrapped from a file on disk (see Set Kubelet parameters via a config file), and you will simply edit a copy of this file (which, as a best practice, should live in version control) while creating the first Kubelet ConfigMap. Today, however, the Kubelet is still bootstrapped with command-line flags. Fortunately, there is a dirty trick you can use to generate a config file containing a Node’s current configuration. The trick involves hitting the Kubelet server’s configz endpoint via the kubectl proxy. This endpoint, in its current implementation, is intended to be used only as a debugging aid, which is part of why this is a dirty trick. There is ongoing work to improve the endpoint, and in the future this will be a less “dirty” operation. This trick also requires the jq command to be installed on your machine, for unpacking and editing the JSON response from the endpoint.

Do the following to generate the file:

1. Pick a Node to reconfigure. We will refer to this Node’s name as NODE_NAME.
2. Start the kubectl proxy in the background with kubectl proxy --port=8001 &
3. Run the following command to download and unpack the configuration from the configz endpoint:

$ export NODE_NAME=the-name-of-the-node-you-are-reconfiguring
$ curl -sSL http://localhost:8001/api/v1/proxy/nodes/${NODE_NAME}/configz | jq '.kubeletconfig|.kind="KubeletConfiguration"|.apiVersion="kubeletconfig/v1alpha1"' > kubelet_configz_${NODE_NAME}

Note that we have to manually add the kind and apiVersion to the downloaded object, as these are not reported by the configz endpoint. This is one of the limitations of the endpoint that is planned to be fixed in the future.

Edit the configuration file

Using your editor of choice, change one of the parameters in the kubelet_configz_${NODE_NAME} file from the previous step. A QPS parameter, eventRecordQPS for example, is a good candidate.

Push the configuration file to the control plane

Push the edited configuration file to the control plane with the following command:

$ kubectl -n kube-system create configmap my-node-config --from-file=kubelet=kubelet_configz_${NODE_NAME} --append-hash -o yaml

You should see a response similar to:

apiVersion: v1
data:
  kubelet: |
    {...}
kind: ConfigMap
metadata:
  creationTimestamp: 2017-09-14T20:23:33Z
  name: my-node-config-gkt4c2m4b2
  namespace: kube-system
  resourceVersion: "119980"
  selfLink: /api/v1/namespaces/kube-system/configmaps/my-node-config-gkt4c2m4b2
  uid: 946d785e-998a-11e7-a8dd-42010a800006

Note that the configuration data must appear under the ConfigMap’s kubelet key.

We create the ConfigMap in the kube-system namespace, which is appropriate because this ConfigMap configures a Kubernetes system component - the Kubelet.

The --append-hash option appends a short checksum of the ConfigMap contents to the name. This is convenient for an edit->push workflow, as it will automatically, yet deterministically, generate new names for new ConfigMaps.

We use the -o yaml output format so that the name, namespace, and uid are all reported following creation. We will need these in the next step. We will refer to the name as CONFIG_MAP_NAME and the uid as CONFIG_MAP_UID.

Authorize your Node to read the new ConfigMap

Now that you’ve created a new ConfigMap, you need to authorize your node to read it. First, create a Role for your new ConfigMap with the following commands:

$ export CONFIG_MAP_NAME=name-from-previous-output
$ kubectl -n kube-system create role ${CONFIG_MAP_NAME}-reader --verb=get --resource=configmap --resource-name=${CONFIG_MAP_NAME}

Next, create a RoleBinding to associate your Node with the new Role:

$ kubectl -n kube-system create rolebinding ${CONFIG_MAP_NAME}-reader --role=${CONFIG_MAP_NAME}-reader --user=system:node:${NODE_NAME}

Once the Node Authorizer is updated to do this automatically, you will be able to skip this step.

Set the Node to use the new configuration

Edit the Node’s reference to point to the new ConfigMap with the following command:

kubectl edit node ${NODE_NAME}

Once in your editor, add the following YAML under spec:

configSource:
    configMapRef:
        name: CONFIG_MAP_NAME
        namespace: kube-system
        uid: CONFIG_MAP_UID

Be sure to specify all three of name, namespace, and uid.

Observe that the Node begins using the new configuration

Retrieve the Node with kubectl get node ${NODE_NAME} -o yaml, and look for the ConfigOK condition in status.conditions. You should see the message Using current (UID: CONFIG_MAP_UID) when the Kubelet starts using the new configuration.

For convenience, you can use the following command (using jq) to filter down to the ConfigOK condition:

$ kubectl get no ${NODE_NAME} -o json | jq '.status.conditions|map(select(.type=="ConfigOK"))'
[
  {
    "lastHeartbeatTime": "2017-09-20T18:08:29Z",
    "lastTransitionTime": "2017-09-20T18:08:17Z",
    "message": "using current (UID: \"2ebc8d1a-9e2a-11e7-a8dd-42010a800006\")",
    "reason": "passing all checks",
    "status": "True",
    "type": "ConfigOK"
  }
]

If something goes wrong, you may see one of several different error conditions, detailed in the Table of ConfigOK Conditions, below. When this happens, you should check the Kubelet’s log for more details.

Edit the configuration file again

To change the configuration again, we simply repeat the above workflow. Try editing the kubelet file, changing the previously changed parameter to a new value.

Push the newly edited configuration to the control plane

Push the new configuration to the control plane in a new ConfigMap with the following command:

$ kubectl create configmap my-node-config --namespace=kube-system --from-file=kubelet=kubelet_configz_${NODE_NAME} --append-hash -o yaml

This new ConfigMap will get a new name, as we have changed the contents. We will refer to the new name as NEW_CONFIG_MAP_NAME and the new uid as NEW_CONFIG_MAP_UID.

Authorize your Node to read the new ConfigMap

Now that you’ve created a new ConfigMap, you need to authorize your node to read it. First, create a Role for your new ConfigMap with the following commands:

$ export NEW_CONFIG_MAP_NAME=name-from-previous-output
$ kubectl -n kube-system create role ${NEW_CONFIG_MAP_NAME}-reader --verb=get --resource=configmap --resource-name=${NEW_CONFIG_MAP_NAME}

Next, create a RoleBinding to associate your Node with the new Role:

$ kubectl -n kube-system create rolebinding ${NEW_CONFIG_MAP_NAME}-reader --role=${NEW_CONFIG_MAP_NAME}-reader --user=system:node:${NODE_NAME}

Once the Node Authorizer is updated to do this automatically, you will be able to skip this step.

Configure the Node to use the new configuration

Once more, edit the Node’s spec.configSource with kubectl edit node ${NODE_NAME}. Your new spec.configSource should look like the following, with name and uid substituted as necessary:

configSource:
    configMapRef:
        name: NEW_CONFIG_MAP_NAME
        namespace: kube-system
        uid: NEW_CONFIG_MAP_UID

Observe that the Kubelet is using the new configuration

Once more, retrieve the Node with kubectl get node ${NODE_NAME} -o yaml, and look for the ConfigOK condition in status.conditions. You should the message Using current (UID: NEW_CONFIG_MAP_UID) when the Kubelet starts using the new configuration.

Deauthorize your Node fom reading the old ConfigMap

Once you know your Node is using the new configuration and are confident that the new configuration has not caused any problems, it is a good idea to deauthorize the node from reading the old ConfigMap. Run the following commands to remove the RoleBinding and Role:

$ kubectl -n kube-system delete rolebinding ${CONFIG_MAP_NAME}-reader
$ kubectl -n kube-system delete role ${CONFIG_MAP_NAME}-reader

Note that this does not necessarily prevent the Node from reverting to the old configuration, as it may locally cache the old ConfigMap for an indefinite period of time.

You may optionally also choose to remove the old ConfigMap:

$ kubectl -n kube-system delete configmap ${CONFIG_MAP_NAME}

Once the Node Authorizer is updated to do this automatically, you will be able to skip this step.

Reset the Node to use its local default configuration

Finally, if you wish to reset the Node to use the configuration it was provisioned with, simply edit the Node with kubectl edit node ${NODE_NAME} and remove the spec.configSource subfield.

Observe that the Node is using its local default configuration

After removing this subfield, you should eventually observe that the ConfigOK condition’s message reverts to either using current (default) or using current (init), depending on how the Node was provisioned.

Deauthorize your Node fom reading the old ConfigMap

Once you know your Node is using the default configuraiton again, it is a good idea to deauthorize the node from reading the old ConfigMap. Run the following commands to remove the RoleBinding and Role:

$ kubectl -n kube-system delete rolebinding ${NEW_CONFIG_MAP_NAME}-reader
$ kubectl -n kube-system delete role ${NEW_CONFIG_MAP_NAME}-reader

Note that this does not necessarily prevent the Node from reverting to the old ConfigMap, as it may locally cache the old ConfigMap for an indefinite period of time.

You may optionally also choose to remove the old ConfigMap:

$ kubectl -n kube-system delete configmap ${NEW_CONFIG_MAP_NAME}

Once the Node Authorizer is updated to do this automatically, you will be able to skip this step.

Kubectl Patch Example

As mentioned above, there are many ways to change a Node’s configSource. Here is an example command that uses kubectl patch:

kubectl patch node ${NODE_NAME} -p "{\"spec\":{\"configSource\":{\"configMapRef\":{\"name\":\"${CONFIG_MAP_NAME}\",\"namespace\":\"kube-system\",\"uid\":\"${CONFIG_MAP_UID}\"}}}}"

Understanding ConfigOK Conditions

The following table describes several of the ConfigOK Node conditions you might encounter in a cluster that has Dynamic Kubelet Config enabled. If you observe a condition with status=False, you should check the Kubelet log for more error details by searching for the message or reason text.