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Kube Controller Manager

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In this section, we will take a look at kube-controller-manager.

Kube Controller Manager manages various controllers in kubernetes.

In kubernetes terms, a controller is a process that continously monitors the state of the components within the system and works towards bringing the whole system to the desired functioning state.

Node Controller

Responsible for monitoring the state of the Nodes and taking necessary actions to keep the application running.
It does that with the help of kube-apiserver. Node Controller checks the status of the Nodes for every 5 seconds.
If a node is unreachable, Node Controller waits for 40 seconds before marking it unreachable.
After a node is marked as unreachable it gives it 5 minutes to come backup.

Replication Controller

It is responsible for monitoring the status of replicasets and ensuring that the desired number of pods are available at all time within the set.

Other Controllers

There are many more such controllers available within kubernetes

Installing Kube-Controller-Manager

When you install kube-controller-manager the different controllers will get installed as well.
Download the kube-controller-manager binary from the kubernetes release page. For example: You can download kube-controller-manager v1.13.0 here kube-controller-manager
```
$ wget https://storage.googleapis.com/kubernetes-release/release/v1.13.0/bin/linux/amd64/kube-controller-manager
```
By default all controllers are enabled, but you can choose to enable specific one from kube-controller-manager.service
```
$ cat /etc/systemd/system/kube-controller-manager.service
```

View kube-controller-manager - kubeadm

kubeadm deploys the kube-controller-manager as a pod in kube-system namespace
```
$ kubectl get pods -n kube-system
```

View kube-controller-manager options - kubeadm

You can see the options within the pod located at /etc/kubernetes/manifests/kube-controller-manager.yaml
```
$ cat /etc/kubernetes/manifests/kube-controller-manager.yaml
```

View kube-controller-manager options - Manual

In a non-kubeadm setup, you can inspect the options by viewing the kube-controller-manager.service
```
$ cat /etc/systemd/system/kube-controller-manager.service
```
You can also see the running process and affective options by listing the process on master node and searching for kube-controller-manager.
```
$ ps -aux |grep kube-controller-manager
```

K8s Referenece Docs: