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Controllers

The "brain" behind k8s.
Processes.
They monitor K8s Objects && respond to the objects.

Replicas

When a single-node, single-pod setup fails, replicas help deal with this.

Replication Controllers

These are K8s objects, in the same way pods are k8s objects.
These help run multiple instances of a single pod.
These can bring up a new pod in a node when a pod fails.
These can create multiple pods to share load.
Replication controllers can even deploy multiple pods across multiple nodes:

Scenario 1, a pod in a node:

  • Node N1, running
    • pod P1
      • running container C1
      • being watched by a replication controller

When the pod traffic gets to be too much, the replication controller duplicates the pod:

  • Node N1, running
    • pod P1
      • running container C1
      • being watched by a replication controller
    • pod P2
      • running container C2
      • being watched by the same replication controller

The replication controller can even scale to add more pods across multiple nodes:

  • Node N1, running
    • pod P1
      • running container C1
      • being watched by a replication controller
    • pod P2
      • running container C2
      • being watched by the same replication controller
  • Node N2, running
    • pod P3
      • running container C3
      • being watched by the same replication controller
    • pod P4
      • running container C4
      • being watched by the same replication controller

Different than a Replica Set

A Controller is the "older" tech being replaced by replica sets.

Creating a Replication Controller

# replication-controller.yml
apiVersion: v1
kind: ReplicationController
metadata:
  name: repl-con
  labels:
    app: repl-app
    type: front-end
spec:
  # provide a pod template here!!
  template:
    metadata:
      name: nginx-pod
      labels:
        app: nginx-app
        type: frontend
    spec:
      containers:
        - name: nginx-container
          image: nginx
  replicas: 3

Notes:

  • same 4 parent fields as a pod definition file
  • the template contents are nearly identical to the pod definition file that the replication controller is "watching"
    • two of the four required root-level yaml config fields are present: metadata and spec
    • two fields are missing: apiVersion and kind
  • the replicas tells the ReplicationController how many pods to make
    • the replicas is a sibling of template
  • run it
Jakes-4:k8s Jake$ kubectl create -f configs/rc/rc.yml
replicationcontroller/first-rc created

# check status
Jakes-4:k8s Jake$ kubectl get replicationcontroller
NAME       DESIRED   CURRENT   READY   AGE
first-rc   3         3         3       25s

Jakes-4:k8s Jake$ kubectl get replicationcontroller/first-rc
NAME       DESIRED   CURRENT   READY   AGE
first-rc   3         3         3       30s

# wider
Jakes-4:k8s Jake$ kubectl get replicationcontroller/first-rc -o wide
NAME       DESIRED   CURRENT   READY   AGE   CONTAINERS        IMAGES   SELECTOR
first-rc   3         3         3       47s   nginx-container   nginx    app=nginx-app,type=front-end

# see the pods
Jakes-4:k8s Jake$ kubectl get pods
NAME             READY   STATUS    RESTARTS   AGE
first-rc-gp5fk   1/1     Running   0          75s
first-rc-jrwzd   1/1     Running   0          75s
first-rc-t6r45   1/1     Running   0          75s

# see the pods, wider
Jakes-4:k8s Jake$ kubectl get pods -o wide
NAME             READY   STATUS    RESTARTS   AGE   IP           NODE       NOMINATED NODE   READINESS GATES
first-rc-gp5fk   1/1     Running   0          95s   172.17.0.5   minikube   <none>           <none>
first-rc-jrwzd   1/1     Running   0          95s   172.17.0.6   minikube   <none>           <none>
first-rc-t6r45   1/1     Running   0          95s   172.17.0.7   minikube   <none>           <none>

Replica Set

A definition file

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: rs
  labels:
    app: rs-app
    type: front-end
spec:
  template:
    metadata:
      name: rs-pod
      labels:
        app: myapp-here
        type: front-end
    spec:
      containers:
        - name: nginx-box
          image: nginx
  replicas: 3
  selector:
    matchLabels:
      type: front-end

Note:

  • selector
    • describe "what pods apply here" - replica sets can manage other pods not explicitly describe in the definition file - epic
    • assumes to be the same as the spec template
    • MAJOR DIFFERENCE betweeen replica set and replication controller
    • hmm, the matchLabels - hmm
  • Replica Sets can be spun up to monitor existing already-running pods
    • CAN create pods if they are not already present
    • on pod failure, re-create the busted pod

Labels and Selectors

Labels are the "shorthand" that replica set "monitors" use to "watch" pods.
When pods are already up && running, the spec:template section of the replSet def file still is required. Interesting and redundant perhaps.

Updating a replica set

say more pods are needed.

  • Update the yaml
  • kubectl replace -f repl-def-file.yml
  • OR
  • kubectl scale --replicas=6 repl-def-file.yml
  • OR
  • another "advanced" way based on load

NOTE: kubectl replace is an interesting and rarely referenced command.

Takeaways

  • kubectl create -f replicated-def.yml
  • kubectl get replicaset
    • see repl sets
    • replicaset should be the name of the set
  • kubectl delete replicaset
    • replicaset should be the name of the set
  • kubectl replace -f replicated-def.yml
  • kubectl scale --replicas=6 -f repl-def.yml
    • updates a running replica set
    • does not alter the replica set difinition file

One more:

Jakes-4:k8s Jake$ kubectl get replicationcontrollers
NAME       DESIRED   CURRENT   READY   AGE
first-rc   3         3         3       115m
Jakes-4:k8s Jake$ kubectl get replicationcontrollers -o wide
NAME       DESIRED   CURRENT   READY   AGE    CONTAINERS        IMAGES   SELECTOR
first-rc   3         3         3       115m   nginx-container   nginx    app=nginx-app,type=front-end

Creating a Replica Set from yaml

See configs/rc/replica-set.yml for an example of a replicaSEt yaml file. Here it is:

apiVersion: apps/v1
kind: ReplicaSet
metadata:
  # no upper-case letters here!
  name: first-replica-set
  labels:
     app: replica-set-app
spec:
  selector:
    matchLabels:
      app: replica-set-app
  replicas: 3
  template:
    metadata:
      name: nginx-from-replica-set
      labels:
        app: replica-set-app
    spec:
      containers:
        - name: nginx-container
          image: nginx

Run it:

# create it
Jakes-4:rc Jake$ kubectl create -f replica-set.yml
replicaset.apps/first-replica-set created

# see the rs
Jakes-4:rc Jake$ kubectl get replicaset
NAME                DESIRED   CURRENT   READY   AGE
first-replica-set   3         3         3       10s

# see the pods
Jakes-4:rc Jake$ kubectl get pods
NAME                      READY   STATUS    RESTARTS   AGE
first-replica-set-6vx5t   1/1     Running   0          118s
first-replica-set-mrs27   1/1     Running   0          118s
first-replica-set-t7fhg   1/1     Running   0          118s

Messing with replica sets

  • see the pods
  • pick one & delete it
  • watch the replica watcher spin up another pod instantaneously
# delete a pod then check on it
Jakes-4:rc Jake$ kubectl delete pod first-replica-set-6vx5t

# see the updates
Jakes-4:rc Jake$ kubectl get pods
NAME                      READY   STATUS    RESTARTS   AGE
# this one is new!!
first-replica-set-ddp57   1/1     Running   0          3s
first-replica-set-mrs27   1/1     Running   0          4m30s
first-replica-set-t7fhg   1/1     Running   0          4m30s

# see stats on the replica set
Jakes-4:rc Jake$ kubectl describe replicaset 
Name:         first-replica-set
Namespace:    default
Selector:     app=replica-set-app
Labels:       app=replica-set-app
Annotations:  <none>
Replicas:     3 current / 3 desired
Pods Status:  3 Running / 0 Waiting / 0 Succeeded / 0 Failed
Pod Template:
  Labels:  app=replica-set-app
  Containers:
   nginx-container:
    Image:        nginx
    Port:         <none>
    Host Port:    <none>
    Environment:  <none>
    Mounts:       <none>
  Volumes:        <none>
Events:
  Type    Reason            Age    From                   Message
  ----    ------            ----   ----                   -------
  Normal  SuccessfulCreate  5m57s  replicaset-controller  Created pod: first-replica-set-mrs27
  Normal  SuccessfulCreate  5m57s  replicaset-controller  Created pod: first-replica-set-6vx5t
  Normal  SuccessfulCreate  5m57s  replicaset-controller  Created pod: first-replica-set-t7fhg
  Normal  SuccessfulCreate  90s    replicaset-controller  Created pod: first-replica-set-ddp57

Create a new pod in a replicaset

  • add a new pod from cli
    • make sure the pod's label matches the app in the replica set
kubectl create -f clone-pod.yml

# 
kubctl get pods

The get pods will show the pod in a terminated state.
The replica set controller "kills" the pod.

Update a replica set

kubectl edit replicaset

That will open a replicaset yaml-looking-file in the terminal.
This file is temporary. In memory. Made by k8s.
Changes here will apply on-save.
Epic.
Here, one could do things like change the replicas value, here from 3 to 4.
Then, kubectl get pods, and see the 4th pod added.

# one-liner, changing replicas in the cli
kubectl scale replicaset first-replica-set --replicas=2

TakeAways

  • get how many pods on a node
    • kubectl get pods
  • get how many replicasets on a node
    • kubectl get replicasets
  • see how many pods are part of a replica set
    • kubectl get pods -o wide
  • make sure metadata:labels:<label+val> match spec:selector:matchLabels:<label+val>
  • update a replica set
    • in real-time, edit the replica set
    • change the template image
    • remove each pod 1-by-1 so that k8s can auto-recreate them using the new image
Tags:
Kubernetes
k8s
controllers
replicas