ReplicaSet

We now know how to create pods, i.e. manually.
But in real world we never want to create a pod manually.
We always use higher level construct to manage pods.
One of the higher level construct that we use to manage pods is ReplicaSet. (Although we will use one more higher level construct that manages ReplicaSet later).
A ReplicaSet ensures the pods it manages are always kept running.
If the pod disappears for any reason, ReplicaSet notices the missing pod and creates a replacement pod.

How does `ReplicaSets` work?

ReplicaSet constantly monitors the list of running pods.
Label selector is used to filter the pods that ReplicaSet manages.
ReplicaSet makes sure the actual number of pods it manages always matches the desired number.
If too few such pods are running, it creates new replicas from a pod template.
If too many such pods are running, it removes the excess replicas.

Defining `ReplicaSet` `yaml` file

apiVersion: apps/v1             # 1                           
kind: ReplicaSet                # 2                      
metadata: # 3               
  name: v1-web                  # 4                    
  labels: # 5               
    version: v1                 # 5                     
    env: prod                   # 5                   
spec: # 6           
  replicas: 3                   # 6                   
  selector: # 8                 
    matchLabels: # 9                   
      version: v1               # 10                      
      env: prod                 # 10                  
  template:
    # After this it just a Pod template we had before                                                                   
    metadata:
      name: v1-web
      labels: # 11                                                
        version: v1             # 12                                                      
        env: prod               # 12                                                    
    spec:
      containers:
        - image: amantuladhar/docker-kubernetes:v1-web
          name: v1-web
          livenessProbe:
            httpGet:
              path: /test/
              port: 8080
          ports:
            - containerPort: 8080

#1: ReplicaSet are defined under apiVersion apps/v1.
#2: We are setting our resource type to ReplicaSet.
#3: Defines the metadata information for our resource.
#4: We are setting the name for our ReplicaSet.
#5: labels includes label added for a ReplicaSet.
#6: spec includes the specification for our resource.
#7: replicas tells ReplicaSet how many copies of pods we want. ReplicaSet make sure that specified number of pods are always running.
#8: selector will have list of labels that ReplicaSet will target. These are the labels that determines which pods are managed by ReplicaSet. This selector must somehow match labels defined in #12, #13.
#9: matchLabels indicate that we are trying to value label key with label value.
- We also have matchExpression which is more expressive. (More on this later).
template is the template for the pod our ReplicaSet manages. When starting the ReplicaSet or adding new Pods, it will use this template to start the Pod.
- Notice: All properties below template is same as Pod yaml file we defined before.

Create `ReplicaSet`

To create a ReplicaSet we simply use kubectl create -f replicaset.yaml.

➜ kubectl create -f replicaset.yaml
replicaset.apps/v1-web created

List `ReplicaSet`.

To list the ReplicaSet we use same command like before but different resource.
kubectl get replicaset or kubectl get rs.
rs is the short form for replicaset.

➜ kubectl get replicaset           
NAME     DESIRED   CURRENT   READY   AGE
v1-web   3         3         3       42s

➜ kubectl get rs        
NAME     DESIRED   CURRENT   READY   AGE
v1-web   3         3         3       44s

List the pods

If you list the running pods, you will see you have three pods running.

➜ kubectl get po
NAME           READY   STATUS    RESTARTS   AGE
v1-web-8s5zw   1/1     Running   0          3m36s
v1-web-r9pkw   1/1     Running   0          3m36s
v1-web-xk9sk   1/1     Running   0          3m36s

po is short form for pods.

Scaling our App

Scale Up

If we ever want to scale up our application, it will be very easy now.
There are many ways you can scale your pods.
- kubectl patch
- kubectl apply
- kubectl edit
- kubectl scale
We already know how to use first three options from previous section.
Let's use fourth one

`kubectl scale`

kubectl scale rs --replicas=@number @rs_name
Using this command you can quickly change the number of replicas you want to run.

➜ kubectl scale rs --replicas=10 v1-web

List the pod again, you will see number of running pods will increase.

➜ kubectl get po
NAME           READY   STATUS    RESTARTS   AGE
v1-web-8s5zw   1/1     Running   0          6m48s
v1-web-fdnwf   1/1     Running   0          8s
v1-web-l6dbh   1/1     Running   0          8s
v1-web-lxkp7   1/1     Running   0          8s
v1-web-n6rfh   1/1     Running   0          8s
v1-web-q9zdq   1/1     Running   0          8s
v1-web-r9pkw   1/1     Running   0          6m48s
v1-web-s455s   1/1     Running   0          8s
v1-web-xctpn   1/1     Running   0          8s
v1-web-xk9sk   1/1     Running   0          6m48s

Scale Down

Scaling down is super easy as well.
Just update the replicas value. Set it to 3 for now.

➜ kubectl scale rs --replicas=3 v1-web

ReplicaSet `matchExpression`

We saw how matchLabels works in previous example.
But ReplicaSet can have more expressive label selector than just matching key and value pairs.

selector:
  matchExpressions:
    - key: @label_key
      operator: @operator_value
      values:
        - @label_value
        - @label_value

When we use matchExpressions we have few option that we can tweak.
We can have multiple matchExpression as well.
key will take a label key.
operator will take one of the pre-defined operators
- In: Label’s value must match one of the specified values.
- NotIn: Label’s value must not match any of the specified values.
- Exists: Pod must include a label with the specified key. When using this operator, you shouldn’t specify the values field.
- DoesNotExist: Pod must not include a label with the specified key. The values property must not be specified.
values: If we use In or NotIn list of values to look for.
Let's generate the scenario where we can work on.
First run the ReplicaSet yaml file you had before.
You will end up with three Pods. Override one of Pod label env=prod to env=dev
The Pod whose label was change is now no longer managed by our ReplicaSet.
Now ReplicaSet will create a new Pod to match the desired state.
If you list the running Pod you will see we have 4 pods. (3 managed by ReplicaSet, 1 not)

➜ kubectl get pods -L version,env                    
NAME           READY   STATUS    RESTARTS   AGE   VERSION   ENV
v1-web-8s5zw   1/1     Running   0          31m   v1        dev
v1-web-njbt9   1/1     Running   0          5s    v1        prod
v1-web-r9pkw   1/1     Running   0          31m   v1        prod
v1-web-xk9sk   1/1     Running   0          31m   v1        prod

How do we tell ReplicaSet to manage Pods with label env=dev and env=prod.
How to not create extra Pod?

Using Operator

Let's update our ReplicaSet label selector to

selector:
  matchExpressions:
    - key: env
      operator: In
      values:
        - dev
        - prod

Using In we can define that ReplicaSet can look for either value dev or prod for label env.
If we update one of the pod label env to dev now, it won't create new Pod. As updated Pod is still managed by ReplicaSet and replicas count match the desired state.

➜ kubectl label po v1-web-6bcgx env=dev --overwrite
pod/v1-web-6bcgx labeled

Now list the pods.

NAME           READY   STATUS    RESTARTS   AGE     VERSION   ENV
v1-web-6bcgx   1/1     Running   0          6m22s   v1        dev
v1-web-mkk79   1/1     Running   0          6m22s   v1        prod
v1-web-sxl59   1/1     Running   0          6m22s   v1        prod