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Deployments

• What is a Deployment?
• Creating a Deployment
• Updating a Deployment
  • Multiple Updates
• Writing a Deployment Spec
  • Pod Template
  • Replicas
  • Selector
  • Strategy
    • Recreate Deployment
    • Rolling Update Deployment
      • Max Unavailable
      • Max Surge
      • Min Ready Seconds
• Alternative to Deployments
  • kubectl rolling update

What is a Deployment?

A Deployment provides declarative updates for Pods and ReplicationControllers. Users describe the desired state in a Deployment object, and the deployment controller changes the actual state to the desired state at a controlled rate. Users can define Deployments to create new resources, or replace existing ones by new ones.

A typical use case is:

• Create a Deployment to bring up a replication controller and pods.
• Later, update that Deployment to recreate the pods (for example, to use a new image).

Creating a Deployment

Here is an example Deployment. It creates a replication controller to bring up 3 nginx pods.

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  replicas: 3
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80

Run the example by downloading the example file and then running this command:

$ kubectl create -f docs/user-guide/nginx-deployment.yaml
deployment "nginx-deployment" created

Running

$ kubectl get deployments

immediately will give:

$ kubectl get deployments
NAME               UPDATEDREPLICAS   AGE
nginx-deployment   0/3               8s

This indicates that the Deployment is trying to update 3 replicas, and has not updated any of them yet.

Running the get again after a minute, should give:

$ kubectl get deployments
NAME               UPDATEDREPLICAS   AGE
nginx-deployment   3/3               1m

This indicates that the Deployment has created all three replicas. Running kubectl get rc and kubectl get pods will show the replication controller (RC) and pods created.

$ kubectl get rc
CONTROLLER                      CONTAINER(S)   IMAGE(S)      SELECTOR                                                        REPLICAS   AGE
REPLICAS   AGE
deploymentrc-1975012602         nginx          nginx:1.7.9   pod-template-hash=1975012602,app=nginx   3          2m

$ kubectl get pods
NAME                            READY          STATUS        RESTARTS       AGE
deploymentrc-1975012602-4f2tb   1/1            Running       0              1m
deploymentrc-1975012602-j975u   1/1            Running       0              1m
deploymentrc-1975012602-uashb   1/1            Running       0              1m

The created RC will ensure that there are three nginx pods at all times.

Updating a Deployment

Suppose that we now want to update the nginx pods to start using the nginx:1.9.1 image instead of the nginx:1.7.9 image. For this, we update our deployment file as follows:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  replicas: 3
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.9.1
        ports:
        - containerPort: 80

We can then apply the Deployment:

$ kubectl apply -f docs/user-guide/new-nginx-deployment.yaml
deployment "nginx-deployment" configured

Running a get immediately will still give:

$ kubectl get deployments
NAME               UPDATEDREPLICAS   AGE
nginx-deployment   3/3               8s

This indicates that deployment status has not been updated yet (it is still showing old status). Running a get again after a minute, should show:

$ kubectl get deployments
NAME               UPDATEDREPLICAS   AGE
nginx-deployment   1/3               1m

This indicates that the Deployment has updated one of the three pods that it needs to update. Eventually, it will update all the pods.

$ kubectl get deployments
NAME               UPDATEDREPLICAS   AGE
nginx-deployment   3/3               3m

We can run kubectl get rc to see that the Deployment updated the pods by creating a new RC, which it scaled up to 3 replicas, and has scaled down the old RC to 0 replicas.

kubectl get rc
CONTROLLER                CONTAINER(S)   IMAGE(S)      SELECTOR                                                         REPLICAS   AGE
deploymentrc-1562004724   nginx          nginx:1.9.1   pod-template-hash=1562004724,app=nginx   3          5m
deploymentrc-1975012602   nginx          nginx:1.7.9   pod-template-hash=1975012602,app=nginx   0          7m

Running get pods should now show only the new pods:

kubectl get pods
NAME                            READY     STATUS    RESTARTS   AGE
deploymentrc-1562004724-0tgk5   1/1       Running   0          9m
deploymentrc-1562004724-1rkfl   1/1       Running   0          8m
deploymentrc-1562004724-6v702   1/1       Running   0          8m

Next time we want to update these pods, we can just update and re-apply the Deployment again.

Deployment ensures that not all pods are down while they are being updated. By default, it ensures that minimum of 1 less than the desired number of pods are up. For example, if you look at the above deployment closely, you will see that it first created a new pod, then deleted some old pods and created new ones. It does not kill old pods until a sufficient number of new pods have come up.

$ kubectl describe deployments
Name:                           nginx-deployment
Namespace:                      default
CreationTimestamp:              Thu, 22 Oct 2015 17:58:49 -0700
Labels:                         app=nginx-deployment
Selector:                       app=nginx
Replicas:                       3 updated / 3 total
StrategyType:                   RollingUpdate
RollingUpdateStrategy:          1 max unavailable, 1 max surge, 0 min ready seconds
OldReplicationControllers:      deploymentrc-1562004724 (3/3 replicas created)
NewReplicationController:       <none>
Events:
  FirstSeen     LastSeen        Count   From                            SubobjectPath   Reason          Message
  10m           10m             1       {deployment-controller }                        ScalingRC       Scaled up rc deploymentrc-1975012602 to 3
  2m            2m              1       {deployment-controller }                        ScalingRC       Scaled up rc deploymentrc-1562004724 to 1
  2m            2m              1       {deployment-controller }                        ScalingRC       Scaled down rc deploymentrc-1975012602 to 1
  1m            1m              1       {deployment-controller }                        ScalingRC       Scaled up rc deploymentrc-1562004724 to 3
  1m            1m              1       {deployment-controller }                        ScalingRC       Scaled down rc deploymentrc-1975012602 to 0

Here we see that when we first created the Deployment, it created an RC and scaled it up to 3 replicas directly. When we updated the Deployment, it created a new RC and scaled it up to 1 and then scaled down the old RC by 1, so that at least 2 pods were available at all times. It then scaled up the new RC to 3 and when those pods were ready, it scaled down the old RC to 0.

Multiple Updates

Each time a new deployment object is observed, a replication controller is created to bring up the desired pods if there is no existing RC doing so. Existing RCs controlling pods whose labels match .spec.selector but whose template does not match .spec.template are scaled down. Eventually, the new RC will be scaled to .spec.replicas and all old RCs will be scaled to 0.

If the user updates a Deployment while an existing deployment is in progress, the Deployment will create a new RC as per the update and start scaling that up, and will roll the RC that it was scaling up previously– it will add it to its list of old RCs and will start scaling it down.

For example, suppose the user creates a deployment to create 5 replicas of nginx:1.7.9, but then updates the deployment to create 5 replicas of nginx:1.9.1, when only 3 replicas of nginx:1.7.9 had been created. In that case, deployment will immediately start killing the 3 nginx:1.7.9 pods that it had created, and will start creating nginx:1.9.1 pods. It will not wait for 5 replicas of nginx:1.7.9 to be created before changing course.

Writing a Deployment Spec

As with all other Kubernetes configs, a Deployment needs apiVersion, kind, and metadata fields. For general information about working with config files, see here, here, and here.

A Deployment also needs a .spec section.

Pod Template

The .spec.template is the only required field of the .spec.

The .spec.template is a pod template. It has exactly the same schema as a pod, except it is nested and does not have an apiVersion or kind.

Replicas

.spec.replicas is an optional field that specifies the number of desired pods. It defaults to 1.

Selector

.spec.selector is an optional field that specifies label selectors for pods targeted by this deployment. Deployment kills some of these pods, if their template is different than .spec.template or if the total number of such pods exceeds .spec.replicas. It will bring up new pods with .spec.template if number of pods are less than the desired number.

Strategy

.spec.strategy specifies the strategy used to replace old pods by new ones. .spec.strategy.type can be “Recreate” or “RollingUpdate”. “RollingUpdate” is the default value.

Recreate Deployment

All existing pods are killed before new ones are created when .spec.strategy.type==Recreate. Note: This is not implemented yet.

Rolling Update Deployment

The Deployment updates pods in a rolling update fashion when .spec.strategy.type==RollingUpdate. Users can specify maxUnavailable, maxSurge and minReadySeconds to control the rolling update process.

Max Unavailable

.spec.strategy.rollingUpdate.maxUnavailable is an optional field that specifies the maximum number of pods that can be unavailable during the update process. The value can be an absolute number (e.g. 5) or a percentage of desired pods (e.g. 10%). The absolute number is calculated from percentage by rounding up. This can not be 0 if .spec.strategy.rollingUpdate.maxSurge is 0. By default, a fixed value of 1 is used.

For example, when this value is set to 30%, the old RC can be scaled down to 70% of desired pods immediately when the rolling update starts. Once new pods are ready, old RC can be scaled down further, followed by scaling up the new RC, ensuring that the total number of pods available at all times during the update is at least 70% of the desired pods.

Max Surge

.spec.strategy.rollingUpdate.maxSurge is an optional field that specifies the maximum number of pods that can be created above the desired number of pods. Value can be an absolute number (e.g. 5) or a percentage of desired pods (e.g. 10%). This can not be 0 if MaxUnavailable is 0. The absolute number is calculated from percentage by rounding up. By default, a value of 1 is used.

For example, when this value is set to 30%, the new RC can be scaled up immediately when the rolling update starts, such that the total number of old and new pods do not exceed 130% of desired pods. Once old pods have been killed, the new RC can be scaled up further, ensuring that the total number of pods running at any time during the update is at most 130% of desired pods.

Min Ready Seconds

.spec.minReadySeconds is an optional field that specifies the minimum number of seconds for which a newly created pod should be ready without any of its containers crashing, for it to be considered available. This defaults to 0 (the pod will be considered available as soon as it is ready). To learn more about when a pod is considered ready, see Container Probes.

Alternative to Deployments

kubectl rolling update

Kubectl rolling update also updates pods and replication controllers in a similar fashion. But deployments is declarative and is server side.