Deployment 101
We looked at ReplicaSets earlier. However, ReplicaSet have one major drawback: once you select the pods that are managed by a ReplicaSet, you cannot change their pod templates.
For example, if you are using a ReplicaSet to deploy four pods with NodeJS running and you want to change the NodeJS image to a newer version, you need to delete the ReplicaSet and recreate it. Restarting the pods causes downtime till the images are available and the pods are running again.
A Deployment resource uses a ReplicaSet to manage the pods. However, it handles updating them in a controlled way. Let’s dig deeper into Deployment Controllers and patterns.
Creating Your First Deployment
The following Deployment definition deploys four pods with nginx as their hosted application:
git clone https://github.com/collabnix/dockerlabs
cd dockerlabs/kubernetes/workshop/Deployment101
kubectl create -f nginx-dep.yaml
deployment.apps/nginx-deployment created
Checking the list of application deployment
To list your deployments use the get deployments command:
$ kubectl get deployments
NAME READY UP-TO-DATE AVAILABLE AGE
nginx-deployment 2/2 2 2 63s
[node1 Deployment101]$ kubectl describe deploy
Name: nginx-deployment
Namespace: default
CreationTimestamp: Mon, 30 Dec 2019 07:10:33 +0000
Labels: <none>
Annotations: deployment.kubernetes.io/revision: 1
Selector: app=nginx
Replicas: 2 desired | 2 updated | 2 total | 0 available | 2 unavailable
StrategyType: RollingUpdate
MinReadySeconds: 0
RollingUpdateStrategy: 25% max unavailable, 25% max surge
Pod Template:
Labels: app=nginx
Containers:
nginx:
Image: nginx:1.7.9
Port: 80/TCP
Host Port: 0/TCP
Environment: <none>
Mounts: <none>
Volumes: <none>
Conditions:
Type Status Reason
---- ------ ------
Available False MinimumReplicasUnavailable
Progressing True ReplicaSetUpdated
OldReplicaSets: <none>
NewReplicaSet: nginx-deployment-6dd86d77d (2/2 replicas created)
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal ScalingReplicaSet 90s deployment-controller Scaled up replica set nginx-deployment-6dd86d77d to 2
We should have 1 Pod. If not, run the command again. This shows:
The DESIRED state is showing the configured number of replicas
The CURRENT state show how many replicas are running now
The UP-TO-DATE is the number of replicas that were updated to match the desired (configured) state
The AVAILABLE state shows how many replicas are actually AVAILABLE to the users
[node1 Deployment101]$ kubectl get deployments
NAME READY UP-TO-DATE AVAILABLE AGE
nginx-deployment 2/2 2 2 2m57s
[node1 Deployment101]$ kubectl get po
NAME READY STATUS RESTARTS AGE
nginx-deployment-6dd86d77d-84fwp 1/1 Running 0 3m44s
nginx-deployment-6dd86d77d-xnrqp 1/1 Running 0 3m44s
Step #2. Scale up/down application deployment
Now let’s scale the Deployment to 4 replicas. We are going to use the kubectl scale command, followed by the deployment type, name and desired number of instances:
Biradars-MacBook-Air-4:~ sangam$ kubectl scale deployments/nginx-deployment --replicas=4
deployment.extensions/nginx-deployment scaled
The change was applied, and we have 4 instances of the application available. Next, let’s check if the number of Pods changed:
Now There should be 4 pods running in the cluster
$ kubectl get deployments
NAME READY UP-TO-DATE AVAILABLE AGE
nginx-deployment 4/4 4 4 4m
There are 4 Pods now, with different IP addresses. The change was registered in the Deployment events log. To check that, use the describe command:
$ kubectl describe deployments/nginx-deployment
Name: nginx-deployment
Namespace: default
CreationTimestamp: Sat, 30 Nov 2019 20:04:34 +0530
Labels: <none>
Annotations: deployment.kubernetes.io/revision: 1
Selector: app=nginx
Replicas: 4 desired | 4 updated | 4 total | 4 available | 0 unavailable
StrategyType: RollingUpdate
MinReadySeconds: 0
RollingUpdateStrategy: 25% max unavailable, 25% max surge
Pod Template:
Labels: app=nginx
Containers:
nginx:
Image: nginx:1.7.9
Port: 80/TCP
Host Port: 0/TCP
Environment: <none>
Mounts: <none>
Volumes: <none>
Conditions:
Type Status Reason
---- ------ ------
Progressing True NewReplicaSetAvailable
Available True MinimumReplicasAvailable
OldReplicaSets: <none>
NewReplicaSet: nginx-deployment-6dd86d77d (4/4 replicas created)
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal ScalingReplicaSet 6m12s deployment-controller Scaled up replica set nginx-deployment-6dd86d77d to 2
Normal ScalingReplicaSet 3m6s deployment-controller Scaled up replica set nginx-deployment-6dd86d77d to 4
Biradars-MacBook-Air-4:~ sangam$
$ kubectl get pods -o wide
NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
nginx-deployment-6dd86d77d-b4v7k 1/1 Running 0 4m32s 10.1.0.237 docker-desktop <none> <none>
nginx-deployment-6dd86d77d-bnc5m 1/1 Running 0 4m32s 10.1.0.236 docker-desktop <none> <none>
nginx-deployment-6dd86d77d-bs6jr 1/1 Running 0 86s 10.1.0.239 docker-desktop <none> <none>
nginx-deployment-6dd86d77d-wbdzv 1/1 Running 0 86s 10.1.0.238 docker-desktop <none> <none>
Biradars-MacBook-Air-4:~ sangam$
You can also view in the output of this command that there are 4 replicas now.
Scaling the service to 2 Replicas
To scale down the Service to 2 replicas, run again the scale command:
$ kubectl scale deployments/nginx-deployment --replicas=2
deployment.extensions/nginx-deployment scaled
$ kubectl get deployments
NAME READY UP-TO-DATE AVAILABLE AGE
nginx-deployment 2/2 2 2 7m23s
Biradars-MacBook-Air-4:~ sangam$
Step #3. Perform rolling updates to application deployment
So far, everything our Deployment did is no different than a typical ReplicaSet. The real power of a Deployment lies in its ability to update the pod templates without causing application outage.
Let’s say that you have finished testing the nginx 1.7.9 , and you are ready to use it in production. The current pods are using the older nginx version . The following command changes the deployment pod template to use the new image:
To update the image of the application to new version, use the set image command, followed by the deployment name and the new image version:
$ kubectl get deployments
NAME READY UP-TO-DATE AVAILABLE AGE
nginx-deployment 2/2 2 2 7m23s
$ kubectl describe pods
Name: nginx-deployment-6dd86d77d-b4v7k
Namespace: default
Priority: 0
PriorityClassName: <none>
Node: docker-desktop/192.168.65.3
Start Time: Sat, 30 Nov 2019 20:04:34 +0530
Labels: app=nginx
pod-template-hash=6dd86d77d
Annotations: <none>
Status: Running
IP: 10.1.0.237
Controlled By: ReplicaSet/nginx-deployment-6dd86d77d
Containers:
nginx:
Container ID: docker://2c739cf9fe4dac53a4cc5c6097207da0c5edc2183f1f36f9f3e5c7057f85da43
Image: nginx:1.7.9
Image ID: docker-pullable://nginx@sha256:e3456c851a152494c3e4ff5fcc26f240206abac0c9d794affb40e0714846c451
Port: 80/TCP
Host Port: 0/TCP
State: Running
Started: Sat, 30 Nov 2019 20:05:28 +0530
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from default-token-ds5tg (ro)
Conditions:
Type Status
Initialized True
Ready True
ContainersReady True
PodScheduled True
Volumes:
default-token-ds5tg:
Type: Secret (a volume populated by a Secret)
SecretName: default-token-ds5tg
Optional: false
QoS Class: BestEffort
Node-Selectors: <none>
Tolerations: node.kubernetes.io/not-ready:NoExecute for 300s
node.kubernetes.io/unreachable:NoExecute for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 10m default-scheduler Successfully assigned default/nginx-deployment-6dd86d77d-b4v7k to docker-desktop
Normal Pulling 10m kubelet, docker-desktop Pulling image "nginx:1.7.9"
Normal Pulled 9m17s kubelet, docker-desktop Successfully pulled image "nginx:1.7.9"
Normal Created 9m17s kubelet, docker-desktop Created container nginx
Normal Started 9m17s kubelet, docker-desktop Started container nginx
Name: nginx-deployment-6dd86d77d-bnc5m
Namespace: default
Priority: 0
PriorityClassName: <none>
Node: docker-desktop/192.168.65.3
Start Time: Sat, 30 Nov 2019 20:04:34 +0530
Labels: app=nginx
pod-template-hash=6dd86d77d
Annotations: <none>
Status: Running
IP: 10.1.0.236
Controlled By: ReplicaSet/nginx-deployment-6dd86d77d
Containers:
nginx:
Container ID: docker://12ab35cbf4fdf78997b106b5eb27135f2fc37c890e723fee44ac820ba1b1fd75
Image: nginx:1.7.9
Image ID: docker-pullable://nginx@sha256:e3456c851a152494c3e4ff5fcc26f240206abac0c9d794affb40e0714846c451
Port: 80/TCP
Host Port: 0/TCP
State: Running
Started: Sat, 30 Nov 2019 20:05:23 +0530
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from default-token-ds5tg (ro)
Conditions:
Type Status
Initialized True
Ready True
ContainersReady True
PodScheduled True
Volumes:
default-token-ds5tg:
Type: Secret (a volume populated by a Secret)
SecretName: default-token-ds5tg
Optional: false
QoS Class: BestEffort
Node-Selectors: <none>
Tolerations: node.kubernetes.io/not-ready:NoExecute for 300s
node.kubernetes.io/unreachable:NoExecute for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 10m default-scheduler Successfully assigned default/nginx-deployment-6dd86d77d-bnc5m to docker-desktop
Normal Pulling 10m kubelet, docker-desktop Pulling image "nginx:1.7.9"
Normal Pulled 9m22s kubelet, docker-desktop Successfully pulled image "nginx:1.7.9"
Normal Created 9m22s kubelet, docker-desktop Created container nginx
Normal Started 9m22s kubelet, docker-desktop Started container nginx
The command notified the Deployment to use a different image for your app and initiated a rolling update. Check the status of the new Pods, and view the old one terminating with the get pods command:
Biradars-MacBook-Air-4:~ sangam$ kubectl set image deployments/nginx-deployment nginx=nginx:1.9.1
deployment.extensions/nginx-deployment image updated
Checking description of pod again
$ kubectl describe pods
Name: nginx-deployment-6dd86d77d-b4v7k
Namespace: default
Priority: 0
PriorityClassName: <none>
Node: docker-desktop/192.168.65.3
Start Time: Sat, 30 Nov 2019 20:04:34 +0530
Labels: app=nginx
pod-template-hash=6dd86d77d
Annotations: <none>
Status: Running
IP: 10.1.0.237
Controlled By: ReplicaSet/nginx-deployment-6dd86d77d
Containers:
nginx:
Container ID: docker://2c739cf9fe4dac53a4cc5c6097207da0c5edc2183f1f36f9f3e5c7057f85da43
Image: nginx:1.7.9
Image ID: docker-pullable://nginx@sha256:e3456c851a152494c3e4ff5fcc26f240206abac0c9d794affb40e0714846c451
Port: 80/TCP
Host Port: 0/TCP
State: Running
Started: Sat, 30 Nov 2019 20:05:28 +0530
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from default-token-ds5tg (ro)
Conditions:
Type Status
Initialized True
Ready True
ContainersReady True
PodScheduled True
Volumes:
default-token-ds5tg:
Type: Secret (a volume populated by a Secret)
SecretName: default-token-ds5tg
Optional: false
QoS Class: BestEffort
Node-Selectors: <none>
Tolerations: node.kubernetes.io/not-ready:NoExecute for 300s
node.kubernetes.io/unreachable:NoExecute for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 16m default-scheduler Successfully assigned default/nginx-deployment-6dd86d77d-b4v7k to docker-desktop
Normal Pulling 16m kubelet, docker-desktop Pulling image "nginx:1.7.9"
Normal Pulled 15m kubelet, docker-desktop Successfully pulled image "nginx:1.7.9"
Normal Created 15m kubelet, docker-desktop Created container nginx
Normal Started 15m kubelet, docker-desktop Started container nginx
Name: nginx-deployment-6dd86d77d-bnc5m
Namespace: default
Priority: 0
PriorityClassName: <none>
Node: docker-desktop/192.168.65.3
Start Time: Sat, 30 Nov 2019 20:04:34 +0530
Labels: app=nginx
pod-template-hash=6dd86d77d
Annotations: <none>
Status: Running
IP: 10.1.0.236
Controlled By: ReplicaSet/nginx-deployment-6dd86d77d
Containers:
nginx:
Container ID: docker://12ab35cbf4fdf78997b106b5eb27135f2fc37c890e723fee44ac820ba1b1fd75
Image: nginx:1.7.9
Image ID: docker-pullable://nginx@sha256:e3456c851a152494c3e4ff5fcc26f240206abac0c9d794affb40e0714846c451
Port: 80/TCP
Host Port: 0/TCP
State: Running
Started: Sat, 30 Nov 2019 20:05:23 +0530
Ready: True
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from default-token-ds5tg (ro)
Conditions:
Type Status
Initialized True
Ready True
ContainersReady True
PodScheduled True
Volumes:
default-token-ds5tg:
Type: Secret (a volume populated by a Secret)
SecretName: default-token-ds5tg
Optional: false
QoS Class: BestEffort
Node-Selectors: <none>
Tolerations: node.kubernetes.io/not-ready:NoExecute for 300s
node.kubernetes.io/unreachable:NoExecute for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 16m default-scheduler Successfully assigned default/nginx-deployment-6dd86d77d-bnc5m to docker-desktop
Normal Pulling 16m kubelet, docker-desktop Pulling image "nginx:1.7.9"
Normal Pulled 15m kubelet, docker-desktop Successfully pulled image "nginx:1.7.9"
Normal Created 15m kubelet, docker-desktop Created container nginx
Normal Started 15m kubelet, docker-desktop Started container nginx
Name: nginx-deployment-784b7cc96d-kxc68
Namespace: default
Priority: 0
PriorityClassName: <none>
Node: docker-desktop/192.168.65.3
Start Time: Sat, 30 Nov 2019 20:20:04 +0530
Labels: app=nginx
pod-template-hash=784b7cc96d
Annotations: <none>
Status: Pending
IP:
Controlled By: ReplicaSet/nginx-deployment-784b7cc96d
Containers:
nginx:
Container ID:
Image: nginx:1.9.1
Image ID:
Port: 80/TCP
Host Port: 0/TCP
State: Waiting
Reason: ContainerCreating
Ready: False
Restart Count: 0
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from default-token-ds5tg (ro)
Conditions:
Type Status
Initialized True
Ready False
ContainersReady False
PodScheduled True
Volumes:
default-token-ds5tg:
Type: Secret (a volume populated by a Secret)
SecretName: default-token-ds5tg
Optional: false
QoS Class: BestEffort
Node-Selectors: <none>
Tolerations: node.kubernetes.io/not-ready:NoExecute for 300s
node.kubernetes.io/unreachable:NoExecute for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 36s default-scheduler Successfully assigned default/nginx-deployment-784b7cc96d-kxc68 to docker-desktop
Normal Pulling 35s kubelet, docker-desktop Pulling image "nginx:1.9.1"
Biradars-MacBook-Air-4:~ sangam$
Step #4. Rollback updates to application deployment
The rollout command reverted the deployment to the previous known state. Updates are versioned and you can revert to any previously know state of a Deployment. List again the Pods:
$ kubectl rollout undo deployments/nginx-deployment
deployment.extensions/nginx-deployment rolled back
$ kubectl rollout status deployments/nginx-deployment
deployment "nginx-deployment" successfully rolled out
After the rollout succeeds, you may want to get the Deployment.
The output shows the update progress until all the pods use the new container image.
The algorithm that Kubernetes Deployments use when deciding how to roll updates is to keep at least 25% of the pods running. Accordingly, it doesn’t kill old pods unless a sufficient number of new ones are up. In the same sense, it does not create new pods until enough pods are no longer running. Through this algorithm, the application is always available during updates.
You can use the following command to determine the update strategy that the Deployment is using:
Biradars-MacBook-Air-4:~ sangam$ kubectl describe deployments | grep Strategy
StrategyType: RollingUpdate
RollingUpdateStrategy: 25% max unavailable, 25% max surge
Biradars-MacBook-Air-4:~ sangam$
Step #5. Cleanup
Finally you can clean up the resources you created in your cluster:
kubectl delete service nginx-deployment
kubectl delete deployment nginx-deployment