What is a Kubernetes Deployment?

Table of Contents

Kubernetes Deployment is a Kubernetes object that defines a desired state for a set of Pods. A Pod is the smallest deployable unit in Kubernetes that can contain one or more containers. A Deployment manages a set of identical Pod replicas, ensuring that the desired number of replicas are running at all times.

A Kubernetes Deployment provides declarative updates for Pods and ReplicaSets. It allows you to specify the number of replicas to be created and the container images to be used for each Pod. It also provides features such as rolling updates and rollbacks, which enable you to update your application without downtime.

With a Deployment, you can specify how many replicas of your application should be running at any given time. If a Pod fails or is terminated, the Deployment will automatically replace it with a new one to maintain the desired number of replicas.

A typical Kubernetes Deployment workflow includes the following steps:

Define the desired state: You define the desired state of your application by creating a Deployment object in Kubernetes. This includes specifying the number of replicas you want to run, the container image to use, and any other relevant configuration.
- Apply the Deployment: Once you have defined the desired state, you apply the Deployment to your Kubernetes cluster using the kubectl command-line tool or through an automation tool like Helm. This creates the necessary resources in the cluster to run your application.
- Kubernetes creates the ReplicaSet: When you apply the Deployment, Kubernetes creates a ReplicaSet, which is responsible for managing the desired number of Pod replicas.
- Pods are created: The ReplicaSet creates the necessary number of Pods based on the desired state you defined in the Deployment. Each Pod contains one or more containers that run your application.
- Monitor the Deployment: You can monitor the status of the Deployment using kubectl or through the Kubernetes dashboard. This allows you to see if the desired number of replicas are running and if any Pods or nodes are experiencing issues.
- Update the Deployment: If you need to update your application, you can update the Deployment object with the new configuration. Kubernetes will then automatically create a new ReplicaSet with the updated configuration and gradually roll out the update to the Pods in a controlled manner.
- Roll back the Deployment: If there are issues with the updated configuration, you can roll back to a previous version by updating the Deployment with the previous configuration. Kubernetes will then roll back the update in a controlled manner, ensuring that the previous version is running without downtime.

This workflow enables you to manage the deployment and scaling of your application in a Kubernetes cluster with minimal downtime and maximum control.

Additionally, Kubernetes Deployments provide features such as scaling, which allows you to scale up or down the number of replicas based on the demand for your application.

In nutshell, a Kubernetes Deployment provides a powerful tool for managing the deployment and scaling of your application in a Kubernetes cluster, enabling you to easily manage the lifecycle of your application and ensure its availability and scalability.

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:

$ 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>

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

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:

$ 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"

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 ou

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:

$ kubectl describe deployments | grep Strategy
StrategyType:           RollingUpdate
RollingUpdateStrategy:  25% max unavailable, 25% max surge

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