# Kubernetes Deployment

## What is Kubernetes Deployments?

> After reading this post you will be understanding the high level of Kubernetes Deployments, its advantages of this, and basic kubectl commands related to Deployments

> If you have not yet checked the previous parts of this series, please go ahead and check this 👉[Link](https://blog.cloudnloud.com/series/kubernetes-deep-dive)

In Kubernetes, Deployment is used to provide declarative updates to Pods as well as ReplicaSets. Also, a Deployment Controller is the higher version of a Replication Controller, as it Removes, Adds, and Updates Pods in ReplicaSets. We need this to work faster and dynamically when we have ReplicaSets with tens of pods, and we are in need to modify them. By using Kubernetes Deployment, this can be achieved with very little effort if used in a correct way.

Deployment sits top of the Kubernetes hierarchy layer. First Pod, then Replica Set, and then Deployment. By using Deployment we can easily upgrade the Pod instances using rolling update and we can easily undo the changes. So Deployment in the Kubernetes cluster is very flexible and gives us more value

### Advantages of Deployment in Kubernetes

1. We can easily scale the application by creating multiple instances of a single Pod.
2. Upgrade the instances seamlessly. The Deployment uses the rolling update concept. The rolling update will update the instance one after another instead of updating all.
3. If the updated instances had some error, then we can easily undo the changes by rolling back to the previous version!

### How Deployment Works

1. First it will create a Deployment. Just a registry for the Pod definition.
2. Then the replica set is created for the Pod definition.
3. Then the desired number of Pods are created using the replica set.

### Two types of deployment strategy
1. Destroy all of them and create a new version
> Note: During the upgradation, time application is not available This is not the default deployment strategy
2. Do not destroy all of them - this is the default deployment Take down the older version and bring up a newer version one by one 

### Deployment Use case Diagram 

This diagram will help you to understand the application and Database deployment in the Kubernetes cluster  

![deployment.jpg](https://cdn.hashnode.com/res/hashnode/image/upload/v1655465078672/6h4zquHGG.jpg align="left")

Create Deployment YAML File
```
apiVersion: apps/v1
kind: Deployment  
metadata:
  name: nodeapp-deployment
  labels:
     app: nodeapp
     type: front-end
spec:
  template:
     metadata:
       name: nodejsapp-pod
       labels: 
         app: nodejsapp
         type: front-end
     spec:
         containers:
           - name: nodejsapp-erp
             image: nginx:1.16.1
  replicas: 3 
  selector:
    matchLabels:
      type: front-end 
```

How to test the deployment status ( you need to check this three status)

```
 kubectl get deploy
 kubectl get rs
 kubectl get pod 
```
Upgrading the Nginx version 

```
kubectl set image deployment/nodeapp-deployment  nodejsapp-erp=nginx:1.19.1
```
you can upgrade the version through the YAML file, change a particular version value and run kubectl apply -f <file.yaml>

Rollout the version 

```
kubectl rollout undo deployment/test-deploy --to-revision=2
```

Rollout history showing command 

```
kubectl rollout history deployment/test-deploy
```

How to check the rollout status 

```
kubectl rollout status deployment/test-deploy
```

Hope you have got an idea about Kubernetes Deployment and how we can implement them in our product environments

Happy Learning 📚

Thank you for reading!!!