While the above kubectl ... CLI commands are useful for your Developer Inner Loop, when it comes time to deploy to Test and Production environments you want something a little more repeatable. We want the declarative configuration of the Deployment we were just exploring in a text based format that we can store alongside our application code in Git. Using kubectl again, we can query that configuration back out of the Kubernetes cluster.
It's a good time to point out that kubectl is just a friendly CLI that gives us convenient access to the Kubernets API Server. When we add or change configuration to Kubernetes with commands like kubectl run ..., there are PUT/POST calls being issued to the Kubernetes API Server. Likewise, once configured, we can retrieve the configuration with an HTTP GET, or kubectl get ....
So there's a lot of good configuration already in our cluster for the Deployment we have. It'd be a shame to lose it, so let's get it out and store it in a file.
Note: Up to now, it didn't matter which directory you were running kubectl commands from, since we were just getting details on our Kubernetes environment, or else communicating directly between docker and the cluster. From here on out we will be using YAML files (included in this repository) to describe the deployment, and so you will need to execute these commands from your root project directory.
kubectl get deployment cn-demo -o yaml > k8s/lab/deployment.yamlThat command semantically says "get me the configuration of the Deployment named cn-demo, format it as YAML and write it to the file k8s/lab/deployment.yaml". Go ahead and open that file in VS Code and look around. Do you see the familiar values you defined with the kubectl CLI earlier for the cloud-native-demo:1 image and the replicas count? There's a lot of other data in there too, some relevant, some not.
Using kubectl get ... is a common technique to bootstrap your configuration without needing to remember all the structure of the YAML document, which in turn is really just the API specification for Kubernetes Deployments. We're going to use a cleaned up version of deployment.yaml for our next steps, but just remember how to use kubectl get ... -oyaml in the future. You'll use it often to troubleshoot your Kubernetes configuration.
Note: In this example we piped the results of
kubectl get ...to a file using the>character. If you leave that part off, the results will be written to your console. You'll do this often when you just need to see the configuration of a particular Kubernetes object.
Before we continue, let's clean up our current deployment. Going forward, we'll work with and deploy using variations of deployment.yaml contained in this repository.
kubectl delete deployment cn-demo