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Kubernetes<\/a> is an orchestration tool for containerized applications. It uses the kubectl<\/em> command line to interact with a cluster.<\/p>\n In this tutorial, we’ll see the difference between the kubectl create<\/em> and kubectl apply<\/em> commands with some examples in a running cluster.<\/p>\n Kubernetes<\/a> (K8s) requires cluster creation and a good understanding of its architecture<\/a>. Furthermore, we should be confident using the API with the kubectl<\/em><\/a> command.<\/p>\n We can set up a K8s cluster in different ways. A good learning experience would be Kubeadm<\/em><\/a> and a local K8s cluster connecting multiple virtual machines on the same network. We might want to try with Archlinux<\/a> or any other Linux distribution.<\/p>\n Nonetheless, we can use a lightweight K8s distribution<\/a> for a quick start. This allows us at least to save time for the cluster configuration setup.<\/p>\n If we want to skip cluster configuration and play with common commands, we can use K8s playgrounds such as Killercoda<\/a> or Play with Kubernetes<\/a>.<\/p>\n In any case, we’ll have to install kubectl<\/em><\/a>. It works on the command line, although it’s not strictly a K8s component. It communicates with the K8s API using the K8s cluster configuration.<\/p>\n Before we discuss create<\/em> and apply,<\/em> let’s see how we can manage objects in a cluster.<\/p>\n K8s is about managing objects<\/a> in a cluster. We might add or update a pod<\/a> that runs, for instance, a web application or a database. We apply operations such as create, read, replace, and delete.<\/p>\n Imperative management means to use verb-driven commands<\/strong>. For example, we create an Nginx deployment:<\/p>\n Likewise, we can extend the same concept to a configuration file:<\/p>\n With declarative commands, we only need a configuration file or a list of files<\/strong>. K8s manages the difference from what is already in the cluster. The apply<\/em> syntax is what we use in this case:<\/p>\n As we’ve already seen, the kubectl create<\/em><\/a> is an imperative command. We can create many resources like services, secrets, ingress, etc.<\/p>\n For easiness, we’ll see a simple Nginx deployment from a YAML configuration file deployment.yaml<\/em>:<\/p>\n We can now use the create<\/em> command:<\/p>\n K8s notifies us of the resource creation:<\/p>\n We can double-check with the kubectl get pods<\/em> command to see if the Nginx container is running:<\/p>\n We’ll get an error if we now try to create, for example, the same deployment:<\/p>\n It’s an option to modify an object before creation. Suppose we want to use a different Nginx version:<\/p>\n We can also output the file and edit it in a second step:<\/p>\n This opens up the default text editor with the YAML file for the new deployment.<\/p>\n Now that our resources are running in the cluster, we might want to follow up and do some operations on them.<\/p>\n Similarly to how we modify before creation, K8s recommends interacting with resources using one imperative command at a time<\/strong>.<\/p>\n So, let’s say we want to scale our deployment to 5<\/em> replicas. We could use the scale<\/em> command:<\/p>\n Instead, we should create another YAML file, for example, deployment_scale.yaml<\/em> from the previous file, and add to the deployment spec:<\/p>\n If we want to update, we can now use the replace<\/em> command:<\/p>\n Likewise, we might want to use the delete<\/em> command to remove an object:<\/p>\n Let’s have a look at some of the create<\/em> options:<\/p>\n The kubectl apply<\/em><\/a> command gives us more flexibility if we want to manage the life cycle of a resource in a cluster.<\/p>\n We can replace the create<\/em> command with apply:<\/em><\/p>\n The deployment works as well. So, what’s the difference between the two commands?<\/p>\n Using the apply<\/em> command with the same configuration produces an unchanged status:<\/p>\n Instead, with a different input, we’ll get a new configuration response:<\/p>\n The apply<\/em> command works with any object, so it’s declarative in the object definition<\/strong>. We know the kind<\/em> of object in the YAML file. It could be a service, a pod, etc. Thus, in the kubectl<\/em> command, we avoid stating the resource’s type.<\/p>\n What’s relevant about the apply<\/em> command is the kubectl.kubernetes.io\/last-applied-configuration<\/em> field in the metadata definition. We can see it from the YAML file configuration:<\/p>\n The output corresponds to the current deployment:<\/p>\n Notably, we can see the last applied configuration with the get<\/em> command:<\/p>\n We omit the last-applied-configuration <\/em>field in the output for brevity:<\/p>\n Let’s say we now update the replicas to 5<\/em>. This creates a new version of the deployment. For example, we can see the generation<\/em>\u00a0and resourceVersion<\/em> change:<\/p>\n This makes K8s able to update the definition of a live object. It works by comparing the current revision with the next to come.<\/p>\n If we don’t use the save-config<\/em> option with the create<\/em> command, we can continue using the apply<\/em> command later on. The create<\/em> options are still valid here. Let’s see some other options for the apply<\/em> command:<\/p>\n We’ve seen the kubectl create<\/em> as an imperative command.<\/strong> Furthermore, if we want to change to an object definition, we’d need to use another command such as kubectl replace<\/em>. It doesn’t strictly require a YAML template.<\/p>\n On the contrary, the kubectl apply<\/em> declarative command matches the object type<\/strong>. K8s knows how to make changes according to the last applied configuration.<\/p>\n However, with the apply<\/em> command, merging<\/a> changes between different versions can be difficult<\/strong> and lead to unexpected results.<\/p>\n It’s worth noting that the create<\/em> command with the –save-config<\/em> option behaves similarly to the apply<\/em> command.<\/p>\n In this article, we saw how the kubectl<\/em> object management works with kubectl create<\/em> and kubectl apply<\/em>.<\/p>\n The apply<\/em> command is more flexible and can apply changes dynamically. The create<\/em> command is imperative, although with straightforward syntax.<\/p>\n In any case, we need to be sure not to mix up different strategies. They’re both mature commands nowadays, so it’s about choosing a direction.<\/p>\n <\/p><\/div>\n\n \u7acb\u523b\u8a3b\u518a\u53ca\u5831\u540d\u96fb\u8166\u88dc\u7fd2\u8ab2\u7a0b\u5427! <\/a> www.ITSec.vip<\/a> www.Sraa.com.hk<\/a> 2. Kubernetes Setup<\/h2>\n
<\/div>\n2.1. Cluster Setup<\/h3>\n
<\/div>\n2.2. kubectl<\/em><\/h3>\n
<\/div>\n3. Declarative vs. Imperative<\/h2>\n
<\/div>\n3.1. Object Management<\/h3>\n
<\/div>\n3.2. Imperative Commands and Objects Configuration<\/h3>\n
<\/div>\n$ kubectl create deployment nginx --image nginx\r\n<\/code><\/pre>\n$ kubectl create -f nginx.yaml\r\n<\/code><\/pre>\n3.3. Declarative Objects Configuration<\/h3>\n
<\/div>\n$ kubectl apply -f deployment.yaml<\/code><\/pre>\n4. kubectl create<\/em><\/h2>\n
<\/div>\napiVersion: apps\/v1\r\nkind: Deployment\r\nmetadata:\r\n name: nginx-deployment\r\nspec:\r\n selector:\r\n matchLabels:\r\n app: nginx\r\n minReadySeconds: 5\r\n template:\r\n metadata:\r\n labels:\r\n app: nginx\r\n spec:\r\n containers:\r\n - name: nginx\r\n image: nginx:1.14.2\r\n ports:\r\n - containerPort: 80<\/code><\/pre>\n$ kubectl create -f deployment.yaml<\/code><\/pre>\ndeployment.apps\/nginx-deployment created<\/code><\/pre>\nNAME READY STATUS RESTARTS AGE\r\nnginx-deployment-86dcfdf4c6-zhkw8 1\/1 Running 0 88s\r\n<\/code><\/pre>\n4.1. Error Creating the Same Resource<\/h3>\n
<\/div>\nError from server (AlreadyExists): error when creating "deployment.yaml": deployments.apps "nginx-deployment" already exists\r\n<\/code><\/pre>\n4.2. Modify Objects Before Creation<\/h3>\n
<\/div>\n$ kubectl create -f deployment.yaml -o yaml --dry-run=client | kubectl set image --local -f - 'nginx=nginx:1.25.2' -o yaml | kubectl create -f -<\/code><\/pre>\n$ kubectl create -f deployment.yaml -o yaml --dry-run=client > deployment_1.yaml && kubectl create --edit -f deployment_1.yaml<\/code><\/pre>\n4.3. One Imperative Command at a Time<\/h3>\n
<\/div>\n$ kubectl scale --replicas=5 deployment nginx-deployment<\/code><\/pre>\n...\r\nspec:\r\n replicas: 2\r\n...<\/code><\/pre>\n$ kubectl replace -f deployment_scale.yml<\/code><\/pre>\n$ kubectl delete -f deployment.yml<\/code><\/pre>\n4.4. Command Options<\/h3>\n
<\/div>\n\n
5. kubectl apply<\/em><\/h2>\n
<\/div>\n5.1. Replace create<\/em> With apply<\/em><\/h3>\n
<\/div>\n$ kubectl apply -f deployment.yaml<\/code><\/pre>\n5.2. No Error Applying the Same Resource<\/h3>\n
<\/div>\ndeployment.apps\/nginx-deployment unchanged<\/code><\/pre>\ndeployment.apps\/nginx-deployment configured\r\n<\/code><\/pre>\n5.3. Object Agnostic<\/h3>\n
<\/div>\n5.4. Keep Track of Objects’ History<\/h3>\n
<\/div>\n$ kubectl apply view-last-applied -f deployment.yaml -o yaml<\/code><\/pre>\napiVersion: apps\/v1\r\nkind: Deployment\r\nmetadata:\r\n annotations: {}\r\n name: nginx-deployment\r\n namespace: default\r\nspec:\r\n minReadySeconds: 5\r\n replicas: 3\r\n selector:\r\n matchLabels:\r\n app: nginx\r\n template:\r\n metadata:\r\n labels:\r\n app: nginx\r\n spec:\r\n containers:\r\n - image: nginx:1.14.2\r\n name: nginx\r\n ports:\r\n - containerPort: 80\r\n<\/code><\/pre>\n5.5. A New Revision for Every Change<\/h3>\n
<\/div>\n$ kubectl get deployments.apps nginx-deployment -o yaml<\/code><\/pre>\napiVersion: apps\/v1\r\nkind: Deployment\r\nmetadata:\r\n annotations:\r\n deployment.kubernetes.io\/revision: "1"\r\n kubectl.kubernetes.io\/last-applied-configuration: |\r\n ... \r\n creationTimestamp: "2023-09-18T14:58:34Z"\r\n generation: 1\r\n name: nginx-deployment\r\n namespace: default\r\n resourceVersion: "154473"\r\n uid: a154ad80-db02-4ec4-888d-0c56281c4816\r\n...<\/code><\/pre>\n... \r\n generation: 2\r\n name: nginx-deployment\r\n namespace: default\r\n resourceVersion: "156401"\r\n...<\/code><\/pre>\n5.6. Move From create<\/em> to apply<\/em><\/h3>\n
<\/div>\n
\n
\nK8s will warn us and set the last applied configuration with the following message:<\/p>\nWarning: resource deployments\/nginx-deployment is missing the kubectl.kubernetes.io\/last-applied-configuration annotation which is required by kubectl apply. \r\nkubectl apply should only be used on resources created declaratively by either kubectl create --save-config or kubectl apply. The missing annotation will be patched automatically.\r\n<\/code><\/pre>\n5.7. Command Options<\/h3>\n
<\/div>\n\n
6. How the apply<\/em> and create<\/em> Commands Differ in kubectl<\/em><\/h2>\n
<\/div>\n7. Conclusion<\/h2>\n
<\/div>\n<\/p>\n
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