Coders Classroom: Kubernetes for Developers #7: Imperative vs. Declarative Kubernetes Objects

kubectl CLI supports both imperative and declarative ways to create and manage Kubernetes objects like Pod, Deployment, Service, namespace etc.

Imperative way:

In this approach, we operate directly on live objects in a cluster using kubectl CLI as arguments or flags.

It follows verb-driven commands to create or manage Kubernetes objects.

Syntax:

 Kubectl create <objecttype> [<subtype>] <name> : # for creating new Kubernetes object
 Kubectl run:  # for creating new Pod to run a container
 Kubectl expose: # used for creating new service object to load balance traffic
                   across multiple Pods
 Kubetl autoscale: # used for creating new Deployment for auto scaling Pods
 Kubectl scale: # used for add/remove Pods by updating replica count
 Kubectl label: # used for add/removing a label from an object
 Kubectl edit: # used for editing configuration in an editor
 Kubectl delete: # used for deleting an object from a cluster
 Kubectl get: # used for getting basic information about an object
 Kubectl describe: # used for getting detailed information about an object
 Kubectl logs: # used for getting stdout and stderr for a container running in a Pod

Example:

> kubectl create ns testnamespace
> kubectl create deployment nginx --image nginx 
> kubectl run my-helloworld --image helloworld:1.0
> kubectl get deployment my-helloworld
> kubectl describe deployment my-helloworld
> kubectl label deployment my-helloworld foo=bar
> kubectl expose deployment my-helloworld --port 80 --target-port 3000
> kubectl logs deployment/my-helloworld
> kubectl scale deployment my-helloworld –replicas=2
> kubectl delete deployment/nginx 

for more info visit @http://millionvisit.blogspot.com/2020/12/kubernetes-for-developers-3-kubectl-cli.html

Advantages:

Commands are simple, easy to learn and remember.
Commands require only a single step to make changes to the cluster.

Disadvantages

Commands cannot be committed to source control system and difficult to review.
Commands do not provide an audit trail associated with changes.
Commands do not provide a template for creating new objects.

Declarative way:

In this approach, we create YAML or JSON files for creating, updating, or deleting Kubernetes objects in a cluster using kubectl CLI.

Syntax:

 kubectl create -f <filename|url>: # used for creating an object from a specified file
 kubectl replace -f <filename|url>: # used for updating a live object from a specified file
 kubectl delete -f <filename|url>: # used for deleting an object from a specified file
 kubectl get -f <filename|url> -o yaml: # used for viewing info about an object from a specified file
 kubectl apply -f <filename|url> : # used for creating/Updating an object from a specified file
 kubectl apply -f <directory> : # used for creating/Updating all objects specified in the directory

Example:

> kubectl create -f nginx.yaml
> kubectl delete -f nginx.yaml -f redis.yaml
> kubectl replace -f nginx.yaml
> kubectl get -f nginx.yaml -o yaml

for more info visit @http://millionvisit.blogspot.com/2020/12/kubernetes-for-developers-3-kubectl-cli.html

Advantages:

Object configurations can be stored in a source control system for reviewing changes before push.
Object configuration provides a template for creating new objects.

Disadvantages

Object configuration requires basic understanding of the object schema.
Object configuration requires the additional step of writing a YAML file.

Convert Imperative to Declarative way:

Option A:

Export the live object to a local object configuration file:

kubectl get {<kind>/<name>} --export -o yaml > {<kind>_<name>}.yaml

Ex:

> kubectl get deployments testnginx --export -o yaml > testnginx2.yaml

Option B:

1. Export the live object to a local object configuration file:

kubectl get {<kind>/<name>} -o yaml > {<kind>_<name>}.yaml

> kubectl get deployments testnginx -o yaml > testnginx2.yaml

2. Remove the status field from the configuration file

3. use replace command to execute the file

kubectl replace -f <kind>_<name>.yaml

> kubectl replace -f testnginx2.yaml

Kubernetes for Developers Journey.

Happy Coding :)

Coders Classroom

Sunday, January 17, 2021

Kubernetes for Developers #7: Imperative vs. Declarative Kubernetes Objects

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