First Deploy

Namespace

First Deploy

Let's launch an application that generates and outputs a hash every 5 seconds or so.

I have prepared one here, you can test it with docker run jakousa/dwk-app1.

Deployment

What is a Deployment?

A Deployment resource takes care of deployment. It's a way to tell Kubernetes what container you want, how they should be running and how many of them should be running.

While we created the Deployment we also created a ReplicaSet object. ReplicaSets are used to tell how many replicas of a Pod you want. It will delete or create Pods until the number of Pods you wanted are running. ReplicaSets are managed by Deployments and you should not have to manually define or modify them. If you want to manage the number of replicas, you can edit the Deployment and it will take care of modifying the ReplicaSet.

kubectl tree

https://github.com/ahmetb/kubectl-tree

$ kubectl tree -n __NAME__ deployment hashgenerator
  NAMESPACE  NAME                                        READY  REASON  AGE
  matti     Deployment/hashgenerator                -               16s
  matti     └─ReplicaSet/hashgenerator-7dbbd57948   -               16s
  matti       └─Pod/hashgenerator-7dbbd57948-7lz2t  True            16s

Scaling up

Things start to get really cumbersome. It is hard to imagine how someone in their right mind could be maintaining multiple applications like that. Thankfully we will now use a declarative approach where we define how things should be rather than how they should change. This is more sustainable in the long term than the iterative approach and will let us keep our sanity.

Before redoing the previous steps via the declarative approach, let's take the existing deployment down.

Declarative Approach

hashgenerator/deployment.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: hashgenerator
spec:
  replicas: 1
  selector:
    matchLabels:
      app: hashgenerator
  template:
    metadata:
      labels:
        app: hashgenerator
    spec:
      containers:
        - name: hashgenerator
          image: jakousa/dwk-app1:latest

This looks a lot like the docker-compose.yamls we have previously written. Let's ignore what we don't know for now, which is mainly labels, and focus on the things that we know:

We're declaring what kind it is (kind: Deployment)
We're declaring it a name as metadata (name: hashgenerator)
We're declaring that there should be one of them (replicas: 1)
We're declaring that it has a container that is from a certain image with a name

Apply the deployment with apply command:

kubectl apply -n __NAME__ -f hashgenerator/deployment.yml

Try with the dir only

kubectl apply -n __NAME__ -f hashgenerator

kubectl apply -n __NAME__ -f hashgenerator -o yaml --dry-run

Notice the namespace:

kind: Deployment
metadata:
  annotations:
    ...
  name: hashgenerator
  namespace: __NAME__
spec:
  replicas: 1
  ...

You have reached the end of this section! Continue to the next section:

2. Second Deploy