Note: If you have missed my previous articles on Docker and Kubernetes, you can find them here.
Application deployment models evolution.
Getting started with Docker.
Docker file and images.
Publishing images to Docker Hub and re-using them.
Docker- Find out what's going on.
Docker Networking- Part 1.
Docker Networking- Part 2.
Docker Swarm-Multi-Host container Cluster.
Docker Networking- Part 3 (Overlay Driver).
Introduction to Kubernetes.
Kubernetes- Diving in (Part 1)-Installing Kubernetes multi-node cluster.
Kubernetes-Diving in (Part2)- Services.
Kubernetes- Infrastructure As Code with Yaml (part 1).
Kubernetes- Infrastructure As Code Part 2- Creating PODs with YAML.
Kubernetes Infrastructure-as-Code part 3- Replicasets with YAML.
Kubernetes Infrastructure-as-Code part 4 - Deployments and Services with YAML.
Deploying a microservices APP with Kubernetes.
Kubernetes- Time based scaling of deployments with python client.
Kubernetes Networking - The Flannel network explained.
Kubernetes- Installing and using kubectl top for monitoring nodes and PoDs
Kubernetes Administration- SchedulingCompute, Storage, networking, and users (administrators/developers) are important pieces of IT infrastructure. Any orchestration system should provide ways to provision, manage, and monitor these pieces. From Kubernetes, perspective PoD is the most fundamental unit of deployment (PoDs are abstracted by replicaSets and deployments). That said:
PoDs consume CPU/Memory resources and should be scheduled on nodes that satisfy requirements: In, Kubernetes Administration- Scheduling I gave a brief overview of how PoDs can be scheduled.
PoDs need to communicate externally and with other PoDs, nodes in the cluster. Container Network Interface specifies requirements of such a solution. One such solution is Flannel. I have attempted to give an overview of the solution here: Kubernetes Networking - The Flannel network explained
The third piece of infrastructure is storage- A PoD runs on a cluster node and utilizes storage. In this article, I hope to give an overview of various storage administration.
I will cover User administration in a future article.
Volumes
To explain the concept of Volumes, I am going to create an HTTP deployment with 2 replicas first.
root@sathish-vm2:/home/sathish# kubectl create deployment mywebserver --image=httpd --replicas=2
deployment.apps/mywebserver created
root@sathish-vm2:/home/sathish# kubectl get deployment --show-labels
NAME READY UP-TO-DATE AVAILABLE AGE LABELS
mywebserver 2/2 2 2 8m41s app=mywebserver
root@sathish-vm2:/home/sathish# kubectl get nodes -o wide
NAME STATUS ROLES AGE VERSION INTERNAL-IP EXTERNAL-IP OS-IMAGE KERNEL-VERSION CONTAINER-RUNTIME
sathish-vm1 Ready <none> 61d v1.19.0 172.28.147.44 <none> Ubuntu 20.04.1 LTS 5.4.0-47-generic docker://19.3.11
sathish-vm2 Ready master 61d v1.19.0 172.28.147.38 <none> Ubuntu 20.04.1 LTS 5.4.0-47-generic docker://19.3.11
root@sathish-vm2:/home/sathish# kubectl get pods -o wide
NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
mywebserver-66465b594f-jjvbg 1/1 Running 0 13m 10.244.0.7 sathish-vm2 <none> <none>
mywebserver-66465b594f-pztcj 1/1 Running 0 14m 10.244.1.18 sathish-vm1 <none> <none>
Now let's create a nodePort service to expose this with this YAML file
apiVersion: v1
kind: Service
metadata:
name: my-webservice
spec:
type: NodePort
selector:
app: mywebserver
ports:
- port: 80
targetPort: 80
nodePort: 30007Accessing the server and it works.
root@sathish-vm2:/home/sathish# curl http://172.28.147.38:30007
<html><body><h1>It works!</h1></body></html>"It works" is the default HTML page from apache.
Let's say I have a custom webpage somewhere on my local storage and I want to display that when the webserver is accessed. The HTML for the webpage is given below
<html>
<body>
<p>Hi from Sathish</p>
</body>
</html>How can I make sure the container serves this webpage instead of the default "it works" page?
Let's look at the Docker Hub page for apache
One of the options is this
docker run -dit --name my-apache-app -p 8080:80 -v "$PWD":/usr/local/apache2/htdocs/ httpd:2.4This essentially mounts contents of current directory of host in /usr/local/apache2/htdocs/ within the container. So, as long I have my custom file somewhere on local storage it is possible to serve this during the container.
So let's recreate the deployment to make this happen.
root@sathish-vm2:/home/sathish/mypage# kubectl get deployment mywebserver -o yaml > mywebserver.yaml
#Deleting existing deployment
root@sathish-vm2:/home/sathish/mypage# kubectl delete deployment mywebserver
deployment.apps "mywebserver" deleted
#Create skeletal YAML file
kubectl create deployment mywebserver --image=httpd --replicas=2 --dry-run=client -o yaml > mywebserver.yaml
#Updated content of mywebserver.yaml file
apiVersion: apps/v1
kind: Deployment
metadata:
labels:
app: mywebserver
name: mywebserver
spec:
replicas: 2
selector:
matchLabels:
app: mywebserver
strategy: {}
template:
metadata:
labels:
app: mywebserver
spec:
containers:
- image: httpd
name: httpd
volumeMounts:
- name: htmlpath
mountPath: /usr/local/apache2/htdocs/
volumes:
- name: htmlpath
hostPath:
path: /home/sathish/mypage/index.html
type: File
#Creating the deployment
root@sathish-vm2:/home/sathish# kubectl create -f mywebserver.yaml
deployment.apps/mywebserver createdLet's look at
volumeMounts: - name: htmlpath mountPath: /usr/local/apache2/htdocs/ volumes: - name: htmlpath hostPath: path: /home/sathish/mypage/index.html type: File
Volume Mount under the container section tells Kubernetes to look for volume by a specific name - "htmlpath" in this case. The contents of the volume will be mounted in "mountPath" which is /usr/local/apache2/htdocs/
Volume section specifies the parameters for Volume which in this case is hostPath. "/home/sathish/mypage/index.html" will be mounted inside the container.
Now when I try to access the webservice it displays my custom webpage
root@sathish-vm2:/home/sathish# curl http://172.28.147.38:30007
<html><body><p>Hi from Sathish</hp></body></html>There is a problem with this- I have to ensure that the mountpath directory is created and the HTML file is available on all nodes where replicas run. This can be solved by having a network mount on all nodes and placing the file there. The network mount could be NFS or any of the cloud file services like Amazon EFS.
The above example is that of a "consumer" PoD that consumes something from the file system and serves it to users.
Let's talk about a different requirement- What about PoDs that require storage for storing and retrieving data (say dynamic data created by server-side scripts). We could create VolumeMounts on nodes and ensure that PoDs are run on nodes with sufficient storage. However, this approach becomes cumbersome when managing 100's or 1000's of PoDs. Each PoD might need a specific amount of storage (say 100 MB) with certain access criteria. Would it be possible to create a bunch of volumes from available storage (say 10 pieces of 100 Mb data) and claim them dynamically as required? This is possible with PersitantVolume and PersistantVolume Claims.
Persistent Volumes and Persistent Volume Claims
Persistent volumes and claims are described here
A persistent volume is a certain amount of physical storage provisioned on a cluster by the administrator. Persistent volume claims can "claim" these volumes. PoDs can use Persistent Volume Claims.
Let's create a PersistentVolume. Here is Yaml file for the same
apiVersion: v1
kind: PersistentVolume
metadata:
name: my-volume
spec:
capacity:
storage: 100Mi
volumeMode: Filesystem
accessModes:
- ReadWriteOnce
hostPath:
path: "/home/sathish/mypage"
root@sathish-vm2:/home/sathish# kubectl get pv
NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE
my-volume 100Mi RWO Retain Available 6m5s"Available" indicates that this volume is available and can be claimed. The different accessModes are described here. As we are using hostPath, read-write-Once is supported mode. If you NFS or cloud storage, other options are supported.
Let's create a PersistantVolumeClaim object and "claim" this volume.
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
name: my-pv-claim
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 50Miroot@sathish-vm2:/home/sathish# kubectl create -f pvc.yaml
persistentvolumeclaim/my-pv-claim created
root@sathish-vm2:/home/sathish# kubectl get pv
NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE
my-volume 100Mi RWO Retain Bound default/my-pv-claim 5m1s
root@sathish-vm2:/home/sathish# kubectl get pvc
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE
my-pv-claim Bound my-volume 100Mi RWO 17s
Few things to note:
Even though the claim required 50Mb, there were no 50MB volumes available. So claim for bound to a volume that satisfied its storage requirement even though the volume itself larger (100 MB)
The access mode between PV and PVC should match (ReadWriteOnce in this case).
Let's recreate the earlier deployment, but this time have it use the PVC. Here is the YAML file
apiVersion: apps/v1
kind: Deployment
metadata:
labels:
app: mywebserver
name: mywebserver
spec:
replicas: 1
selector:
matchLabels:
app: mywebserver
strategy: {}
template:
metadata:
creationTimestamp: null
labels:
app: mywebserver
spec:
volumes:
- name: webcontent
persistentVolumeClaim:
claimName: my-pv-claim
containers:
- image: httpd
name: httpd
volumeMounts:
- mountPath: "/usr/local/apache2/htdocs/"
name: webcontent
nodeName: sathish-vm2Notes:
"nodeName" selector is used as this is where I have created the directory for PV. In my case, this happens to be the master node. I have labelled my master node like so:
root@sathish-vm2:/home/sathish# kubectl label node sathish-vm2 name=sathish-vm2
node/sathish-vm2 labeled
By defaults PoDs cannot be scheduled on master node due to the presence of NoSchedule taint. This can be removed with:
root@sathish-vm2:/home/sathish# kubectl taint node sathish-vm2 node-role.kubernetes.io/master:NoSchedule-
Let's check things out
#Rewriting index.html webpage with custom page
root@sathish-vm2:/home/sathish/mypage# kubectl exec mywebserver-6d6c47b597-jqtfc -- sh -c "echo 'Hi from sathish-vm2 pvc' > /usr/local/apache2/htdocs/index.html"
#Checking access
root@sathish-vm2:/home/sathish/mypage# curl http://172.28.147.38:30007
Hi from sathish-vm2 pvc
#The webpage is created on local storage referenced by PV
root@sathish-vm2:/home/sathish/mypage# pwd
/home/sathish/mypage
root@sathish-vm2:/home/sathish/mypage# cat index.html
Hi from sathish-vm2 pvc
That's it for today folks. Hope this was useful. Thanks for your time and have a good weekend.
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