container.training/slides/k8s/volumes.md

Volumes

• Volumes are special directories that are mounted in containers

• Volumes can have many different purposes:

  • share files and directories between containers running on the same machine

  • share files and directories between containers and their host

  • centralize configuration information in Kubernetes and expose it to containers

  • manage credentials and secrets and expose them securely to containers

  • store persistent data for stateful services

  • access storage systems (like Ceph, EBS, NFS, Portworx, and many others)

---

Kubernetes volumes vs. Docker volumes

• Kubernetes and Docker volumes are very similar

  (the Kubernetes documentation says otherwise ...
  but it refers to Docker 1.7, which was released in 2015!)

• Docker volumes allow to share data between containers running on the same host

• Kubernetes volumes allow us to share data between containers in the same pod

• Both Docker and Kubernetes volumes allow us access to storage systems

• Kubernetes volumes are also used to expose configuration and secrets

• Docker has specific concepts for configuration and secrets

  (but under the hood, the technical implementation is similar)

• If you're not familiar with Docker volumes, you can safely ignore this slide!

---

A simple volume example

apiVersion: v1
kind: Pod
metadata:
  name: nginx-with-volume
spec:
  volumes:
  - name: www
  containers:
  - name: nginx
    image: nginx
    volumeMounts:
    - name: www
      mountPath: /usr/share/nginx/html/

---

A simple volume example, explained

• We define a standalone Pod named nginx-with-volume

• In that pod, there is a volume named www

• No type is specified, so it will default to emptyDir

  (as the name implies, it will be initialized as an empty directory at pod creation)

• In that pod, there is also a container named nginx

• That container mounts the volume www to path /usr/share/nginx/html/

---

A volume shared between two containers

.small[

apiVersion: v1
kind: Pod
metadata:
  name: nginx-with-volume
spec:
  volumes:
  - name: www
  containers:
  - name: nginx
    image: nginx
    volumeMounts:
    - name: www
      mountPath: /usr/share/nginx/html/
  - name: git
    image: alpine
    command: [ "sh", "-c", "apk add --no-cache git && git clone https://github.com/octocat/Spoon-Knife /www" ]
    volumeMounts:
    - name: www
      mountPath: /www/
  restartPolicy: OnFailure

]

---

Sharing a volume, explained

• We added another container to the pod

• That container mounts the www volume on a different path (/www)

• It uses the alpine image

• When started, it installs git and clones the octocat/Spoon-Knife repository

  (that repository contains a tiny HTML website)

• As a result, NGINX now serves this website

---

Sharing a volume, in action

• Let's try it!

.exercise[

• Create the pod by applying the YAML file:

  kubectl apply -f ~/container.training/k8s/nginx-with-volume.yaml
  

• Check the IP address that was allocated to our pod:

  kubectl get pod nginx-with-volume -o wide
  IP=$(kubectl get pod nginx-with-volume -o json | jq -r .status.podIP)
  

• Access the web server:

  curl $IP
  

]

---

The devil is in the details

• The default restartPolicy is Always

• This would cause our git container to run again ... and again ... and again

  (with an exponential back-off delay, as explained in the documentation)

• That's why we specified restartPolicy: OnFailure

• There is a short period of time during which the website is not available

  (because the git container hasn't done its job yet)

• This could be avoided by using Init Containers

  (we will see a live example in a few sections)

---

Volume lifecycle

• The lifecycle of a volume is linked to the pod's lifecycle

• This means that a volume is created when the pod is created

• This is mostly relevant for emptyDir volumes

  (other volumes, like remote storage, are not "created" but rather "attached" )

• A volume survives across container restarts

• A volume is destroyed (or, for remote storage, detached) when the pod is destroyed