Host Path

  • We learned how to deploy our Stateless app using Kubernetes.
  • But we may need to deploy the app that has state, that accesses the database to save the records.
  • Making a Stateful app run using Kubernetes is a bit tricky, solely because our app can run in different nodes.
  • Kubernetes out of the box has different types of volumes, but based on cloud provider you use you will have way to save your data.

Kubernetes Host Path Volume

  • Kubernetes has a simple volume type called hostPath which gives you persistence storage.
  • But hostPath has one big limitation i.e. hostPath volume mounts a file or directory from the host node’s filesystem into your Pod.
  • The main point here is that, hostPath volume type mounts the volumes to node.
  • Our cluster can have multiple nodes, and two nodes doesn't share a filesystem.
  • Pods with identical configuration may behave differently on different nodes due to different files on the nodes
  • Because we are developing our app locally we have to use hostPath, and it doesn't affect us. We are running only one node.

Using Host Path

apiVersion: apps/v1
kind: Deployment
# metadata
spec:
  #replicas & metadata
  template:
    # metadata
    spec:
      volumes:                #1
        - name: volume-name   #2
          hostPath:           #3
            path: /absolute/host/path   #4
      containers:
        - image: amantuladhar/docker-kubernetes:v1-web
          name: web-co
          volumeMounts:                 #5
              - name: volume-name       #6
                mountPath: /containerPath/    #7
  • #1In above Deployment definition, we are using volumes properties to define the volumes we want to use. (This is not a proper way to define volume, but let's go step by step)
  • #2 We are giving name to our volume so that we can use it later.
  • #3 hostPath property. This is the type of volume we are using. For local development we are using hostPath but this will be provider specific volume type.
  • #4 Child property of hostPath. This is an absolute path which points which folder from host to mount.
    • Remember for minikube host path is inside VM.
    • Here's a minikube documentation to mount your actual host path to minikube VM, then to mount that volume to Kubernetes.
    • TLDR; use minikube mount actual/host/path:/minikube/vm/path
  • #5 Using volumeMounts property we can specify which volumes our container needs to mount.
  • #6 Using name property we specify the name of the volume we want to mount.
    • For your purposes it needs to match #2
  • #7 mountPath take the path inside the container. This is the place where container will access external data.

We are using Deployment here. If you just want to create a Pod you remember, everything below from template property is basically Pod definition.

  • After you create a resource, we can go inside the Pod and play around with it.
  • Let's create a file inside the container and see if that file is available on our host.
  • And let's delete the Pod and see if new Pod can see that file.
kubectl create -f hostPath.yaml
  • Creating file inside Pod
# Get inside Pod
➜ kubectl exec -it web-dep-89fd5db9-897dr sh

# Change directory to path where we mounted the volume
/myApp # cd /containerPath/

# Create a file with some text
/containerPath # echo "Very important data" >> file.txt

# Check if file exist (inside container)
/containerPath # ls
file.txt
  • Check if file is visible in your host path. You will see the file if everything went well.
  • Now, restart the container and see if file exist.
kubectl delete pod --all
  • This will delete all the Pods.
  • But Deployment will spawn new Pod immediately.
  • Go inside new Pod and see if file exist.
# Getting inside new Pod
➜ kubectl exec -it web-dep-89fd5db9-689sw sh

# Checking if file exist
/myApp # ls /containerPath/
file.txt

# Checking the content of file
/myApp # cat /containerPath/file.txt
Very important data
  • We learned how we can easily mount volume to Kubernetes by using hostPath.
  • Make sure if you use hostPath on production you know what you are doing.
  • There may be a scenario where we need this, but for general purposes like database this is not a solution.
  • Next we will learn how we can improve this example and what problems this solution has.