Kubernetes - DB in Pods?

How to use DB in Kubernetes?

Containers do not maintain data from before restart when they restart.
Therefore, for services that need to be Stateful like databases, you can mount volumes to external file systems to maintain state.

Container volume mounting in Docker

In Docker, you can specify volumes to mount when running containers in the form of -v {external volume}:{internal volume in container}.

docker run --name {name} -v {external volume}:{container volume} -d -p {ext port}:{int port} {image}:{version}

Example) docker run --name mongo -v C:\\mongo-mount:/data/db -d -p 27017:27017 wichan-mongo:0.0.2

In this case, even if the container terminates, you can continue the service by referencing the mount path when restarting, so it's appropriate.

Pod (container) volume mounting in k8s

Since k8s is ultimately a tool for running containers, you can maintain state by mounting pod volumes to nodes.
However, additional considerations are needed when using replicaset functionality.
If multiple replicas that mount the same volume are created, conflicts will occur.

Reference1: https://bcho.tistory.com/1306
Reference2: https://velog.io/@hoonki/%EC%BF%A0%EB%B2%84%EB%84%A4%ED%8B%B0%EC%8A%A4k8s-Persistent-Storage%EB%9E%80

Let's try deploying DB in k8s

Reference: https://zzsza.github.io/development/2019/01/27/kubernetes-statefulset/

Try registering with Deployment

apiVersion: apps/v1
kind: Deployment      # deployment > replicaset > pod
metadata:
  name: mongo-deployment       # deployment's name
  labels:
    app: mongo                 # labels can be attached to resources
spec:                 # define deployment spec
  replicas: 4         # create 4 replica pods
  selector:           # method for deployment to find pods to manage
    matchLabels:      # select pods with app=mongo label
      app: mongo
  template:                    # define meta, spec for pods to be created
    metadata:
      labels:
        app: mongo             # attach app=mongo label to pod
    spec:
      containers:              # create one container
      - name: mongo            # value of name field is mongo
        image: mongo:latest    # use latest mongo image
        ports:
        - containerPort: 27017    # specify container port
kubectl apply -f mongo-deployment.yaml

structure

Register with Statefulset

Let's create mongo pods with statefulset.

kind: StatefulSet         # create statefulset
apiVersion: apps/v1       # statefulset is supported from apps/v1
metadata:                 # specify sfs metadata
  name: mongo-primary-0   # sfs name, but pods are named {sfs name}-{number} when created
spec:                     # specify sfs spec
  replicas: 1             # specify number of replicas
  selector:               # specify selector to map pods
    matchLabels:
      app: mongo-primary
  template:               
    metadata:             # specify pod metadata
      labels:
        app: mongo-primary
    spec:                 # specify pod spec
      volumes:
        - name: mongodata # find volume named mongodata and mount to /app/mongo on node
          hostPath:
            path: /app/mongo
      containers:
        - name: mongo-primary-container
          image: mongo:latest
          ports:
            - containerPort: 27017
              protocol: TCP
          volumeMounts:
            - name: mongodata
              mountPath: /data/db         # specify pod's /data/db as mongodata name
MSAKubernetesDB