Multi Container Pod

Multi-container pods are used in cases where it’s better to have separate code for some added functionality along with the application code. Example: log agent alongside the application to collect logs and send them to a centralized log server.

Containers inside a pod share the same:

Lifecycle - created and destroyed together
Network - can refer each other as localhost
Storage - can access the same storage volumes

All the containers inside a pod must keep running for the pod to remain in running state. If any of them fails, the pod restarts.

Sharing volumes in multi-container pods


apiVersion: v1
kind: Pod
metadata:
	name: web-pod
spec:
	volumes:
		- name: shared-vol
			emptyDir: {}
	containers:
		- name: webapp
			image: webapp
			volumeMounts:
		    - name: shared-vol
		      mountPath: /files
		- name: log-agent
			image: log-agent
			volumeMounts:
		    - name: shared-vol
		      mountPath: /logs

Init Containers

Init containers run once during the pod initialization and they terminate after that. When a POD is first created, the init container is run, and the process in the init container must run to a completion before the real container hosting the application starts. We can have multiple init containers. In that case each init container is run one at a time in sequential order. If any of the init containers fail to complete, Kubernetes restarts the Pod repeatedly until all the init containers succeed.

For example: a process that pulls a code or binary from a repository that will be used by the main web application or a process that waits for an external service or database to be up before the actual application starts.

Init containers are defined just like other containers inside a pod definition file. However, they are defined under initContainers section.


apiVersion: v1
kind: Pod
metadata:
  name: myapp-pod
  labels:
    app: myapp
spec:
  containers:
  - name: myapp-container
    image: busybox:1.28
    command: ['sh', '-c', 'echo The app is running! && sleep 3600']
  initContainers:
  - name: init-myservice
    image: busybox:1.28
    command: ['sh', '-c', 'until nslookup myservice; do echo waiting for myservice; sleep 2; done;']
  - name: init-mydb
    image: busybox:1.28
    command: ['sh', '-c', 'until nslookup mydb; do echo waiting for mydb; sleep 2; done;']

Sidecar Containers

They are secondary containers that run along with the main container within the same Pod.

You have an application that is hard-coded to write log output to a file on disk.

You add a secondary container to the Pod that reads the log file from a shared volume and prints it to the console so the log output will appear in the container log.