Multi-Container Pods

In general we want to define 1 container per pod. That is the case when we want to operate a microservice on that container.

But there are reasons to run multiple ones. For example, helper containers, run setup scripts, stuff like that.

Design Patterns

Emerged from pod requirements

  • Init Container: initialization logic, that need to be run before the main app starts.

    • Example: Downloading config files required by the application.

  • Sidecar: containers not part of the main traffic the app receives, but will run along side the main app

    • Example: Watcher capabilities.

  • Adapter: Transform output produced by app into another format or smth that makes it more usable for another program.

    • Example: Massaging log data

  • Ambassador: Provides a proxy for communicating with external services. To abstract complexity.

    • Example: providing credentials for auth.

Init Container

To define an initContainer you need to add this to your yaml.

apiVersion: v1
kind: Pod
metadata:
  name: myapp
spec:
  initContainers:
  - name: app-init
    image: busybox:1.28
    command: ['sh', '-c', "wget something.com/res.png"]
  containers:
  - name: myapp-container
    image: busybox:1.28
    command: ['sh', '-c', 'echo The app is running! && sleep 3600']

When we start the pod, under STATUS you can see Init:0/1 meaning it is creating the containers needed.

We can talk with any of the containers using --contianer or -c.

k --container=init
k --container=app