It's Time for the Populator

Declarative initialization of kubernetes persistent storage

Adam Litke - alitke@redhat.com
Principle Software Engineer - Red Hat
FOSDEM 2019 - 02 February 2019

Kubernetes

• The de-facto way to run containers at scale.
• Declarative model using the Controller pattern
• Extensible using Custom Resources and Controllers

Nodes

• A Kubernetes worker machine
• Have the services necessary to run Pods (container runtime, kubelet and kube-proxy)
• Managed by master components

Pods

• One or more containers
• Run together on a Node
• Shared storage and networking

Storage Resources

• Persistent Volume: a piece of provisioned storage with a type
• Persistent Volume Claim: a request for a piece of storage

Why do we need PVC Initialization?

• Clone your database
• Big data processing
• Restore from backup
• Mutable persistent data (ie. VM disk)

Current options

• `kubectl cp` into a running container
• Run a special Pod first to prepare PVC
• Use an initContainer in your application Pod

Initcontainer example

apiVersion: v1
kind: Pod
metadata:
  name: fosdemo-pod
spec:
  containers:
  - name: fosdemo-nginix
    image: nginx
  initContainers:
  - name: init-content
    image: busybox
    command: ['sh', '-c',
              'wget -O /usr/share/nginx/html http://fosdem.org']

Is it good enough?

• You will want more features
  • Error handling
  • Authorization and Security
  • Logging and monitoring
• You need to manage it
  • Ordering
  • ReplicaSets and auto-scaling
  • Error handling

A Better Approach

• Declarative, the kubernetes way
• A Custom Resource describes a population strategy
• PVC references Custom Resource
• A Populator Controller manages the process
• Can be used to collaborate on many populators

Demo

Declarative Population

If you request a populated PVC, kubernetes will not make that PVC available to Pods until it has been populated.

Taints and Tolerations

• Reuse existing Kubernetes pattern
• New PVCs with a DataSource defined receive a Taint
• Signifies the PVC has not been populated yet
• Regular Pods will not schedule while a PVC is tainted
• Populator pods express a Toleration and may run
• Populator Controller removes the Taint on success

Next Steps

• Allow CRs in PVC DataSource field
• PVC Taints and Tolerations
• Populator SDK

Populator Examples

• Git Repo
• HTTP using k8s secrets for authentication
• Clone existing PVC
• Upload data from client
• Restore from backup

Thank you!

• Check out:
  github.com/kubevirt/containerized-data-importer