Concepts

Detailed explanations of Kubernetes system concepts and abstractions.

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Init Containers

This feature has exited beta in 1.6. Init Containers can be specified in the PodSpec alongside the app containers array. The beta annotation value will still be respected and overrides the PodSpec field value.

This page provides an overview of Init Containers, which are specialized Containers that run before app Containers and can contain utilities or setup scripts not present in an app image.

Understanding Init Containers

A Pod can have multiple Containers running apps within it, but it can also have one or more Init Containers, which are run before the app Containers are started.

Init Containers are exactly like regular Containers, except:

If an Init Container fails for a Pod, Kubernetes restarts the Pod repeatedly until the Init Container succeeds. However, if the Pod has a restartPolicy of Never, it is not restarted.

To specify a Container as an Init Container, add the initContainers field on the PodSpec as a JSON array of objects of type v1.Container alongside the app containers array. The status of the init containers is returned in status.initContainerStatuses field as an array of the container statuses (similar to the status.containerStatuses field).

Differences from regular Containers

Init Containers support all the fields and features of app Containers, including resource limits, volumes, and security settings. However, the resource requests and limits for an Init Container are handled slightly differently, which are documented in Resources below. Also, Init Containers do not support readiness probes because they must run to completion before the Pod can be ready.

If multiple Init Containers are specified for a Pod, those Containers are run one at a time in sequential order. Each must succeed before the next can run. When all of the Init Containers have run to completion, Kubernetes initializes the Pod and runs the application Containers as usual.

What can Init Containers be used for?

Because Init Containers have separate images from app Containers, they have some advantages for start-up related code:

Examples

Here are some ideas for how to use Init Containers:

More detailed usage examples can be found in the StatefulSets documentation and the Production Pods guide.

Init Containers in use

The following yaml file for Kubernetes 1.5 outlines a simple Pod which has two Init Containers. The first waits for myservice and the second waits for mydb. Once both containers complete, the Pod will begin.

apiVersion: v1
kind: Pod
metadata:
  name: myapp-pod
  labels:
    app: myapp
  annotations:
    pod.beta.kubernetes.io/init-containers: '[
        {
            "name": "init-myservice",
            "image": "busybox",
            "command": ["sh", "-c", "until nslookup myservice; do echo waiting for myservice; sleep 2; done;"]
        },
        {
            "name": "init-mydb",
            "image": "busybox",
            "command": ["sh", "-c", "until nslookup mydb; do echo waiting for mydb; sleep 2; done;"]
        }
    ]'
spec:
  containers:
  - name: myapp-container
    image: busybox
    command: ['sh', '-c', 'echo The app is running! && sleep 3600']

There is a new syntax in Kubernetes 1.6, although the old annotation syntax still works. We have moved the declaration of init containers to spec:

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

1.5 syntax still works on 1.6, but we recommend using 1.6 syntax. In Kubernetes 1.6, Init Containers were made a field in the API. The beta annotation is still respected but will be deprecated in future releases.

Yaml file below outlines the mydb and myservice services:

kind: Service
apiVersion: v1
metadata:
  name: myservice
spec:
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9376
---
kind: Service
apiVersion: v1
metadata:
  name: mydb
spec:
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9377

This Pod can be started and debugged with the following commands:

$ kubectl create -f myapp.yaml
pod "myapp-pod" created
$ kubectl get -f myapp.yaml
NAME        READY     STATUS     RESTARTS   AGE
myapp-pod   0/1       Init:0/2   0          6m
$ kubectl describe -f myapp.yaml 
Name:          myapp-pod
Namespace:     default
[...]
Labels:        app=myapp
Status:        Pending
[...]
Init Containers:
  init-myservice:
[...]
    State:         Running
[...]
  init-mydb:
[...]
    State:         Waiting
      Reason:      PodInitializing
    Ready:         False
[...]
Containers:
  myapp-container:
[...]
    State:         Waiting
      Reason:      PodInitializing
    Ready:         False
[...]
Events:
  FirstSeen    LastSeen    Count    From                      SubObjectPath                           Type          Reason        Message
  ---------    --------    -----    ----                      -------------                           --------      ------        -------
  16s          16s         1        {default-scheduler }                                              Normal        Scheduled     Successfully assigned myapp-pod to 172.17.4.201
  16s          16s         1        {kubelet 172.17.4.201}    spec.initContainers{init-myservice}     Normal        Pulling       pulling image "busybox"
  13s          13s         1        {kubelet 172.17.4.201}    spec.initContainers{init-myservice}     Normal        Pulled        Successfully pulled image "busybox"
  13s          13s         1        {kubelet 172.17.4.201}    spec.initContainers{init-myservice}     Normal        Created       Created container with docker id 5ced34a04634; Security:[seccomp=unconfined]
  13s          13s         1        {kubelet 172.17.4.201}    spec.initContainers{init-myservice}     Normal        Started       Started container with docker id 5ced34a04634
$ kubectl logs myapp-pod -c init-myservice # Inspect the first init container
$ kubectl logs myapp-pod -c init-mydb      # Inspect the second init container

Once we start the mydb and myservice services, we can see the Init Containers complete and the myapp-pod is created:

$ kubectl create -f services.yaml
service "myservice" created
service "mydb" created
$ kubectl get -f myapp.yaml
NAME        READY     STATUS    RESTARTS   AGE
myapp-pod   1/1       Running   0          9m

This example is very simple but should provide some inspiration for you to create your own Init Containers.

Detailed behavior

During the startup of a Pod, the Init Containers are started in order, after the network and volumes are initialized. Each Container must exit successfully before the next is started. If a Container fails to start due to the runtime or exits with failure, it is retried according to the Pod restartPolicy. However, if the Pod restartPolicy is set to Always, the Init Containers use RestartPolicy OnFailure.

A Pod cannot be Ready until all Init Containers have succeeded. The ports on an Init Container are not aggregated under a service. A Pod that is initializing is in the Pending state but should have a condition Initializing set to true.

If the Pod is restarted, all Init Containers must execute again.

Changes to the Init Container spec are limited to the container image field. Altering an Init Container image field is equivalent to restarting the Pod.

Because Init Containers can be restarted, retried, or re-executed, Init Container code should be idempotent. In particular, code that writes to files on EmptyDirs should be prepared for the possibility that an output file already exists.

Init Containers have all of the fields of an app Container. However, Kubernetes prohibits readinessProbe from being used because Init Containers cannot define readiness distinct from completion. This is enforced during validation.

Use activeDeadlineSeconds on the Pod and livenessProbe on the Container to prevent Init Containers from failing forever. The active deadline includes Init Containers.

The name of each app and Init Container in a Pod must be unique; a validation error is thrown for any Container sharing a name with another.

Resources

Given the ordering and execution for Init Containers, the following rules for resource usage apply:

Quota and limits are applied based on the effective Pod request and limit.

Pod level cgroups are based on the effective Pod request and limit, the same as the scheduler.

Pod restart reasons

A Pod can restart, causing re-execution of Init Containers, for the following reasons:

Support and compatibility

A cluster with Apiserver version 1.6.0 or greater supports Init Containers using the spec.initContainers field. Previous versions support Init Containers using the alpha or beta annotations. The spec.initContainers field is also mirrored into alpha and beta annotations so that Kubelets version 1.3.0 or greater can execute Init Containers, and so that a version 1.6 apiserver can safely be rolled back to version 1.5.x without losing Init Container functionality for existing created pods.

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