- Overview
- What are Init Containers Good For?
- Detailed Behavior
- Pod Restart Reasons
- Support and compatibilty
In addition to having one or more main containers (or app containers), a pod can also have one or more init containers which run before the app containers. Init containers allow you to reduce and reorganize setup scripts and “glue code”.
Overview
An init container is exactly like a regular container, except that it always
runs to completion and each init container must complete successfully before
the next one is started. If the init container fails, Kubernetes will restart
the pod until the init container succeeds. If a pod is marked as RestartNever,
the pod will fail if the init container fails.
You specify a container as an init container by adding an annotation
The annotation key is pod.beta.kubernetes.io/init-containers. The annotation
value is a JSON array of objects of type v1.Container
Once the feature exits beta, the init containers will be specified on the Pod
Spec alongside the app containers array.
The status of the init containers is returned as another annotation -
pod.beta.kubernetes.io/init-container-statuses – as an array of the
container statuses (similar to the status.containerStatuses field).
Init containers support all of the same features as normal containers, including resource limits, volumes, and security settings. The resource requests and limits for an init container are handled slightly differently. Init containers do not support readiness probes since they will run to completion before the pod can be ready. An init container has all of the fields of an app container.
If you specify multiple init containers for a pod, those containers run one at a time in sequential order. Each must succeed before the next can run. Once all init containers have run to completion, Kubernetes initializes the pod and runs the application containers as usual.
What are Init Containers Good For?
Because init containers have separate images from application containers, they have some advantages for start-up related code. These include:
- they can contain utilities that are not desirable to include in the app container image for security reasons,
- they can contain utilities or custom code for setup that is not present in an app
image. (No need to make an image
FROManother image just to use a tool likesed,awk,python,dig, etc during setup). - the application image builder and the deployer roles can work independently without the need to jointly build a single app image.
Because init containers have different filesystem view (Linux namespaces) from app containers, they can be given access to Secrets that the app containers are not able to access.
Since init containers run to completion before any app containers start, and since app containers run in parallel, they provide an easier way to block or delay the startup of application containers until some precondition is met.
Because init containers run in sequence and there can be multiple init containers, they can be composed easily.
Here are some ideas for how to use init containers:
- Wait for a service to be created with a shell command like:
for i in {1..100}; do sleep 1; if dig myservice; then exit 0; fi; exit 1
- Register this pod with a remote server with a command like:
curl -X POST http://$MANAGEMENT_SERVICE_HOST:$MANAGEMENT_SERVICE_PORT/register -d 'instance=$(POD_NAME)&ip=$(POD_IP)'
using POD_NAME and POD_IP from the downward API.
- Wait for some time before starting the app container with a command like sleep 60.
- Clone a git repository into a volume
- Place values like a POD_IP into a configuration file, and run a template tool (e.g. jinja)
to generate a configuration file to be consumed by the main app contianer.
```
Complete usage examples can be found in the PetSets guide and the Production Pods guide.
Detailed Behavior
Each pod may have 0..N init containers defined along with the existing 1..M app containers.
On startup of the pod, after the network and volumes are initialized, the init containers are started in order. Each container must exit successfully before the next is invoked. If a container fails to start (due to the runtime) or exits with failure, it is retried according to the pod RestartPolicy, except when the pod restart policy is RestartPolicyAlways, in which case just the init containers use RestartPolicyOnFailure.
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 being
initialized is in the Pending phase but should has 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 a init container image field is equivalent to restarting the pod.
Because init containers can be restarted, retried, or reexecuted, 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.
An init container has all of the fields of an app container. The following fields are prohibited from being used on init containers by validation:
readinessProbe- init containers must exit for pod startup to continue, are not included in rotation, and so cannot define readiness distinct from completion.
Init container authors may 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 - it is a validation error for any container to share a name.
Resources
Given the ordering and execution for init containers, the following rules for resource usage apply:
- The highest of any particular resource request or limit defined on all init containers is the effective init request/limit
- The pod’s effective request/limit for a resource is the higher of:
- sum of all app containers request/limit for a resource
- effective init request/limit for a resource
- Scheduling is done based on effective requests/limits, which means init containers can reserve resources for initialization that are not used during the life of the pod.
- QoS tier of the pod’s effective QoS tier is the QoS tier for init containers and app containers alike.
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 may “restart”, causing reexecution of init containers, for the following reasons:
- An init container image is changed by a user updating the Pod Spec.
- App container image changes only restart the app container.
- The pod infrastructure container is restarted
- This is uncommon and would have to be done by someone with root access to nodes.
- All containers in a pod are terminated, requiring a restart (RestartPolicyAlways) AND the record of init container completion has been lost due to garbage collection.
Support and compatibilty
A cluster with Kubelet and Apiserver version 1.4.0 or greater supports init containers with the beta annotations. Support varies for other combinations of Kubelet and Apiserver version; see the release notes for details.