--- reviewers: - erictune title: Init Containers content_template: templates/concept weight: 40 --- {{% capture overview %}} This page provides an overview of init containers: specialized containers that run before app containers in a {{< glossary_tooltip text="Pod" term_id="pod" >}}. Init containers can contain utilities or setup scripts not present in an app image. You can specify init containers in the Pod specification alongside the `containers` array (which describes app containers). {{% /capture %}} {{% capture body %}} ## Understanding init containers A {{< glossary_tooltip text="Pod" term_id="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: * Init containers always run to completion. * Each init container must complete successfully before the next one starts. If a Pod's init container fails, Kubernetes repeatedly restarts the Pod until the init container succeeds. However, if the Pod has a `restartPolicy` of Never, Kubernetes does not restart the Pod. To specify an init container for a Pod, add the `initContainers` field into the Pod specification, as an array of objects of type [Container](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#container-v1-core), 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 differently, as documented in [Resources](#resources). Also, init containers do not support readiness probes because they must run to completion before the Pod can be ready. If you specify multiple init containers for a Pod, Kubelet runs each init container sequentially. Each init container must succeed before the next can run. When all of the init containers have run to completion, Kubelet initializes the application containers for the Pod and runs them as usual. ## Using init containers Because init containers have separate images from app containers, they have some advantages for start-up related code: * Init containers can contain utilities or custom code for setup that are not present in an app image. For example, there is no need to make an image `FROM` another image just to use a tool like `sed`, `awk`, `python`, or `dig` during setup. * The application image builder and deployer roles can work independently without the need to jointly build a single app image. * Init containers can run with a different view of the filesystem than app containers in the same Pod. Consequently, they can be given access to {{< glossary_tooltip text="Secrets" term_id="secret" >}} that app containers cannot access. * Because init containers run to completion before any app containers start, init containers offer a mechanism to block or delay app container startup until a set of preconditions are met. Once preconditions are met, all of the app containers in a Pod can start in parallel. * Init containers can securely run utilities or custom code that would otherwise make an app container image less secure. By keeping unnecessary tools separate you can limit the attack surface of your app container image. ### Examples Here are some ideas for how to use init containers: * Wait for a {{< glossary_tooltip text="Service" term_id="service">}} to be created, using a shell one-line command like: ```shell for i in {1..100}; do sleep 1; if dig myservice; then exit 0; fi; done; exit 1 ``` * Register this Pod with a remote server from the downward API with a command like: ```shell curl -X POST http://$MANAGEMENT_SERVICE_HOST:$MANAGEMENT_SERVICE_PORT/register -d 'instance=$()&ip=$()' ``` * Wait for some time before starting the app container with a command like ```shell sleep 60 ``` * Clone a Git repository into a {{< glossary_tooltip text="Volume" term_id="volume" >}} * Place values into a configuration file and run a template tool to dynamically generate a configuration file for the main app container. For example, place the `POD_IP` value in a configuration and generate the main app configuration file using Jinja. #### Init containers in use This example defines a simple Pod that has two init containers. The first waits for `myservice`, and the second waits for `mydb`. Once both init containers complete, the Pod runs the app container from its `spec` section. ```yaml 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.$(cat /var/run/secrets/kubernetes.io/serviceaccount/namespace).svc.cluster.local; do echo waiting for myservice; sleep 2; done"] - name: init-mydb image: busybox:1.28 command: ['sh', '-c', "until nslookup mydb.$(cat /var/run/secrets/kubernetes.io/serviceaccount/namespace).svc.cluster.local; do echo waiting for mydb; sleep 2; done"] ``` You can start this Pod by running: ```shell kubectl apply -f myapp.yaml ``` ``` pod/myapp-pod created ``` And check on its status with: ```shell kubectl get -f myapp.yaml ``` ``` NAME READY STATUS RESTARTS AGE myapp-pod 0/1 Init:0/2 0 6m ``` or for more details: ```shell 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 ``` To see logs for the init containers in this Pod, run: ```shell kubectl logs myapp-pod -c init-myservice # Inspect the first init container kubectl logs myapp-pod -c init-mydb # Inspect the second init container ``` At this point, those init containers will be waiting to discover Services named `mydb` and `myservice`. Here's a configuration you can use to make those Services appear: ```yaml --- apiVersion: v1 kind: Service metadata: name: myservice spec: ports: - protocol: TCP port: 80 targetPort: 9376 --- apiVersion: v1 kind: Service metadata: name: mydb spec: ports: - protocol: TCP port: 80 targetPort: 9377 ``` To create the `mydb` and `myservice` services: ```shell kubectl apply -f services.yaml ``` ``` service/myservice created service/mydb created ``` You'll then see that those init containers complete, and that the `myapp-pod` Pod moves into the Running state: ```shell kubectl get -f myapp.yaml ``` ``` NAME READY STATUS RESTARTS AGE myapp-pod 1/1 Running 0 9m ``` This simple example should provide some inspiration for you to create your own init containers. [What's next](#what-s-next) contains a link to a more detailed example. ## Detailed behavior During the startup of a Pod, each init container starts in order, after the network and volumes are initialized. Each container must exit successfully before the next container starts. 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 `Initialized` set to true. If the Pod [restarts](#pod-restart-reasons), or 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: * 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: * the sum of all app containers request/limit for a resource * the 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. * The QoS (quality of service) 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 control groups (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: * A user updates the Pod specification, causing the init container image to change. Any changes to the init container image restarts the Pod. 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 while `restartPolicy` is set to Always, forcing a restart, and the init container completion record has been lost due to garbage collection. {{% /capture %}} {{% capture whatsnext %}} * Read about [creating a Pod that has an init container](/docs/tasks/configure-pod-container/configure-pod-initialization/#creating-a-pod-that-has-an-init-container) * Learn how to [debug init containers](/docs/tasks/debug-application-cluster/debug-init-containers/) {{% /capture %}}