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Improve init containers concept page #14281

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238 changes: 140 additions & 98 deletions content/en/docs/concepts/workloads/pods/init-containers.md
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Expand Up @@ -7,99 +7,103 @@ weight: 40
---

{{% capture overview %}}
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.
{{% /capture %}}

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.

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, however, they are deprecated in 1.6 and 1.7.
In 1.8, the annotations are no longer supported and must be converted to the PodSpec field.
You can specify init containers in the Pod specification alongside the `containers`
array (which describes app containers).
{{% /capture %}}

{{% capture body %}}
## Understanding Init Containers

A [Pod](/docs/concepts/workloads/pods/pod-overview/) 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.
## Understanding init containers
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Excellent de-capitalization. 👍


Init Containers are exactly like regular Containers, except:
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.

* They always run to completion.
* Each one must complete successfully before the next one is 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.
* Init containers always run to completion.
* Each init container must complete successfully before the next one starts.

To specify a Container as an Init Container, add the `initContainers` field on the PodSpec as
a JSON array of objects of type
[Container](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#container-v1-core)
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
### Differences from regular containers

Init Containers support all the fields and features of app 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](#resources) below. Also, Init Containers do not
support readiness probes because they must run to completion before the Pod can
be ready.
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 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.
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.

## What can Init Containers be used for?
## Using init containers

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

* They can contain and run 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
* 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.
* Init containers can securely run utilities that would make an app container image less secure.
* The application image builder and deployer roles can work independently without
the need to jointly build a single app image.
* They use Linux namespaces so that they have different filesystem views from app Containers.
Consequently, they can be given access to Secrets that app Containers are not able to
access.
* They run to completion before any app Containers start, whereas app
Containers run in parallel, so Init Containers provide an easy way to block or
delay the startup of app Containers until some set of preconditions are met.
* 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.

### Examples
Here are some ideas for how to use Init Containers:

* Wait for a service to be created with a shell command like:
### Examples
Here are some ideas for how to use init containers:

for i in {1..100}; do sleep 1; if dig myservice; then exit 0; fi; done; exit 1
* 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
```
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* 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=$(<POD_NAME>)&ip=$(<POD_IP>)'
```

* Wait for some time before starting the app container with a command like
```shell
sleep 60
```

`curl -X POST http://$MANAGEMENT_SERVICE_HOST:$MANAGEMENT_SERVICE_PORT/register -d 'instance=$(<POD_NAME>)&ip=$(<POD_IP>)'`
* Clone a Git repository into a {{< glossary_tooltip text="Volume" term_id="volume" >}}

* Wait for some time before starting the app Container with a command like `sleep 60`.
* Clone a git repository into a 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
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.

More detailed usage examples can be found in the [StatefulSets documentation](/docs/concepts/workloads/controllers/statefulset/)
and the [Production Pods guide](/docs/tasks/configure-pod-container/configure-pod-initialization/).
#### Init containers in use

### Init Containers in use

The following yaml file 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.
This example defines a simple Pod that has two init containers.
The first waits for `myservice`, and the second waits for `mydb`. Once both
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init containers complete, the Pod runs the app container from its `spec` section.

```yaml
apiVersion: v1
Expand All @@ -122,7 +126,8 @@ spec:
command: ['sh', '-c', 'until nslookup mydb; do echo waiting for mydb; sleep 2; done;']
```

Yaml file below outlines the `mydb` and `myservice` services:

The following YAML file outlines the `mydb` and `myservice` services:

```yaml
apiVersion: v1
Expand All @@ -146,7 +151,7 @@ spec:
targetPort: 9377
```

This Pod can be started and debugged with the following commands:
You can start this Pod by running:

```shell
kubectl apply -f myapp.yaml
Expand All @@ -155,6 +160,7 @@ kubectl apply -f myapp.yaml
pod/myapp-pod created
```

And check on its status with:
```shell
kubectl get -f myapp.yaml
```
Expand All @@ -163,6 +169,7 @@ NAME READY STATUS RESTARTS AGE
myapp-pod 0/1 Init:0/2 0 6m
```

or for more details:
```shell
kubectl describe -f myapp.yaml
```
Expand Down Expand Up @@ -200,13 +207,42 @@ Events:
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
```

Once we start the `mydb` and `myservice` services, we can see the Init Containers
complete and the `myapp-pod` is created:
At this point, those init containers will be waiting to discover Services named
`mydb` and `myservices`.

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
Expand All @@ -216,91 +252,97 @@ 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 example is very simple but should provide some inspiration for you to
create your own Init Containers.
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, 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
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.
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
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](#pod-restart-reasons), all Init Containers must
execute again.
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.
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
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
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.
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.
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
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 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 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
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.
* 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 cgroups are based on the effective Pod request and limit, the
same as the scheduler.
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
A Pod can restart, causing re-execution of init containers, for the following
reasons:

* A user updates the PodSpec 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.
* 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
forcing a restart, and the init container completion record has been lost due
to garbage collection.

{{% /capture %}}


{{% capture whatsnext %}}

* [Creating a Pod that has an Init Container](/docs/tasks/configure-pod-container/configure-pod-initialization/#creating-a-pod-that-has-an-init-container)
* 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 %}}