website/docs/user-guide/walkthrough/k8s201.md

---
assignees:
- janetkuo
- mikedanese

---

## Labels, Deployments, Services and Health Checking

If you went through [Kubernetes 101](/docs/user-guide/walkthrough/), you learned about kubectl, Pods, Volumes, and multiple containers.
For Kubernetes 201, we will pick up where 101 left off and cover some slightly more advanced topics in Kubernetes, related to application productionization, Deployment and
scaling.

In order for the kubectl usage examples to work, make sure you have an examples directory locally, either from [a release](https://github.com/kubernetes/kubernetes/releases) or [the source](https://github.com/kubernetes/kubernetes).

* TOC
{:toc}


## Labels

Having already learned about Pods and how to create them, you may be struck by an urge to create many, many Pods.  Please do!  But eventually you will need a system to organize these Pods into groups.  The system for achieving this in Kubernetes is Labels.  Labels are key-value pairs that are attached to each object in Kubernetes.  Label selectors can be passed along with a RESTful `list` request to the apiserver to retrieve a list of objects which match that label selector.

To add a label, add a labels section under metadata in the Pod definition:

```yaml
  labels:
    app: nginx
```

For example, here is the nginx Pod definition with labels ([pod-nginx-with-label.yaml](/docs/user-guide/walkthrough/pod-nginx-with-label.yaml)):

{% include code.html language="yaml" file="pod-nginx-with-label.yaml" ghlink="/docs/user-guide/walkthrough/pod-nginx-with-label.yaml" %}

Create the labeled Pod ([pod-nginx-with-label.yaml](/docs/user-guide/walkthrough/pod-nginx-with-label.yaml)):

```shell
kubectl create -f docs/user-guide/walkthrough/pod-nginx-with-label.yaml
```

List all Pods with the label `app=nginx`:

```shell
kubectl get pods -l app=nginx
```

For more information, see [Labels](/docs/user-guide/labels/).
They are a core concept used by two additional Kubernetes building blocks: Deployments and Services.


## Deployments

Now that you know how to make awesome, multi-container, labeled Pods and you want to use them to build an application, you might be tempted to just start building a whole bunch of individual Pods, but if you do that, a whole host of operational concerns pop up.  For example: how will you scale the number of Pods up or down?  How will you roll out a new release?

The answer to those questions and more is to use a [_Deployment_](/docs/user-guide/deployments/#what-is-a-deployment) to manage maintaining and updating your running _Pods_.

A Deployment object defines a Pod creation template (a "cookie-cutter" if you will) and desired replica count.  The Deployment uses a label selector to identify the Pods it manages, and will create or delete Pods as needed to meet the replica count.  Deployments are also used to manage safely rolling out changes to your running Pods.

Here is a Deployment that instantiates two nginx Pods:

{% include code.html language="yaml" file="deployment.yaml" ghlink="/docs/user-guide/walkthrough/deployment.yaml" %}


#### Deployment Management

Create an nginx Deployment:

Download the `deployment.yaml` above by clicking on the file name and copy to your local directory.

```shell
kubectl create -f ./deployment.yaml
```

List all Deployments:

```shell
kubectl get deployment
```

List the Pods created by the Deployment:

```shell
kubectl get pods -l app=nginx
```

Upgrade the nginx container from 1.7.9 to 1.8 by changing the Deployment and calling `apply`.  The following config
contains the desired changes:

{% include code.html language="yaml" file="deployment-update.yaml" ghlink="/docs/user-guide/walkthrough/deployment-update.yaml" %}

Download ./deployment-update.yaml and copy to your local directory.

```shell
kubectl apply -f ./deployment-update.yaml
```

Watch the Deployment create Pods with new names and delete the old Pods:

```shell
kubectl get pods -l app=nginx
```

Delete the Deployment by name:

```shell
kubectl delete deployment nginx-deployment
```

For more information, such as how to rollback Deployment changes to a previous version, see [_Deployments_](/docs/user-guide/deployments/).


## Services

Once you have a replicated set of Pods, you need an abstraction that enables connectivity between the layers of your application.  For example, if you have a Deployment managing your backend jobs, you don't want to have to reconfigure your front-ends whenever you re-scale your backends.  Likewise, if the Pods in your backends are scheduled (or rescheduled) onto different machines, you can't be required to re-configure your front-ends.  In Kubernetes, the service abstraction achieves these goals.  A service provides a way to refer to a set of Pods (selected by labels) with a single static IP address. It may also provide load balancing, if supported by the provider.

For example, here is a service that balances across the Pods created in the previous nginx Deployment example ([service.yaml](/docs/user-guide/walkthrough/service.yaml)):

{% include code.html language="yaml" file="service.yaml" ghlink="/docs/user-guide/walkthrough/service.yaml" %}


#### Service Management

Create an nginx service ([service.yaml](/docs/user-guide/walkthrough/service.yaml)):

```shell
kubectl create -f docs/user-guide/walkthrough/service.yaml
```

List all services:

```shell
kubectl get services
```

On most providers, the service IPs are not externally accessible. The easiest way to test that the service is working is to create a busybox Pod and exec commands on it remotely. See the [command execution documentation](/docs/user-guide/kubectl-overview/) for details.

Provided the service IP is accessible, you should be able to access its http endpoint with wget on the exposed port:

```shell{% raw %}
$ export SERVICE_IP=$(kubectl get service nginx-service -o go-template='{{.spec.clusterIP}}')
$ export SERVICE_PORT=$(kubectl get service nginx-service -o go-template='{{(index .spec.ports 0).port}}')
$ echo "$SERVICE_IP:$SERVICE_PORT"
$ kubectl run busybox  --generator=run-pod/v1 --image=busybox --restart=Never --tty -i --env "SERVICE_IP=$SERVICE_IP,SERVICE_PORT=$SERVICE_PORT"
u@busybox$ wget -qO- http://$SERVICE_IP:$SERVICE_PORT # Run in the busybox container
u@busybox$ exit # Exit the busybox container
$ kubectl delete pod busybox # Clean up the pod we created with "kubectl run"
{% endraw %}```

To delete the service by name:

```shell
kubectl delete service nginx-service
```

When created, each service is assigned a unique IP address.  This address is tied to the lifespan of the Service, and will not change while the Service is alive.  Pods can be configured to talk to the service, and know that communication to the service will be automatically load-balanced out to some Pod that is a member of the set identified by the label selector in the Service.

For more information, see [Services](/docs/user-guide/services/).


## Health Checking

When I write code it never crashes, right?  Sadly the [Kubernetes issues list](https://github.com/kubernetes/kubernetes/issues) indicates otherwise...

Rather than trying to write bug-free code, a better approach is to use a management system to perform periodic health checking
and repair of your application.  That way a system outside of your application itself is responsible for monitoring the
application and taking action to fix it.  It's important that the system be outside of the application, since if
your application fails and the health checking agent is part of your application, it may fail as well and you'll never know.
In Kubernetes, the health check monitor is the Kubelet agent.

#### Process Health Checking

The simplest form of health-checking is just process level health checking.  The Kubelet constantly asks the Docker daemon
if the container process is still running, and if not, the container process is restarted.  In all of the Kubernetes examples
you have run so far, this health checking was actually already enabled.  It's on for every single container that runs in
Kubernetes.

#### Application Health Checking

However, in many cases this low-level health checking is insufficient.  Consider, for example, the following code:

```go
lockOne := sync.Mutex{}
lockTwo := sync.Mutex{}

go func() {
  lockOne.Lock();
  lockTwo.Lock();
  ...
}()

lockTwo.Lock();
lockOne.Lock();
```

This is a classic example of a problem in computer science known as ["Deadlock"](https://en.wikipedia.org/wiki/Deadlock). From Docker's perspective your application is
still operating and the process is still running, but from your application's perspective your code is locked up and will never respond correctly.

To address this problem, Kubernetes supports user implemented application health-checks.  These checks are performed by the
Kubelet to ensure that your application is operating correctly for a definition of "correctly" that _you_ provide.

Currently, there are three types of application health checks that you can choose from:

   * HTTP Health Checks - The Kubelet will call a web hook.  If it returns between 200 and 399, it is considered success, failure otherwise. See health check examples [here](/docs/user-guide/liveness/).
   * Container Exec - The Kubelet will execute a command inside your container.  If it exits with status 0 it will be considered a success. See health check examples [here](/docs/user-guide/liveness/).
   * TCP Socket - The Kubelet will attempt to open a socket to your container.  If it can establish a connection, the container is considered healthy, if it can't it is considered a failure.

In all cases, if the Kubelet discovers a failure the container is restarted.

The container health checks are configured in the `livenessProbe` section of your container config. There you can also specify an `initialDelaySeconds` that is a grace period from when the container is started to when health checks are performed, to enable your container to perform any necessary initialization.

Here is an example config for a Pod with an HTTP health check ([pod-with-http-healthcheck.yaml](/docs/user-guide/walkthrough/pod-with-http-healthcheck.yaml)):

{% include code.html language="yaml" file="pod-with-http-healthcheck.yaml" ghlink="/docs/user-guide/walkthrough/pod-with-http-healthcheck.yaml" %}


For more information about health checking, see [Container Probes](/docs/user-guide/pod-states/#container-probes).


## What's Next?

For a complete application see the [guestbook example](https://github.com/kubernetes/kubernetes/tree/{{page.githubbranch}}/examples/guestbook/).