website/content/en/docs/concepts/configuration/overview.md

---
reviewers:
- mikedanese
title: Configuration Best Practices
content_template: templates/concept
weight: 10
---

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This document highlights and consolidates configuration best practices that are introduced throughout the user guide, Getting Started documentation, and examples.

This is a living document. If you think of something that is not on this list but might be useful to others, please don't hesitate to file an issue or submit a PR.
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## General Configuration Tips

- When defining configurations, specify the latest stable API version.

- Configuration files should be stored in version control before being pushed to the cluster. This allows you to quickly roll back a configuration change if necessary. It also aids cluster re-creation and restoration.

- Write your configuration files using YAML rather than JSON. Though these formats can be used interchangeably in almost all scenarios, YAML tends to be more user-friendly.

- Group related objects into a single file whenever it makes sense. One file is often easier to manage than several. See the [guestbook-all-in-one.yaml](https://github.com/kubernetes/examples/tree/{{< param "githubbranch" >}}/guestbook/all-in-one/guestbook-all-in-one.yaml) file as an example of this syntax.

- Note also that many `kubectl` commands can be called on a directory. For example, you can call `kubectl apply` on a directory of config files.

- Don't specify default values unnecessarily: simple, minimal configuration will make errors less likely.

- Put object descriptions in annotations, to allow better introspection.


## "Naked" Pods vs ReplicaSets, Deployments, and Jobs

- Don't use naked Pods (that is, Pods not bound to a [ReplicaSet](/docs/concepts/workloads/controllers/replicaset/) or [Deployment](/docs/concepts/workloads/controllers/deployment/)) if you can avoid it. Naked Pods will not be rescheduled in the event of a node failure.

  A Deployment, which both creates a ReplicaSet to ensure that the desired number of Pods is always available, and specifies a strategy to replace Pods (such as [RollingUpdate](/docs/concepts/workloads/controllers/deployment/#rolling-update-deployment)), is almost always preferable to creating Pods directly, except for some explicit [`restartPolicy: Never`](/docs/concepts/workloads/pods/pod-lifecycle/#restart-policy) scenarios. A [Job](/docs/concepts/workloads/controllers/jobs-run-to-completion/) may also be appropriate.


## Services

- Create a [Service](/docs/concepts/services-networking/service/) before its corresponding backend workloads (Deployments or ReplicaSets), and before any workloads that need to access it. When Kubernetes starts a container, it provides environment variables pointing to all the Services which were running when the container was started. For example, if a Service named `foo` exists, all containers will get the following variables in their initial environment:

  ```shell
  FOO_SERVICE_HOST=<the host the Service is running on>
  FOO_SERVICE_PORT=<the port the Service is running on>
  ```

  *This does imply an ordering requirement* - any `Service` that a `Pod` wants to access must be created before the `Pod` itself, or else the environment variables will not be populated.  DNS does not have this restriction.

- An optional (though strongly recommended) [cluster add-on](/docs/concepts/cluster-administration/addons/) is a DNS server.  The
DNS server watches the Kubernetes API for new `Services` and creates a set of DNS records for each.  If DNS has been enabled throughout the cluster then all `Pods` should be able to do name resolution of `Services` automatically.

- Don't specify a `hostPort` for a Pod unless it is absolutely necessary. When you bind a Pod to a `hostPort`, it limits the number of places the Pod can be scheduled, because each <`hostIP`, `hostPort`, `protocol`> combination must be unique. If you don't specify the `hostIP` and `protocol` explicitly, Kubernetes will use `0.0.0.0` as the default `hostIP` and `TCP` as the default `protocol`.

  If you only need access to the port for debugging purposes, you can use the [apiserver proxy](/docs/tasks/access-application-cluster/access-cluster/#manually-constructing-apiserver-proxy-urls) or [`kubectl port-forward`](/docs/tasks/access-application-cluster/port-forward-access-application-cluster/).

  If you explicitly need to expose a Pod's port on the node, consider using a [NodePort](/docs/concepts/services-networking/service/#nodeport) Service before resorting to `hostPort`.

- Avoid using `hostNetwork`, for the same reasons as `hostPort`.

- Use [headless Services](/docs/concepts/services-networking/service/#headless-
services) (which have a `ClusterIP` of `None`) for easy service discovery when you don't need `kube-proxy` load balancing.

## Using Labels

- Define and use [labels](/docs/concepts/overview/working-with-objects/labels/) that identify __semantic attributes__ of your application or Deployment, such as `{ app: myapp, tier: frontend, phase: test, deployment: v3 }`. You can use these labels to select the appropriate Pods for other resources; for example, a Service that selects all `tier: frontend` Pods, or all `phase: test` components of `app: myapp`. See the [guestbook](https://github.com/kubernetes/examples/tree/{{< param "githubbranch" >}}/guestbook/) app for examples of this approach.

A Service can be made to span multiple Deployments by omitting release-specific labels from its selector. [Deployments](/docs/concepts/workloads/controllers/deployment/) make it easy to update a running service without downtime.

A desired state of an object is described by a Deployment, and if changes to that spec are _applied_, the deployment controller changes the actual state to the desired state at a controlled rate.

- You can manipulate labels for debugging. Because Kubernetes controllers (such as ReplicaSet) and Services match to Pods using selector labels, removing the relevant labels from a Pod will stop it from being considered by a controller or from being served traffic by a Service. If you remove the labels of an existing Pod, its controller will create a new Pod to take its place. This is a useful way to debug a previously "live" Pod in a "quarantine" environment. To interactively remove or add labels, use [`kubectl label`](/docs/reference/generated/kubectl/kubectl-commands#label).

## Container Images

The [imagePullPolicy](/docs/concepts/containers/images/#updating-images) and the tag of the image affect when the [kubelet](/docs/admin/kubelet/) attempts to pull the specified image.

- `imagePullPolicy: IfNotPresent`: the image is pulled only if it is not already present locally.

- `imagePullPolicy: Always`: the image is pulled every time the pod is started.

- `imagePullPolicy` is omitted and either the image tag is `:latest` or it is omitted: `Always` is applied.

- `imagePullPolicy` is omitted and the image tag is present but not `:latest`: `IfNotPresent` is applied.

- `imagePullPolicy: Never`: the image is assumed to exist locally. No attempt is made to pull the image.

{{< note >}}
To make sure the container always uses the same version of the image, you can specify its [digest](https://docs.docker.com/engine/reference/commandline/pull/#pull-an-image-by-digest-immutable-identifier), for example `sha256:45b23dee08af5e43a7fea6c4cf9c25ccf269ee113168c19722f87876677c5cb2`. The digest uniquely identifies a specific version of the image, so it is never updated by Kubernetes unless you change the digest value.
{{< /note >}}

{{< note >}}
You should avoid using the `:latest` tag when deploying containers in production as it is harder to track which version of the image is running and more difficult to roll back properly.
{{< /note >}}

{{< note >}}
The caching semantics of the underlying image provider make even `imagePullPolicy: Always` efficient. With Docker, for example, if the image already exists, the pull attempt is fast because all image layers are cached and no image download is needed.
{{< /note >}}

## Using kubectl

- Use `kubectl apply -f <directory>`. This looks for Kubernetes configuration in all `.yaml`, `.yml`, and `.json` files in `<directory>` and passes it to `apply`.

- Use label selectors for `get` and `delete` operations instead of specific object names. See the sections on [label selectors](/docs/concepts/overview/working-with-objects/labels/#label-selectors) and [using labels effectively](/docs/concepts/cluster-administration/manage-deployment/#using-labels-effectively).

- Use `kubectl run` and `kubectl expose` to quickly create single-container Deployments and Services. See [Use a Service to Access an Application in a Cluster](/docs/tasks/access-application-cluster/service-access-application-cluster/) for an example.

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