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Configuration Best Practices

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.

• General Configuration Tips
• “Naked” Pods vs ReplicaSets, Deployments, and Jobs
• Services
• Using Labels
• Container Images
• Using kubectl

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 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 or 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), is almost always preferable to creating Pods directly, except for some explicit restartPolicy: Never scenarios. A Job may also be appropriate.

Services

• Create a 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:

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

If you are writing code that talks to a Service, don’t use these environment variables; use the DNS name of the Service instead. Service environment variables are provided only for older software which can’t be modified to use DNS lookups, and are a much less flexible way of accessing Services.

• 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 or kubectl port-forward.

If you explicitly need to expose a Pod’s port on the node, consider using a NodePort Service before resorting to hostPort.

• Avoid using hostNetwork, for the same reasons as hostPort.

• Use 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 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 app for examples of this approach.

A Service can be made to span multiple Deployments by omitting release-specific labels from its selector. Deployments 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.

Container Images

The imagePullPolicy and the tag of the image affect when the 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, 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: 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: 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.

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 and 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 for an example.

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