What is the Difference Between "Always" and "On Failure" in Kubernetes Restart Policy?

Understanding the difference between the “Always” and “On Failure” restart policies in Kubernetes is important for managing applications well. The “Always” policy makes sure that containers restart no matter what their exit status is. This gives high availability for important applications. On the other hand, the “On Failure” policy only restarts containers that exit with a non-zero status. This is helpful for dealing with temporary errors without restarting too much. Knowing these Kubernetes restart policies can help us improve our deployment plans and reduce downtime.

In this article, we will look at the differences between the “Always” and “On Failure” restart policies in Kubernetes. We will talk about how each policy works. We will also discuss when to use them and best practices for using them in our Kubernetes setup. The topics we will cover are:

  • What is the Difference Between Always and On Failure in Kubernetes Restart Policy
  • Understanding Kubernetes Restart Policy Options
  • How Does Always Restart Policy Work in Kubernetes
  • How Does On Failure Restart Policy Work in Kubernetes
  • When to Use Always Restart Policy in Kubernetes
  • When to Use On Failure Restart Policy in Kubernetes
  • Frequently Asked Questions

By the end of this article, we will understand how to use these restart policies well to improve our Kubernetes applications.

Understanding Kubernetes Restart Policy Options

Kubernetes gives us different restart policies. These policies control what happens to Pods when containers fail. The main restart policies are:

  1. Always: We will restart the container no matter what its exit status is. We usually use this policy for services that need to run all the time.

  2. OnFailure: We restart the container only if it exits with a non-zero exit status. This means it failed. This policy is good for batch jobs or tasks that should not restart when they finish successfully.

  3. Never: We do not restart the container no matter what. This is best for tasks that run just once. We do not want Kubernetes to try and restart these.

Example Configuration

Here is how we can set a restart policy in a Pod manifest:

apiVersion: v1
kind: Pod
metadata:
  name: example-pod
spec:
  restartPolicy: Always  # Options: Always, OnFailure, Never
  containers:
  - name: example-container
    image: my-image:latest

Key Points

  • Always is the default restart policy for Deployments.
  • We use OnFailure for jobs that should retry only if they fail.
  • Never is good for tasks that should run one time without restarting.

Kubernetes also lets us set backoff limits and other settings. This helps us adjust how Pods restart. For more details, we can check the Kubernetes documentation.

How Does Always Restart Policy Work in Kubernetes

In Kubernetes, the “Always” restart policy makes sure that a container restarts again and again when it stops. It does not matter why the container stopped. This policy is great for services that need to run all the time.

Configuration

We can set the “Always” restart policy in the Pod specification YAML file like this:

apiVersion: v1
kind: Pod
metadata:
  name: my-application
spec:
  containers:
  - name: my-container
    image: my-image:latest
  restartPolicy: Always

Behavior

When we use the “Always” restart policy:

  • If the container stops, whether it fails or finishes, Kubernetes will try to restart it by itself.
  • It does not matter if the container stopped with a success status (0) or an error status (non-zero).
  • Kubernetes will keep trying to restart it until we delete the Pod or scale down the application.

Use Cases

  • This policy is good for services that need to stay running. For example, web servers or background workers.
  • We should not use it for batch jobs or one-time tasks. This policy will keep restarting the container even after the job is done.

For more understanding of Kubernetes deployment strategies, we can check how does Kubernetes differ from Docker Swarm.

How Does On Failure Restart Policy Work in Kubernetes

In Kubernetes, the OnFailure restart policy lets a container restart by itself if it fails. This policy is helpful for apps that might have short problems and can fix them without us having to do anything.

Configuration

We can set the OnFailure restart policy in the Pod or Deployment YAML file. Here is an example of how to set up a Pod with the OnFailure restart policy:

apiVersion: v1
kind: Pod
metadata:
  name: my-pod
spec:
  restartPolicy: OnFailure
  containers:
  - name: my-container
    image: my-image:latest
    command: ["./my-application"]

Behavior

  • The container will restart if it stops with a non-zero status code. This means it has failed.
  • If the container stops successfully with a zero status code, it will not restart.
  • The number of restart attempts is limited by Kubernetes settings. The default is unlimited unless we say otherwise.

Use Case

The OnFailure policy works best for batch jobs or microservices. These might need to restart after a failure but do not need to run all the time. It helps make sure that workloads can recover from short failures. This way, the apps running in Kubernetes become stronger.

For more details about Kubernetes deployment practices, we can check how to deploy a Kubernetes cluster on AWS EKS.

When to Use Always Restart Policy in Kubernetes

We use the “Always” restart policy in Kubernetes to keep a Pod running all the time. If the Pod fails, Kubernetes will restart it automatically. It does not matter why it stopped. This policy is very helpful in some situations:

  • Critical Applications: We should use the “Always” policy for important applications. These are things like web servers or databases. This way, they can recover from problems without us needing to do anything.

  • Stateless Services: This policy works well for stateless apps. These apps do not need to keep one instance running. If one fails, it will start a new one right away.

  • Development and Testing: When we are developing, using “Always” helps us find problems quickly. It reduces the downtime because the Pod restarts automatically after a failure.

  • DaemonSets: If we deploy daemon sets for system services, the “Always” policy makes sure these services are always available on all nodes.

Example Configuration

Here is an example of a Pod configuration that uses the “Always” restart policy:

apiVersion: v1
kind: Pod
metadata:
  name: my-app
spec:
  restartPolicy: Always
  containers:
  - name: my-app-container
    image: my-app-image:latest
    ports:
    - containerPort: 80

In this configuration:

  • The restartPolicy is set to Always. This means Kubernetes will always try to keep the Pod running.
  • If the Pod crashes or fails for any reason, Kubernetes will restart it automatically.

Using the “Always” restart policy can really improve how reliable and available our applications are in Kubernetes. For more details on Kubernetes policies and configurations, we can check out this Kubernetes tutorial.

When to Use On Failure Restart Policy in Kubernetes

The “On Failure” restart policy in Kubernetes helps us restart containers only when they stop with a non-zero status. This means there was an error. We use this policy when we want our application to fail nicely without restarting for small issues. Here are some times we should use the “On Failure” policy:

  • Batch Jobs: When we run batch jobs, we may want to restart the job only if it fails. This way, we do not waste resources on jobs that finish successfully.

  • Transient Errors: We can use this policy for apps that might have temporary errors. If the app can fix itself after a small problem, it should restart only when it fails, not when it stops without issues.

  • Resource Management: If we have limited resources, using “On Failure” helps us restart failed pods without always using resources to restart pods that are working fine.

Configuration Example

To set up a pod with the “On Failure” restart policy, we can use this YAML snippet:

apiVersion: v1
kind: Pod
metadata:
  name: example-pod
spec:
  restartPolicy: OnFailure
  containers:
  - name: example-container
    image: example-image

Additional Considerations

  • Exit Codes: We need to make sure our application gives back the right exit codes. A non-zero exit code will make Kubernetes restart the pod. A zero exit code will not.

  • Retry Limits: By default, Kubernetes keeps restarting the container forever on failure unless we say otherwise. It is good to set a limit on how many times it retries to stop endless restart loops.

  • Logging and Monitoring: We should check logs to find out why failures happen. This helps us improve the app and its environment to lower failure rates.

For more information on Kubernetes policies and configurations, you can check this article.

Frequently Asked Questions

What is the purpose of Kubernetes Restart Policies?

Kubernetes Restart Policies help us manage Pods. They make sure our applications stay available and strong. The main policies are “Always” and “On Failure.” These tell Kubernetes how to react when a container has a problem. Knowing the difference between these policies is important. It helps us keep our applications running well in Kubernetes.

When should I use the “Always” restart policy in Kubernetes?

We should use the “Always” restart policy for important apps that need to be available all the time. This policy restarts the container no matter what happens. It works well for services that need to run all the time like web servers or databases. If you want a full guide on how to manage your apps, check the article on Kubernetes Deployments.

How does the “On Failure” restart policy work in Kubernetes?

The “On Failure” restart policy will restart a container only if it stops with an error. This means it has a non-zero status. This policy is good for batch jobs or apps where we need to look into the failure before restarting. To know more about managing your Kubernetes apps, see how to troubleshoot issues in Kubernetes deployments.

What are the implications of using “Always” versus “On Failure” in Kubernetes?

When we pick between “Always” and “On Failure” restart policies, it can really affect our app’s stability and how we use resources. The “Always” policy can waste resources if the app keeps failing. On the other hand, “On Failure” lets us check the issue first before restarting. For more insights on managing Kubernetes, look at Kubernetes best practices.

Can I change the restart policy after a Pod is created in Kubernetes?

After we create a Pod in Kubernetes, we can’t change its restart policy directly. But we can change the deployment or replica set settings. Then we make a new version of the Pod with the restart policy we want. This way, our apps can change with our needs. For more on managing Kubernetes resources, see the article on how to manage the lifecycle of a Kubernetes Pod.