What is Kubernetes rollout?

What is Kubernetes Rollout?

Kubernetes rollout is a process of deploying new versions of an application or updating the existing one in a controlled and managed way. It is a crucial feature in Kubernetes that allows developers and operators to update their applications with minimal downtime and ensure a smooth transition between versions.

Kubernetes Deployment and Rollout

In Kubernetes, a Deployment is a higher-level abstraction that manages the lifecycle of a set of Pods, which are the smallest deployable units in Kubernetes. When you create a Deployment, Kubernetes will manage the creation, scaling, and updating of the Pods based on the desired state defined in the Deployment configuration.

The rollout process in Kubernetes is closely tied to the Deployment resource. When you update the Deployment, Kubernetes will automatically create a new ReplicaSet (a set of Pods with the same label selector) and gradually transition the Pods from the old ReplicaSet to the new one, ensuring that the application remains available during the update.

Rollout Strategies

Kubernetes provides several rollout strategies that you can use to control the update process:

Recreate: This strategy will first terminate all the existing Pods and then create new Pods with the updated image. This approach is simple, but it results in downtime during the update process.
Rolling Update: This is the default rollout strategy in Kubernetes. It gradually replaces the old Pods with new Pods, ensuring that the application remains available during the update. The rollout can be controlled by specifying the maximum number of Pods that can be unavailable or the maximum number of new Pods that can be created at the same time.

graph LR A[Old Pods] --> B[New Pods] B --> C[New Pods] C --> D[New Pods] D --> E[New Pods] E --> F[New Pods]

Blue-Green Deployment: This strategy involves maintaining two identical environments, often referred to as "blue" and "green". The new version of the application is deployed to the "green" environment, while the "blue" environment continues to serve the current version. Once the new version is verified, the traffic is switched to the "green" environment, and the "blue" environment is decommissioned.

graph LR A[Blue Environment] --> B[Green Environment] B --> C[Green Environment] C --> D[Green Environment] D --> E[Green Environment] E --> F[Green Environment]

Canary Deployment: In this strategy, the new version of the application is first deployed to a small subset of users or instances, often referred to as the "canary" group. This allows you to test the new version and gather feedback before rolling it out to the entire user base. Once the new version is validated, the remaining instances are updated.

graph LR A[Canary Pods] --> B[Existing Pods] B --> C[Existing Pods] C --> D[Existing Pods] D --> E[Existing Pods] E --> F[Existing Pods]

The choice of rollout strategy depends on the specific requirements of your application, such as the need for zero-downtime updates, the ability to quickly roll back to a previous version, and the risk tolerance of your organization.

Rollout Management

Kubernetes provides several commands and tools to manage the rollout process:

kubectl rollout status: This command allows you to check the status of a rollout and see if it has completed successfully.
kubectl rollout undo: This command allows you to roll back to a previous version of the application.
kubectl rollout history: This command allows you to view the history of previous rollouts and the changes made to the Deployment.

By using these tools, you can effectively manage the rollout process, monitor the progress, and quickly roll back to a previous version if necessary.

In summary, Kubernetes rollout is a powerful feature that enables you to update your applications in a controlled and managed way, ensuring high availability and a smooth transition between versions. By understanding the different rollout strategies and the tools provided by Kubernetes, you can effectively manage the deployment of your applications and ensure a seamless user experience.