Both Deployments and ReplicaSets are designed to manage stateless applications in Kubernetes. This means that they are not responsible for managing any application state or data storage. Instead, they manage the deployment and scaling of stateless pods that run the application code.
In a stateless application, each pod is independent and interchangeable, and can be scaled up or down as needed. The application state is typically stored in a separate data store, such as a database, that is managed independently of the pods.
How Stateless?
Deployments and ReplicaSets achieve statelessness by using pod templates to create identical copies of the application code. When a new replica is needed, the controller creates a new pod from the template, and when a pod needs to be removed, the controller deletes the pod.
MongoDB Scenario
In the case of MongoDB in Kubernetes, you can use a StatefulSet controller to manage the stateful MongoDB instances. A StatefulSet controller manages the creation, scaling, and deletion of a set of stateful pods, and ensures that each pod has a stable hostname and persistent storage.
To deploy a stateless MongoDB instance in Kubernetes using a Deployment, you would typically use a MongoDB replica set. A replica set is a group of MongoDB instances that maintain the same data set, and provide redundancy and high availability.
To deploy a MongoDB replica set using a Deployment in Kubernetes, you would create a Deployment YAML file that includes a pod template with the MongoDB image, and a service that exposes the pods to the network. The Deployment controller would then manage the creation and scaling of the MongoDB pods based on the specified replica set configuration.
While the MongoDB pods themselves may be stateless, the underlying database is still stateful and requires persistent storage. To ensure that the data stored in the MongoDB instance is not lost when a pod is deleted or scaled down, you would need to configure persistent storage using a persistent volume and persistent volume claim.