Deployments:-

Deployments are usually used for stateless applications. However, you can save the state of deployment by attaching a Persistent Volume to it and make it stateful, but all the pods of a deployment will be sharing the same Volume and data across all of them will be same.

If you create a deployment with 1 replica, it will check that the desired state of ReplicaSet is 1 and current state is 0, so it will create a ReplicaSet, which will further create the pod. If you create a deployment with name counter, it will create a ReplicaSet with name counter-<replica-set-id>, which will further create a Pod with name counter-<replica-set->-<pod-id>.

Deployments, as discussed, creates a ReplicaSet which then creates a Pod so whenever you update the deployment using RollingUpdate(default) strategy, a new ReplicaSet is created and the Deployment moves the Pods from the old ReplicaSet to the new one at a controlled rate. Rolling Update means that the previous ReplicaSet doesn’t scale to 0 unless the new ReplicaSet is up & running ensuring 100% uptime. If an error occurs while updating, the new ReplicaSet will never be in Ready state, so old ReplicaSet will not terminate again ensuring 100% uptime in case of a failed update

Statefulsets :-

It is a Kubernetes resource used to manage stateful applications. It manages the deployment and scaling of a set of Pods, and provides guarantee about the ordering and uniqueness of these Pods.

it doesn’t create ReplicaSet rather itself creates the Pod with a unique naming convention. e.g. If you create a StatefulSet with name counter, it will create a pod with name counter-0, and for multiple replicas of a statefulset, their names will increment like counter-0, counter-1, counter-2, etc

Every replica of a stateful set will have its own state, and each of the pods will be creating its own PVC(Persistent Volume Claim). So a statefulset with 3 replicas will create 3 pods, each having its own Volume, so total 3 PVCs.

StatefulSets don’t create ReplicaSet or anything of that sort, so you cant rollback a StatefulSet to a previous version. You can only delete or scale up/down the Statefulset. If you update a StatefulSet, it also performs RollingUpdate i.e. one replica pod will go down and the updated pod will come up, then the next replica pod will go down in same manner e.g. If I change the image of the above StatefulSet, the counter-2 will terminate and once it terminates completely, then counter-2 will be recreated and counter-1 will be terminated at the same time, similarly for next replica i.e. counter-0. If an error occurs while updating, so only counter-2 will be down, counter-1 & counter-0 will still be up, running on previous stable version. Unlike Deployments, you cannot roll back to any previous version of a StatefulSet.

StatefulSets are useful in case of Databases especially when we need Highly Available Databases in production as we create a cluster of Database replicas with one being the primary replica and others being the secondary replicas. The primary will be responsible for read/write operations and secondary for read only operations and they will be syncing data with the primary one.

DaemonSet

A DaemonSet is a controller that ensures that the pod runs on all the nodes of the cluster. If a node is added/removed from a cluster, DaemonSet automatically adds/deletes the pod.

Some typical use cases of a DaemonSet is to run cluster level applications like:

• Monitoring Exporters: You would want to monitor all the nodes of your cluster so you will need to run a monitor on all the nodes of the cluster like NodeExporter.

• Logs Collection Daemon: You would want to export logs from all nodes so you would need a DaemonSet of log collector like Fluentd to export logs from all your nodes.

However, Daemonset automatically doesn’t run on nodes which have a taint e.g. Master. You will have to specify the tolerations for it on the pod.

Reference : https://cloudyuga.guru/hands_on_lab/Deployments-vs-StatefulSets-vs-DaemonSets

https://medium.com/stakater/k8s-deployments-vs-statefulsets-vs-daemonsets-60582f0c62d4