Why StatefulSets? Can't a stateless Pod use persistent volumes?

Question

I am trying to understand Stateful Sets. How does their use differ from the use of "stateless" Pods with Persistent Volumes? That is, assuming that a "normal" Pod may lay claim to persistent storage, what obvious thing am I missing that requires this new construct (with ordered start/stop and so on)?

Answer 1

1: Why StatefulSets?

Stateless app: Usually, frontend components have completely different scaling requirements than the backends, so we tend to scale them individually. Not to mention the fact that backends such as databases are usually much harder to scale compared to (stateless) frontend web servers. Yes, the term “stateless” means that no past data nor state is stored or needs to be persistent when a new container is created

Stateful app: Stateful applications typically involve some database, such as Cassandra, MongoDB, or MySQL and processes a read and/or write to it.

2: Can't a stateless Pod use persistent volumes?

Basically, there are few ways by which you can do it. However, it has its own disadvantages.

1: USING ONE REPLICASET PER POD INSTANCE

• you could create multiple ReplicaSets—one for each pod with each ReplicaSet’s desired replica count set to one, and each ReplicaSet’s pod template referencing a dedicated PersistentVolumeClaim.

• Although this takes care of the automatic rescheduling in case of node failures or accidental pod deletions, it’s much more cumbersome compared to having a single ReplicaSet.

• For example, think about how you’d scale the pods in that case. You couldn’t change the desired replica count you’d have to create additional ReplicaSets instead. Using multiple ReplicaSets is therefore not the best solution.

2: USING MULTIPLE DIRECTORIES IN THE SAME VOLUME

• A trick you can use is to have all pods use the same PersistentVolume, but then have a separate file directory inside that volume for each pod Because you can’t configure pod replicas differently from a single pod template, you can’t tell each instance what directory it should use, but you can make each instance automatically select (and possibly also create) a data directory that isn’t being used by any other instance at that time.

• This solution does require coordination between the instances, and isn’t easy to do correctly. It also makes the shared storage volume the bottleneck.

That's why one should encourage to use statefulsets