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I am having a stateful application - I am keeping data in user's sessions (basically data in HttpSession object) - but I do not have any requirement to write anything to persistent disk.

From what I have read so far - StatefulSet workloads are meant for stateful applications, but my understanding so far is that even though my application is a stateful application but Deployment workloads can also suffice my requirement because I do not want to write anything to persistent disks.

However, one point I am not sure about is that suppose I use Deployment workload and a lot of user data is present in my HttpSession object, now due to some reason Kubernetes restarts my Pod then of course all that user session data will be lost. So, my question are following:

  • Does StatefulSet handles this situation any better than Deployment workload?
  • So, only difference between Deployment workload and StatefulSet workload is about absence/presence of persistent disk or there is something to do with application session management as well in case of StatefulSet?
pjj
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1 Answers1

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Does StatefulSet handles this situation any better than Deployment workload?

No. Neither Deployment nor StatefulSet will preserve memory contents. To preserve session information, you'll need to store it somewhere. One common approach is to use Redis.

So, only difference between Deployment workload and StatefulSet workload is about absence/presence of persistent disk or there is something to do with application session management as well in case of StatefulSet?

No, there are other differences:

  • StatefulSets create (and re-create) deterministic, consistent pod names (identifiers).
  • StatefulSets, are deployed, scaled, and updated one by one in a deterministic, consistent order. The next pod will be created only after the previous one reached the Running state.

Additionally, it's worth mentioning that persistent disks can be attached to pods that aren't part of a StatefulSet. It's just that it's convenient to have disks always be attached to a pod with a consistent id. For instance if you have pods running a replicated database, you can use StatefulSets to ensure that the master replica's disk is always attached to pod #1.

Edit:

Link to official documentation about StatefulSets

From the documentation:

Like a Deployment, a StatefulSet manages Pods that are based on an identical container spec. Unlike a Deployment, a StatefulSet maintains a sticky identity for each of their Pods. These pods are created from the same spec, but are not interchangeable: each has a persistent identifier that it maintains across any rescheduling.

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StatefulSets are valuable for applications that require one or more of the following.

  • Stable, unique network identifiers.
  • Stable, persistent storage.
  • Ordered, graceful deployment and scaling.
  • Ordered, automated rolling updates.

In the above, stable is synonymous with persistence across Pod (re)scheduling. If an application doesn't require any stable identifiers or ordered deployment, deletion, or scaling, you should deploy your application using a workload object that provides a set of stateless replicas. Deployment or ReplicaSet may be better suited to your stateless needs.

Malt
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  • Ok thanks. So, to sum up (1.) StatefulSets allows you to attach a persistent disk (2.) StatefulSets are deterministic (3.) StatefulSets have consistent Pod name. Right? And one question please - can I attach persistent disk in a Deployment workload as well? And one request please - if you would elaborate more on StatefulSets (I know I can Google and read more, but it would be helpful for future visitors as well). – pjj Mar 14 '21 at 18:22
  • @pjj There's nothing special about StatfulSets with regard to persistent disks. A persistent volume can be attached to any pod, including one that was created using a regular `Deployment`. I mentioned that in the answer. I'll add some relevant documentation to my answer as well as a link to the official documentation. Hope it'll be useful. – Malt Mar 14 '21 at 19:28