Sablier is not stateless / HA ready in Kubernetes

[dixneuf19]

Describe the bug

Hi, I joined a new team in the process of deploying your product to automatically stop and start some software stacks running in Kubernetes, depending on your great open-source software.

My coworker deployed Sablier as a StatefulSet, with only one replica, a PVC, and the file storage feature activated. This setup effectively maintains its state across restarts, making it somewhat functional.

However, it is quite a sub-optimal setup for Kubernetes. Since it is a StatefulSet with one replica, updating Sablier or just losing the pod means the whole Sablier service becomes unavailable for a while.

The StatefulSet with one replica is necessary because the state is saved in memory in the tinykv store. If we have several Sablier pods with a random load balancer (i.e., what you have with a Deployment and N replicas), the state would be inconsistent between pods, leading to premature stops for some apps.

I find all of this a bit brittle for a good Kubernetes deployment, ideally using as many stateless pods as possible with a remote distributed store. Another solution is to have a truly stateful Sablier app, with built-in clustering. And a last one would be going full into a Kubernetes Operator, using CRD and leases to acheive HA and leader election.

Anyway, here are my questions:

• Are you aware of these limitations regarding using Sablier in Kubernetes? If yes, should we amend the documentation accordingly, and also change the official Helm chart which tries to deploy a Deployment for an inherently stateful app?
• Do you have any plans to have better support for an HA deployment on Kubernetes, either by having an external kvstore such as Redis, or going more into a Kubernetes operator, with CRD and leases?

I think that your tool is a very interesting approach for on-demand environments and a good way to reduce load in cloud environments. I would be happy to help better support this kind of usage on Kubernetes.

Here is some context, but my point is quite agnostic of the version/reverse proxy used.

Context

Sablier version: 1.8.1
Provider: Kubernetes
Reverse proxy: Traefik v2.11
Running as a StatefulSet in Kubernetes

Anyway, thanks for your time on this FOSS software!

[acouvreur]

Hello @dixneuf19 !

Are you aware of these limitations regarding using Sablier in Kubernetes? If yes, should we amend the documentation accordingly, and also change the official Helm chart which tries to deploy a Deployment for an inherently stateful app?

I am aware of these limitations, we should definitely change the chart to be a StatefulSet.

Do you have any plans to have better support for an HA deployment on Kubernetes, either by having an external kvstore such as Redis, or going more into a Kubernetes operator, with CRD and leases?

I initially planned to go with Redis but didn't want people to be bothered by setting this up. And I think this project needs to be able to run with a redis backend (that replaces tinykv). I will look into that in the coming months.

For the Kubernetes operator, I planned on doing it but mainly for auto configuration based on known reverse proxies. What did you have in mind exactly ? Also how would you imagine configuring this through CRD ?

For leases, I'm not familiar with that, could you please detail what you have in mind ?

Thanks!

[dixneuf19]

Hi,

Regarding the redis backend I think it would be a good and easy solution, since there is already a Store interface in the code.
Furthermore, having a Redis along Sablier app in Kubernetes is not much an bother. For example Argocd does it, it includes Bitnami Redis as a subchart in its official Helm chart. We should just be careful around persistence.

On the operator it could either be

• a full fledged operator to select and configure the shutdown behavior namespace/app, and even configure known reverse proxies. However for this last point it might conflict with the existing way of configuring their plugin.
• just use CRD as a way to store some state in Kubernetes etcd. For example you could have a CRD for each group/session, and then save last request/timeout info in this resource. If some action could conflict if several Sablier app acts at the same time (for example shutdown a namespace), Kubernetes Leases are a simple way to elect a leader without going full raft or another complex cluster protocol.

Anyway the first solution should be the simplest for the moment !

[dixneuf19]

Yes Redis might be a "Maslow’s Hammer" for DevOps community, a familiar and abused tool. And the whole license change and ValKey alternatives should challenge this it a bit.

Honestly, as long as the solution is lightweight and easy to operate, I am fine with it.