Issue warning when configuration incompatible with sticky session option is provided

[jnoordsij]

Welcome!

• Yes, I've searched similar issues on GitHub and didn't find any.
• Yes, I've searched similar issues on the Traefik community forum and didn't find any.

What did you expect to see?

Traefik allows enabling sticky sessions for services, which set a cookie to identify the server handling the request. However, in some configurations, this makes little sense and might even be confusing to end-users.

The concrete confusing example that I found is using sticky sessions combined with the nativeLB option on a Kubernetes IngressRoute (or the new nativeLBByDefault option). In this case, the user might expect the cookie to ensure traffic is routed to the same pod, while in practice this does not happen as all traffic is routed through the "Kubernetes-level" service and no pod choice can be made. It would be nice to warn users that this combination of settings may not yield the results they expect.

I suspect there might be other cases where a warning could be relevant (e.g. cases where there's only a single backing service by configuration, making the sticky sessions flag obsolete), so the scope of this issue could be expanded to add additional validation.

[nmengin]

Hy @jnoordsij,

Thank you for reaching out.

The idea sounds great, such documentation would help a lot of people.

Do not hesitate to open a PR: Traefik is an Open Source project and any help is appreciated (and, concerning the documentation, it's more than appreciated 😅 ).