Support for layering ingresses

[simskij]

Enhancement Proposal

We should make sure that traefik supports the ingress relation both as a provider and as a consumer, allowing us to layer ingresses.

Acceptance Criteria

• Traefik is able to successfully relate to another Traefik over the ingress relation
• The traffic of traefik (lol) is proxied through both layers of ingress
• The ingress URL of anything ingressed by the downstream traefik is decided by the upstream traefik

cc @jnsgruk

[jnsgruk]

A good test for this will be deploying cos-lite (complete with it's traefik), then deploying another traefik upstream of it and ensuring you can access each of the cos components through it.

Looking at how things work now, really this should mean that the config option for external hostname is ignored when an ingress relation is presented that will provide a URL (i think!)

[dstathis]

So is the idea that the url would be for example "https://example.com/cos-traefik/cos-grafana"? I am not sure I entirely understand the benefit of this.

[jnsgruk]

The intended use case is a situation where someone deploys your "solution" which ships with a traefik, but needs to expose it within some infrastructure a different way (for example, in our datacentre with another charm such as a cloudflare/similar charm which presents the ingress interface).

In this particular case, the person deploying the solution won't have the rights to the actual ingress which controls the in-flow of traffic into the datacentre, so they would likely be consuming a CMR.

By layering the ingress, we avoid people having to reconfigure all the ingress for COS lite which is already configured.