Allow setting RoutingType on a per-route level

[jukie]

Description:
Currently, RoutingType is only configurable at the EnvoyProxy level which means that if using merged gateways there's no way to enable service IP routing for a single backend. I propose adding a similar field to BackendTrafficPolicy so that users can define endpoint or service routing at a more granular level.

My use case is in relation to an ingress-nginx migration and the nginx.ingress.kubernetes.io/service-upstream annotation where I have a few users who would like to enable routing to the service instead of backend pods but all other users want to use the default endpoint-based routing. The only workaround I'm aware of is to setup an isolated GatewayClass for these users.

[optional Relevant Links:]

• https://kubernetes.github.io/ingress-nginx/user-guide/nginx-configuration/annotations/#service-upstream
• https://gateway.envoyproxy.io/docs/api/extension_types/#routingtype

[nareddyt]

Curious - will all Envoy Gateway features work when RoutingType = Service?

nginx specifically calls out:

Sticky Sessions will not work as only round-robin load balancing is supported.

Does Envoy Gateway have any features that require RoutingType=Endpoint?

Trying to understand what validations we need to perform, and whether 1 application enabling RoutingType=Service for a single backend would have side-effects on supported features for other backends.

[jukie]

That's a good question and I believe the same restriction applies in this case. We can add documentation or explicitly call out what features won't work alongside but RoutingType is already in EnvoyProxy API so I don't think adding it to BTP would introduce any side-effects that don't already exist.