Have the ability to have different TypeUrl from Discovery Request vs Discovery Response

[lita]

Currently, go-control-plane doesn't provide a way for us to send v3 configuration through the v2 protocol, as supported by envoy for a migration.

For Fetch, it always sets the DiscoveryRequest TypeUrl.

For Streaming, it always uses the v2 resources when using the v2/cache.go module.

In order to support the v2->v3 migration, we will need to add functionality here to allow the TypeUrl to be changed. It's not clear to me if the passthrough work will handle this or not, since it is still using the request TypeUrl.

[jyotimahapatra]

I don't think envoy accepts v3 response in return for a v2 request or vice versa, based on the observation here.

[lita]

After talking with @jessicayuen, it looks like the Passthrough struct takes the raw discovery proto.

[lita]

@jyotima I believe Envoy added code to do the v2 -> v3 conversion since the last time you did that test.

[jyotimahapatra]

@lita That's possible, but i saw that the issue is still open and not linked to any PR. Worth retesting it again to check if this is solved.

[jyotimahapatra]

In order to perform migration, go-control-plane enables registering the v3 endpoint. Although the cache management will require more code until envoyproxy/envoy#10776 is done.

[lita]

I think fully mixed is not supported. But sending v3 resources over v2 transport is supported.

envoyproxy/envoy#10776 (comment)

https://www.envoyproxy.io/docs/envoy/latest/faq/api/envoy_v3

https://github.com/envoyproxy/envoy/blob/master/source/common/protobuf/utility.cc#L544-L547

[jyotimahapatra]

That is true. The grpc endpoints are chosen based on type_url , based on resource_api_version .
The transport_api_version is only really used here to scrub deprecated fields.

I feel there is not much correlation between typeurl and transport version.
A v2 resource version request requires a v2 resource response and v3 resource needs a v3 resource version in response. In other words, control plane needs to register both v2 and v3 grpc endpoints for different resource versions.

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