> I could indeed use a custom error to convey this scenario, but I feel that the semantics of the ErrNoSubConnAvailable error match my current situation well

[alialobidm]

I could indeed use a custom error to convey this scenario, but I feel that the semantics of the ErrNoSubConnAvailable error match my current situation well

I don't think so. You really should be returning an error in this case. Also, things generally don't work well if you have multiple versions of a service deployed. All clients should use the old version of the service until you know all servers have been upgraded, and only then should you update your clients to use the new version. Otherwise you will run into trouble with things like this.

ErrNoSubConnAvailable is not intended for an end user to see. It is just part of the API between the LB policy and the ClientConn.

Originally posted by @dfawley in grpc/grpc-go#7143 (comment)