tl;dr This is a followup from a discussion that was initiated on gitter grpc/grpc channel. Currently the grpc/java library reuses the User-Agent from the channel for each RPC and discards User-Agent by treating it as a reserved header.
However, User-Agent is not a reserved header and this creates complications when trying to write a proxy-like gRPC service for HTTP endpoints that care about User-Agent for response customization. Posting it here to get more ideas about how to resolve this. quoting the conversation below: Asad @asadali Jun 04 13:10 What was the underlying reason for the restriction on not allowing User-Agent to be overridden on a per-call basis? can't seem to find a spec which reserves the User-Agent string for gRPC/HTTP2 and yet there is code in place in the libraries (grpc-java/ grpc-go ..) to discard any user-supplied metadata regarding User-Agent and always use the channel's value eg: Utils.convertServerHeaders Asad @asadali Jun 04 13:16 use-case: client ---> httpSVC-A ---> grpcSVC-B ---> httpSVC-C how can the client's user-agent be conveyed to httpSVC-C? if A-B have ONLY one channel open between them with a channel-level User-Agent that can't be overridden @ejona86 ^ question regarding user-agent behavior Eric Anderson @ejona86 Jun 04 14:34 @asadali, user-agent is a built-in feature as gRPC itself sends it. There is an API to change what gRPC sends, but there didn't seem to be any need to allow it to be changed per-RPC. Asad @asadali Jun 04 14:43 @ejona86 we seem to have a use-case in which a per-RPC user-agent will make things easier for us. The alternate is to use custom metadata fields to preserve this information. that approach seems non-standard and we were hoping to avoid it. Will it be possible to include a per-RPC user-agent in gRPC? i will be happy to code it up. but based on what i read in past issues, this request was repeatedly turned down. Eric Anderson @ejona86 Jun 04 14:44 That is a cross-language decision. You would need to make clear what the use-case for it is. Right now, it isn't clear what the use-case is. Oh. I see now. You want to communicate the origin client's user-agent to SVC-C Yeah. That's not appropriate for user-agent. Asad @asadali Jun 04 14:45 ack Eric Anderson @ejona86 Jun 04 14:46 .... unless you are making something closer to a proxy. Maybe. It sort of seems like a can of worms. It just makes a mess of things. But I think I understand now. Asad @asadali Jun 04 14:48 so the intermediate gateways aren't pure proxies but maybe more like aggregators. in the non-GRPC world, the implementation made an assumption that User-Agent is the originating client's user-agent. and all intermediate hops honored that. I agree, that this is a very loose reading of the spec. I feel the more logical method is to update the user-agent on each hop however, systems built around that assumption aren't happy when they lose this info :( IMO, gRPC clients can default to per-channel behavior but the choice should ultimately be left to the user if they want to override it Eric Anderson @ejona86 Jun 04 14:51 Well, today the application can't set the entire user-agent. gRPC will always include itself in the user-agent. I'm trying to check what HTTP says to do for user-agent and proxies. Asad @asadali Jun 04 14:52 yeah i can use another opinion on this. and current gRPC behavior is what I am trying to rationalize. does it need to always include its user-agent? Eric Anderson @ejona86 Jun 04 17:31 @asadali, proxies do forward the user-agent. We do want to enable grpc proxies, so that does mean we should forward the user-agent. Although on the server, any compatibility quirks would generally be with the proxy, not the end-client. So it still seems muddled, but it does seem we should consider it. > -- You received this message because you are subscribed to the Google Groups "grpc.io" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/grpc-io/96b765b5-c7fe-4ead-a9a9-fd1d0f9a84e2%40googlegroups.com.
