Golang: Is it possible to get header (metadata) in StreamClientInterceptor?

[温明浩]

Hi all,

Currently I have a rpc like this:

rpc SayHello (HelloRequest) returns (stream HelloReply) {}

Now I add a StreamClientInterceptor to catch response initial metadata,

but it block forever, and server shows that no request get, at least not into the rpc function.

The interceptor is similar to this example:

https://github.com/grpc/grpc-go/blob/master/examples/features/interceptor/client/main.go#L90

and I just add a line under L107 :

responseMeta, err := s.Header()  //<-- block here

Im sure that I can done in Python code,  which is implement similar to:

https://github.com/grpc/grpc/blob/master/examples/python/interceptors/headers/generic_client_interceptor.py#L33

and adds code under L37:

print(response_it.initial_metadata())

Can anyone tell me what is wrong of my Golang code?

Actually I can get header by wrapping grpc.ClientStream and received in RecvMsg()

and I observed that I can only get header after SendMsg, but it doesn't make sense :(

[Amit Saha]

Have you tried the metadata.FromIncomingContext() function?

You haven't shared your example code - i think that will be useful for others to help you.

[温明浩]

Hi amits, 

Sorry for the late replay.

I have write a minimal example to express my question.

https://gist.github.com/codingbaobao/f89291866024a97c1b476a682f82c029

The main purpose is trying to manage request&response header in interceptor, in order to share come context or status.

I have try metadata.FromIncomingContext() and seems empty.

Is there have any method to send request header first without sending payload?

Since this concept is documented in here: https://grpc.io/docs/what-is-grpc/core-concepts/#unary-rpc

Or, any method that can let me access response header without wrapping a ClientStream?

thanks!

amits...@gmail.com 在 2022年4月1日 星期五上午5:49:54 [UTC+8] 的信中寫道：

[温明浩]

Sorry for pushing up manually,

but I really want to know if there's method to do it like in Python.

Yes I can do it by wrapping a `ClientStream`, however it create a new instance in every call that I needs to use a pointer or somehow to manage together with my `ClientInterceptor`, which seems a bit dirty 

thanks.

温明浩 在 2022年4月6日 星期三上午11:03:45 [UTC+8] 的信中寫道：

[Easwar Swaminathan]

Apologies for the late reply.

> Is there have any method to send request header first without sending payload?

You can do so by calling grpc.SendHeader, or for streaming handlers by calling stream.SendHeader


The grpc.Header call option only works for unary RPCs and the header metadata will be available only after the RPC completes. For streaming RPCs, the client can call the stream.Header when they know that header metadata will be available, or after the first call to stream.RecvMsg has returned.

Hope this helps. If not, please feel free to post a question on our GitHub repo.

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