advanced-go-programming-book/vendor/github.com/grpc-ecosystem/grpc-gateway/docs/_docs/customizingyourgateway.md

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Customizing your gateway	documentation	101

Customizing your gateway

Message serialization

You might want to serialize request/response messages in MessagePack instead of JSON, for example.

1. Write a custom implementation of Marshaler
2. Register your marshaler with WithMarshalerOption e.g.

   var m your.MsgPackMarshaler
   mux := runtime.NewServeMux(runtime.WithMarshalerOption("application/x-msgpack", m))
   

You can see the default implementation for JSON for reference.

Mapping from HTTP request headers to gRPC client metadata

You might not like the default mapping rule and might want to pass through all the HTTP headers, for example.

1. Write a HeaderMatcherFunc.

2. Register the function with WithIncomingHeaderMatcher

   e.g.

   func yourMatcher(headerName string) (mdName string, ok bool) {
   	...
   }
   ...
   mux := runtime.NewServeMux(runtime.WithIncomingHeaderMatcher(yourMatcher))
   
   

Mapping from gRPC server metadata to HTTP response headers

ditto. Use WithOutgoingHeaderMatcher

Mutate response messages or set response headers

You might want to return a subset of response fields as HTTP response headers; You might want to simply set an application-specific token in a header. Or you might want to mutate the response messages to be returned.

1. Write a filter function.

   func myFilter(ctx context.Context, w http.ResponseWriter, resp proto.Message) error {
   	w.Header().Set("X-My-Tracking-Token", resp.Token)
   	resp.Token = ""
   	return nil
   }
   

2. Register the filter with WithForwardResponseOption

   e.g.

   mux := runtime.NewServeMux(runtime.WithForwardResponseOption(myFilter))
   

Error handler

http://mycodesmells.com/post/grpc-gateway-error-handler

Replace a response forwarder per method

You might want to keep the behavior of the current marshaler but change only a message forwarding of a certain API method.

1. write a custom forwarder which is compatible to ForwardResponseMessage or ForwardResponseStream.

2. replace the default forwarder of the method with your one.

   e.g. add forwarder_overwrite.go into the go package of the generated code,

   package generated
   
   import (
   	"net/http"
   
   	"github.com/grpc-ecosystem/grpc-gateway/runtime"
   	"github.com/golang/protobuf/proto"
   	"golang.org/x/net/context"
   )
   
   func forwardCheckoutResp(ctx context.Context, mux *runtime.ServeMux, marshaler runtime.Marshaler, w http.ResponseWriter, req *http.Request, resp proto.Message, opts ...func(context.Context, http.ResponseWriter, proto.Message) error) {
   	if someCondition(resp) {
   		http.Error(w, "not enough credit", http. StatusPaymentRequired)
   		return
   	}
   	runtime.ForwardResponseMessage(ctx, mux, marshaler, w, req, resp, opts...)
   }
   
   func init() {
   	forward_MyService_Checkout_0 = forwardCheckoutResp
   }