Can you use a HTTP2 request to successfully call a gRPC service?

Since grpc is built on top of http2, can you use an http request to directly call a grpc service?

For example, if you start a grpc service with the following proto:

option java_package = "io.grpc.examples.helloworld";
option java_outer_classname = "HelloWorldProto";
option objc_class_prefix = "HLW";

package helloworld;

// The greeting service definition.
service Greeter {
  // Sends a greeting
  rpc SayHello (HelloRequest) returns (HelloReply) {}
}

// The request message containing the user's name.
message HelloRequest {
  string name = 1;
}

// The response message containing the greetings
message HelloReply {
  string message = 1;
}

Then send a curl request to that service at localhost and the port number:

curl --http2 -H "Content-Type: application/grpc+proto" -d "{\"name\": \"test\"}" http://localhost:50051/io.grpc.examples.helloworld.Greeter/SayHello

Is this suppported? I get the following error:

io.grpc.netty.shaded.io.netty.handler.codec.http2.Http2Exception: Unexpected HTTP/1.x request: POST /io.grpc.examples.helloworld.Greeter/SayHello

I'm not sure if I am making the call incorrectly, or if this isn't possible. Besides using curl, I've also tried using Java's HttpClient, which gave me the same error. Any info on this is appreciated, thanks

You can, but you need to follow gRPC's message framing format which is binary. Also, most grpc servers don't support JSON-encoded messages. So here's a protobuf version:

$ echo -en '\x00\x00\x00\x00\x06\x0a\x04test' | curl --http2-prior-knowledge --header "Content-Type: application/grpc" --header "TE: trailers" --data-binary @- http://localhost:50051/helloworld.Greeter/SayHello | xxd
00000000: 0000 0000 0c0a 0a48 656c 6c6f 2074 6573  .......Hello tes
00000010: 74                                       t

The request's break-down:

• \x00\x00\x00\x00\x06. 5 byte gRPC frame header. The first 00 is a flag byte with no flags set. The last four bytes are the message length of 6 bytes.
• \x0a\x04test. The protobuf message. 0a contains the field type and tag number. The 04 is the length of the following string "test".

You can see how the response was also binary grpc and protobuf in the same format as the request.

The error you were seeing is fixed by --http2-prior-knowledge. It tells curl not to use the plaintext HTTP/1.1 Upgrade to HTTP/2 and instead just start directly with HTTP/2. RFC 9113 deprecated the HTTP/1.1 Upgrade approach for HTTP/2 in favor of this prior knowledge approach.