grpc/proxy/README.md

# gRPC Proxy

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[gRPC Go](https://github.com/grpc/grpc-go) Proxy server

## Project Goal

Build a transparent reverse proxy for gRPC targets that will make it easy to expose gRPC services
over the internet. This includes:
 * no needed knowledge of the semantics of requests exchanged in the call (independent rollouts)
 * easy, declarative definition of backends and their mappings to frontends
 * simple round-robin load balancing of inbound requests from a single connection to multiple backends

The project now exists as a **proof of concept**, with the key piece being the `proxy` package that
is a generic gRPC reverse proxy handler.

## Proxy Handler

The package [`proxy`](proxy/) contains a generic gRPC reverse proxy handler that allows a gRPC server to
not know about registered handlers or their data types. Please consult the docs, here's an exaple usage.

You can call `proxy.NewProxy` to create a `*grpc.Server` that proxies requests.
```go
proxy := proxy.NewProxy(clientConn)
``` 

More advanced users will want to define a `StreamDirector` that can make more complex decisions on what
to do with the request.
```go
director = func(ctx context.Context, fullMethodName string) (context.Context, *grpc.ClientConn, error) {
    md, _ := metadata.FromIncomingContext(ctx)
    outCtx = metadata.NewOutgoingContext(ctx, md.Copy())
    return outCtx, cc, nil
	
    // Make sure we never forward internal services.
    if strings.HasPrefix(fullMethodName, "/com.example.internal.") {
        return outCtx, nil, status.Errorf(codes.Unimplemented, "Unknown method")
    }
    
    if ok {
        // Decide on which backend to dial
        if val, exists := md[":authority"]; exists && val[0] == "staging.api.example.com" {
            // Make sure we use DialContext so the dialing can be cancelled/time out together with the context.
            return outCtx, grpc.DialContext(ctx, "api-service.staging.svc.local", grpc.WithCodec(proxy.Codec())), nil
        } else if val, exists := md[":authority"]; exists && val[0] == "api.example.com" {
            return outCtx, grpc.DialContext(ctx, "api-service.prod.svc.local", grpc.WithCodec(proxy.Codec())), nil
        }
    }
    return outCtx, nil, status.Errorf(codes.Unimplemented, "Unknown method")
}
```

Then you need to register it with a `grpc.Server`. The server may have other handlers that will be served
locally.

```go
server := grpc.NewServer(
    grpc.CustomCodec(proxy.Codec()),
    grpc.UnknownServiceHandler(proxy.TransparentHandler(director)))
pb_test.RegisterTestServiceServer(server, &testImpl{})
```

## Testing
To make debugging a bit simpler, there are some helpers.

`testservice` contains a method `TestTestServiceServerImpl` which performs a complete test against
the reference implementation of the `TestServiceServer`.

In `proxy_test.go`, the test framework spins up a `TestServiceServer` that it tests the proxy 
against. To make debugging a bit simpler (eg. if the developer needs to step into 
`google.golang.org/grpc` methods), this `TestServiceServer` can be provided by a server by 
passing `-test-backend=addr` to `go test`. A simple, local-only implementation of 
`TestServiceServer` exists in [`testservice/server`](./testservice/server).


## License

`grpc-proxy` is released under the Apache 2.0 license. See [LICENSE.txt](LICENSE.txt).