// Copyright 2016 Michal Witkowski. All Rights Reserved.
// See LICENSE for licensing terms.

/*
`middleware` is a collection of gRPC middleware packages: interceptors, helpers and tools.

Middleware

gRPC is a fantastic RPC middleware, which sees a lot of adoption in the Golang world. However, the
upstream gRPC codebase is relatively bare bones.

This package, and most of its child packages provides commonly needed middleware for gRPC:
client-side interceptors for retires, server-side interceptors for input validation and auth,
functions for chaining said interceptors, metadata convenience methods and more.

Chaining

By default, gRPC doesn't allow one to have more than one interceptor either on the client nor on
the server side. `middleware` provides convenient chaining methods

Simple way of turning a multiple interceptors into a single interceptor. Here's an example for
server chaining:

	myServer := grpc.NewServer(
	    grpc.StreamInterceptor(middleware.ChainStreamServer(loggingStream, monitoringStream, authStream)),
	    grpc.UnaryInterceptor(middleware.ChainUnaryServer(loggingUnary, monitoringUnary, authUnary),
	)

These interceptors will be executed from left to right: logging, monitoring and auth.

Here's an example for client side chaining:

	clientConn, err = grpc.Dial(
	    address,
	        grpc.WithUnaryInterceptor(middleware.ChainUnaryClient(monitoringClientUnary, retryUnary)),
	        grpc.WithStreamInterceptor(middleware.ChainStreamClient(monitoringClientStream, retryStream)),
	)
	client = testpb.NewTestServiceClient(clientConn)
	resp, err := client.PingEmpty(s.ctx, &myservice.Request{Msg: "hello"})

These interceptors will be executed from left to right: monitoring and then retry logic.

The retry interceptor will call every interceptor that follows it whenever when a retry happens.

Writing Your Own

Implementing your own interceptor is pretty trivial: there are interfaces for that. But the interesting
bit exposing common data to handlers (and other middleware), similarly to HTTP Middleware design.
For example, you may want to pass the identity of the caller from the auth interceptor all the way
to the handling function.

For example, a client side interceptor example for auth looks like:

	func FakeAuthUnaryInterceptor(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
	   newCtx := context.WithValue(ctx, "user_id", "john@example.com")
	   return handler(newCtx, req)
	}

Unfortunately, it's not as easy for streaming RPCs. These have the `context.Context` embedded within
the `grpc.ServerStream` object. To pass values through context, a wrapper (`WrappedServerStream`) is
needed. For example:

	func FakeAuthStreamingInterceptor(srv interface{}, stream grpc.ServerStream, info *grpc.StreamServerInfo, handler grpc.StreamHandler) error {
	   newStream := middleware.WrapServerStream(stream)
	   newStream.WrappedContext = context.WithValue(ctx, "user_id", "john@example.com")
	   return handler(srv, newStream)
	}
*/
package middleware
