doc.go

/*	Package rpc provides access to the exported methods of an object across a
	network or other I/O connection.  A server registers an object, making it visible
	as a service with the name of the type of the object.  After registration, exported
	methods of the object will be accessible remotely.  A server may register multiple
	objects (services) of different types but it is an error to register multiple
	objects of the same type.

	Only methods that satisfy these criteria will be made available for remote access;
	other methods will be ignored:

		- the method's type is exported.
		- the method is exported.
		- the method has three arguments, all exported (or builtin) types.
		- the method's first argument is a context.Context
		- the method's third argument is a pointer.
		- the method has return type error.

	In effect, the method must look schematically like

		func (t *T) MethodName(ctx context.Context, argType T1, replyType *T2) error

	where T1 and T2 can be marshaled by encoding/gob.
	These requirements apply even if a different codec is used.
	(In the future, these requirements may soften for custom codecs.)

	The method's second argument represents the arguments provided by the caller; the
	third argument represents the result parameters to be returned to the caller.
	The method's return value, if non-nil, is passed back as a string that the client
	sees as if created by errors.New.  If an error is returned, the reply parameter
	will not be sent back to the client.

	The server may handle requests on a single connection by calling ServeConn.  More
	typically it will create a network listener and call Accept.

	A client wishing to use the service establishes a connection and then invokes
	NewClient on the connection.  The convenience function Dial performs
	both steps for a raw network connection.  The resulting
	Client object has two methods, Call and Go, that specify the service and method to
	call, a context, a pointer containing the arguments, and a pointer to receive the result
	parameters.

	The Call method waits for the remote call to complete or the context.Context to complete while the Go method
	launches the call asynchronously and signals completion using the Call
	structure's Done channel.

	Unless an explicit codec is set up, package encoding/gob is used to
	transport the data.

	Here is a simple example.  A server wishes to export an object of type Arith:

		package server

		import "errors"

		type Args struct {
			A, B int
		}

		type Quotient struct {
			Quo, Rem int
		}

		type Arith int

		func (t *Arith) Multiply(ctx context.Context, args *Args, reply *int) error {
			*reply = args.A * args.B
			return nil
		}

		func (t *Arith) Divide(ctx context.Context, args *Args, quo *Quotient) error {
			if args.B == 0 {
				return errors.New("divide by zero")
			}
			quo.Quo = args.A / args.B
			quo.Rem = args.A % args.B
			return nil
		}

	The server calls (for HTTP service):

		arith := new(Arith)
		rpc.Register(arith)

		l, e := net.Listen("tcp", ":1234")
		if e != nil {
			log.Fatal("listen error:", e)
		}
		go rpc.Accept(context.Background(), l)

	At this point, clients can see a service "Arith" with methods "Arith.Multiply" and
	"Arith.Divide".  To invoke one, a client first dials the server:

		ctx, cancel := context.WithTimeout(context.Background(), time.Second*10)
		client, err := rpc.Dial(ctx, "tcp", serverAddress + ":1234")

		if err != nil {
			log.Fatal("dialing:", err)
		}

	Then it can make a remote call:

		// Synchronous call
		args := &server.Args{7,8}
		var reply int
		err = client.Call(ctx, "Arith.Multiply", args, &reply)
		if err != nil {
			log.Fatal("arith error:", err)
		}
		fmt.Printf("Arith: %d*%d=%d", args.A, args.B, reply)

	or

		// Asynchronous call
		quotient := new(Quotient)
		divCall := client.Go(ctx, "Arith.Divide", args, quotient, nil)
		replyCall := <-divCall.Done	// will be equal to divCall
		// check errors, print, etc.

	A server implementation will often provide a simple, type-safe wrapper for the
	client.

*/
package rpc