Remote Procedure Calls using Interfaces

Distributed Objects, RPC, and Capabilities

So far, we've used Cap'n Proto as a faster version of Protocol Buffers. This is already a huge improvement, but the real power of Cap'n Proto lies in its high-performance RPC protocol.

At its core, Cap'n Proto RPC is a distributed object protocol that allows you to call the methods of objects residing in remote hosts. To do this, you first obtain a reference to the remote object, called a capability. Calling a capability's methods performs RPC against the remote object.

Cap'n Proto capabilities are first-class objects. This means you can:

1. embed them in a Cap'n Proto struct,
2. store them in a List type,
3. pass them as parameters to RPC calls (i.e. pass them as arguments to method calls on other capabilities), and
4. return them from RPC calls.

This is a huge difference from typical protocols. JSON-RPC, Go's net/rpc, gRPC and Thrift only allow you to address global URLs or singleton objects that are exported at startup. In contrast, Cap'n Proto RPC allows you to create new objects at runtime, and share references to these objects over the network. In other words: object-oriented programming (OOP).

This pattern of Object Capabilities provides a powerful framework for writing secure, performant protocols. We'll explore object capabilities in greater detail in an advanced RPC tutorial.

For now, let's skip over the theory and proceed by example.

Declaring an RPC Capability in your Schema

Capnp RPC operates on interface types in your schema. We will begin by reproducing the Arith RPC server from the Go Standard Library RPC package documentation.

Open a new file called arith.capnp, and copy/paste the following schema:

using Go = import "/go.capnp";

@0xf454c62f08bc504b;

$Go.package("arith");
$Go.import("arith");

interface Arith {
	multiply @0 (a :Int64, b :Int64) -> (:product :Int64);
	divide   @1 (num :Int64, denom :Int64) -> (quo :Int64, rem :Int64);
}

Now, compile the schema as before:

capnp compile -I$GOPATH/src/capnproto.org/go/capnp/std -ogo arith.capnp

Server Implementation

You should take a moment to inspect the generated types in arith.capnp.go. For interface declarations in your schema, the capnp compiler generates several types:

1. A server interface: Arith_Server.
2. A client struct: Arith
3. Param, Result and Future types for each method of Arith

Notice that the only missing piece is the Arith_Server implementation. We'll define that below.

☝️ NOTE: the ability to provide multiple implementations of an RPC server is an extremely powerful pattern. It allows you to create such things as mock implementations for testing, and restricted/revokable interface providers for capability-based security.

Let's define our Arith server. In the same directory, create an arith.go file and paste in the following code:

package arith

import capnp "capnproto.org/go/capnp/v3"

type ArithServer struct{}

func (Arith) Multiply(ctx context.Context, call Arith_multiply) error {
	res, err := call.AllocResults()  // allocate the results struct
	if err != nil {
		return err
	}

	res.SetProduct(call.Args().A() * call.Args().B())
	return nil
}

func (Arith) Divide(ctx context.Context, call Arith_divide) error {
	if call.Args().B() == 0 {
		return errors.New("divide by zero")
	}

	res, err := call.AllocResults()
	if err != nil {
		return err
	}

	res.SetQuo(call.Args().A() / call.Args().B())
	res.SetRem(call.Args().A() % call.Args().B())
	return nil
}

Making RPC calls

We now have a working RPC server implementation for our schema interface. Let's begin by starting a server and listening for incoming RPC calls.

The following snippet instantiates an arith.Arith server, and exports it over a bidirectional stream.

// Create a new locally implemented arith server.
server := arith.Arith_ServerToClient(arith.ArithServer{})

// Listen for calls, using the server as the bootstrap interface.
// The 'rwc' parameter can be any io.ReadWriteCloser, usually net.Conn.
conn := rpc.NewConn(rpc.NewStreamTransport(rwc), &rpc.Options{
	// The BootstrapClient is the RPC interface that will be made available
	// to the remote endpoint by default.  In this case, Arith.
	BootstrapClient: capnp.Client(server),
})
defer conn.Close()

// Block until the connection terminates.
select {
case <-conn.Done():
	return nil
case <-ctx.Done():
	return conn.Close()
}

And here's the corresponding client setup and RPC call:

// As before, rwc can be any io.ReadWriteCloser, and will typically be
// a net.Conn.  The rpc.Options can be nil, if you don't want to override
// the defaults.
conn := rpc.NewConn(rpc.NewStreamTransport(rwc), nil)
defer conn.Close()

// Now we wait until we receive the bootstrap interface from the ArithServer.
// The context can be used to time-out or otherwise abort the bootstrap call.
// It is safe to cancel the context after `Bootstrap` returns.
a := Arith(conn.Bootstrap(ctx))

// Okay! Let's make an RPC call!
//
// There are a few things to notice here:
//  1. We pass a callback function to set parameters on the RPC call.  If the
//     call takes no arguments, you MAY pass nil.
//  2. We return a Future type, representing the in-flight RPC call.  We also
//     return a release function, which MUST be called when you're done with
//     the RPC call and its results.
f, release := a.Multiply(ctx, func(ps arith.Arith_multiply_Params) error {
	ps.SetA(2)
	ps.SetB(42)
	return nil
})
defer release()

// You can do other things while the RPC call is in-flight, but we're going to
// block until the call completes.
res, err := f.Struct()
if err != nil {
	return err
}

log.Println(res.Product())  // prints 84

And that's it! Let's reiterate the key points about calling RPC methods:

1. For the sake of simplicity, this example uses an in-memory pipe, but you can use TCP connections, Unix pipes, or any other type that implements io.ReadWriteCloser.
2. The return type for a client call is a promise, not an immediate value. It isn't until the Struct() method is called on a method that the client function blocks on the remote side.

A few additional words on the Future type are in order. If your RPC method returns another interface type, you can use the Future to immediately make calls against that as-of-yet-unreturned interface. This relies on a feature of the Cap'n Proto RPC protocol called promise pipelining, the advantage of which is that Cap'n Proto can often optimize away the additional network round-trips when such method calls are chained. This is one of Cap'n Proto's key advantages.

Next

Now that you've learned the basics of Cap'n Proto RPC, you are ready to learn more about object capabilities and advanced RPC.