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The Cap’n Proto C++ RPC layer sits on top of the serialization layer and implements the RPC protocol.

Current Status

As of version 0.4, Cap’n Proto’s C++ RPC implementation is a Level 1 implementation. Persistent capabilities, three-way introductions, and distributed equality are not yet implemented.

Sample Code

The Calculator example implements a fully-functional Cap’n Proto client and server.

KJ Concurrency Framework

RPC naturally requires a notion of concurrency. Unfortunately, all concurrency models suck.

Cap’n Proto’s RPC is based on the KJ library’s event-driven concurrency framework. The core of the KJ asynchronous framework (events, promises, callbacks) is defined in kj/async.h, with I/O interfaces (streams, sockets, networks) defined in kj/async-io.h.

Event Loop Concurrency

KJ’s concurrency model is based on event loops. While multiple threads are allowed, each thread must have its own event loop. KJ discourages fine-grained interaction between threads as synchronization is expensive and error-prone. Instead, threads are encouraged to communicate through Cap’n Proto RPC.

KJ’s event loop model bears a lot of similarity to the Javascript concurrency model. Experienced Javascript hackers – especially node.js hackers – will feel right at home.

As of version 0.4, the only supported way to communicate between threads is over pipes or socketpairs. This will be improved in future versions. For now, just set up an RPC connection over that socketpair. :)


Function calls that do I/O must do so asynchronously, and must return a “promise” for the result. Promises – also known as “futures” in some systems – are placeholders for the results of operations that have not yet completed. When the operation completes, we say that the promise “resolves” to a value, or is “fulfilled”. A promise can also be “rejected”, which means an exception occurred.

// Example promise-based interfaces.

kj::Promise<kj::String> fetchHttp(kj::StringPtr url);
// Asynchronously fetches an HTTP document and returns
// the content as a string.

kj::Promise<void> sendEmail(kj::StringPtr address,
    kj::StringPtr title, kj::StringPtr body);
// Sends an e-mail to the given address with the given title
// and body.  The returned promise resolves (to nothing) when
// the message has been successfully sent.

As you will see, KJ promises are very similar to the evolving Javascript promise standard, and much of the wisdom around it can be directly applied to KJ promises.


If you want to do something with the result of a promise, you must first wait for it to complete. This is normally done by registering a callback to execute on completion. Luckily, C++11 just introduced lambdas, which makes this far more pleasant than it would have been a few years ago!

kj::Promise<kj::String> contentPromise =

kj::Promise<int> lineCountPromise =
    contentPromise.then([](kj::String&& content) {
  return countChars(content, '\n');

The callback passed to then() takes the promised result as its parameter and returns a new value. then() itself returns a new promise for that value which the callback will eventually return. If the callback itself returns a promise, then then() actually returns a promise for the resolution of the latter promise – that is, Promise<Promise<T>> is automatically reduced to Promise<T>.

Note that then() consumes the original promise: you can only call then() once. This is true of all of the methods of Promise. The only way to consume a promise in multiple places is to first “fork” it with the fork() method, which we don’t get into here. Relatedly, promises are linear types, which means they have move constructors but not copy constructors.

Error Propagation

then() takes an optional second parameter for handling errors. Think of this like a catch block.

kj::Promise<int> lineCountPromise =
    promise.then([](kj::String&& content) {
  return countChars(content, '\n');
}, [](kj::Exception&& exception) {
  // Error!  Pretend the document was empty.
  return 0;

Note that the KJ framework coerces all exceptions to kj::Exception – the exception’s description (as returned by what()) will be retained, but any type-specific information is lost. Under KJ exception philosophy, exceptions always represent an error that should not occur under normal operation, and the only purpose of exceptions is to make software fault-tolerant. In particular, the only reasonable ways to handle an exception are to try again, tell a human, and/or propagate to the caller. To that end, kj::Exception contains information useful for reporting purposes and to help decide if trying again is reasonable, but typed exception hierarchies are not useful and not supported.

It is recommended that Cap’n Proto code use the assertion macros in kj/debug.h to throw exceptions rather than use the C++ throw keyword. These macros make it easy to add useful debug information to an exception and generally play nicely with the KJ framework. In fact, you can even use these macros – and propagate exceptions through promises – if you compile your code with exceptions disabled. See the headers for more information.


It is illegal for code running in an event callback to wait, since this would stall the event loop. However, if you are the one responsible for starting the event loop in the first place, then KJ makes it easy to say “run the event loop until this promise resolves, then return the result”.

kj::EventLoop loop;
kj::WaitScope waitScope(loop);

kj::Promise<kj::String> contentPromise =

kj::String content = contentPromise.wait(waitScope);

int lineCount = countChars(content, '\n');

Using wait() is common in high-level client-side code. On the other hand, it is almost never used in servers.


If you discard a Promise without calling any of its methods, the operation it was waiting for is canceled, because the Promise itself owns that operation. This means than any pending callbacks simply won’t be executed. If you need explicit notification when a promise is canceled, you can use its attach() method to attach an object with a destructor – the destructor will be called when the promise either completes or is canceled.

Other Features

KJ supports a number of primitive operations that can be performed on promises. The complete API is documented directly in the kj/async.h header. Additionally, see the kj/async-io.h header for APIs for performing basic network I/O – although Cap’n Proto RPC users typically won’t need to use these APIs directly.

Generated Code

Imagine the following interface:

interface Directory {
  create @0 (name :Text) -> (file :File);
  open @1 (name :Text) -> (file :File);
  remove @2 (name :Text);

capnp compile will generate code that looks like this (edited for readability):

struct Directory {
  Directory() = delete;

  class Client;
  class Server;

  struct CreateParams;
  struct CreateResults;
  struct OpenParams;
  struct OpenResults;
  struct RemoveParams;
  struct RemoveResults;
  // Each of these is equivalent to what would be generated for
  // a Cap'n Proto struct with one field for each parameter /
  // result.

class Directory::Client
    : public virtual capnp::Capability::Client {
  Client(kj::Own<Directory::Server> server);
  Client(kj::Promise<Client> promise);
  Client(kj::Exception exception);

  capnp::Request<CreateParams, CreateResults> createRequest();
  capnp::Request<OpenParams, OpenResults> openRequest();
  capnp::Request<RemoveParams, RemoveResults> removeRequest();

class Directory::Server
    : public virtual capnp::Capability::Server {
  typedef capnp::CallContext<CreateParams, CreateResults> CreateContext;
  typedef capnp::CallContext<OpenParams, OpenResults> OpenContext;
  typedef capnp::CallContext<RemoveParams, RemoveResults> RemoveContext;
  // Convenience typedefs.

  virtual kj::Promise<void> create(CreateContext context);
  virtual kj::Promise<void> open(OpenContext context);
  virtual kj::Promise<void> remove(RemoveContext context);
  // Methods for you to implement.


The generated Client type represents a reference to a remote Server. Clients are pass-by-value types that use reference counting under the hood. (Warning: For performance reasons, the reference counting used by Clients is not thread-safe, so you must not copy a Client to another thread, unless you do it by means of an inter-thread RPC.)

A Client can be implicitly constructed from any of:

For each interface method foo(), the Client has a method fooRequest() which creates a new request to call foo(). The returned capnp::Request object has methods equivalent to a Builder for the parameter struct (FooParams), with the addition of a method send(). send() sends the RPC and returns a capnp::RemotePromise<FooResults>.

This RemotePromise is equivalent to kj::Promise<capnp::Response<FooResults>>, but also has methods that allow pipelining. Namely:

In other words, the RemotePromise effectively implements a subset of the eventual results’ Reader interface – one that only allows access to interfaces and sub-structs.

The RemotePromise eventually resolves to capnp::Response<FooResults>, which behaves like a Reader for the result struct except that it also owns the result message.

Directory::Client dir = ...;

// Create a new request for the `open()` method.
auto request = dir.openRequest();

// Send the request.
auto promise = request.send();

// Make a pipelined request.
auto promise2 = promise.getFile().getSizeRequest().send();

// Wait for the full results.
auto promise3 = promise2.then(
    [](capnp::Response<File::GetSizeResults>&& response) {
  cout << "File size is: " << response.getSize() << endl;

For generic methods, the fooRequest() method will be a template; you must explicitly specify type parameters.


The generated Server type is an abstract interface which may be subclassed to implement a capability. Each method takes a context argument and returns a kj::Promise<void> which resolves when the call is finished. The parameter and result structures are accessed through the context – context.getParams() returns a Reader for the parameters, and context.getResults() returns a Builder for the results. The context also has methods for controlling RPC logistics, such as cancellation – see capnp::CallContext in capnp/capability.h for details.

Accessing the results through the context (rather than by returning them) is unintuitive, but necessary because the underlying RPC transport needs to have control over where the results are allocated. For example, a zero-copy shared memory transport would need to allocate the results in the shared memory segment. Hence, the method implementation cannot just create its own MessageBuilder.

class DirectoryImpl final: public Directory::Server {
  kj::Promise<void> open(OpenContext context) override {
    auto iter = files.find(context.getParams().getName());

    // Throw an exception if not found.
    KJ_REQUIRE(iter != files.end(), "File not found.");


    return kj::READY_NOW;

  // Any method which we don't implement will simply throw
  // an exception by default.

  std::map<kj::StringPtr, File::Client> files;

On the server side, generic methods are NOT templates. Instead, the generated code is exactly as if all of the generic parameters were bound to AnyPointer. The server generally does not get to know exactly what type the client requested; it must be designed to be correct for any parameterization.

Initializing RPC

Cap’n Proto makes it easy to start up an RPC client or server using the “EZ RPC” classes, defined in capnp/ez-rpc.h. These classes get you up and running quickly, but they hide a lot of details that power users will likely want to manipulate. Check out the comments in ez-rpc.h to understand exactly what you get and what you miss. For the purpose of this overview, we’ll show you how to use EZ RPC to get started.

Starting a client

A client should typically look like this:

#include <capnp/ez-rpc.h>
#include "my-interface.capnp.h"
#include <iostream>

int main(int argc, const char* argv[]) {
  // We expect one argument specifying the server address.
  if (argc != 2) {
    std::cerr << "usage: " << argv[0] << " HOST[:PORT]" << std::endl;
    return 1;

  // Set up the EzRpcClient, connecting to the server on port
  // 5923 unless a different port was specified by the user.
  capnp::EzRpcClient client(argv[1], 5923);
  auto& waitScope = client.getWaitScope();

  // Request the bootstrap capability from the server.
  MyInterface::Client cap = client.getMain<MyInterface>();

  // Make a call to the capability.
  auto request = cap.fooRequest();
  auto promise = request.send();

  // Wait for the result.  This is the only line that blocks.
  auto response = promise.wait(waitScope);

  // All done.
  std::cout << response.getResult() << std::endl;
  return 0;

Note that for the connect address, Cap’n Proto supports DNS host names as well as IPv4 and IPv6 addresses. Additionally, a Unix domain socket can be specified as unix: followed by a path name.

For a more complete example, see the calculator client sample.

Starting a server

A server might look something like this:

#include <capnp/ez-rpc.h>
#include "my-interface-impl.h"
#include <iostream>

int main(int argc, const char* argv[]) {
  // We expect one argument specifying the address to which
  // to bind and accept connections.
  if (argc != 2) {
    std::cerr << "usage: " << argv[0] << " ADDRESS[:PORT]"
              << std::endl;
    return 1;

  // Set up the EzRpcServer, binding to port 5923 unless a
  // different port was specified by the user.  Note that the
  // first parameter here can be any "Client" object or anything
  // that can implicitly cast to a "Client" object.  You can even
  // re-export a capability imported from another server.
  capnp::EzRpcServer server(kj::heap<MyInterfaceImpl>(), argv[1], 5923);
  auto& waitScope = server.getWaitScope();

  // Run forever, accepting connections and handling requests.

Note that for the bind address, Cap’n Proto supports DNS host names as well as IPv4 and IPv6 addresses. The special address * can be used to bind to the same port on all local IPv4 and IPv6 interfaces. Additionally, a Unix domain socket can be specified as unix: followed by a path name.

For a more complete example, see the calculator server sample.


If you’ve written a server and you want to connect to it to issue some calls for debugging, perhaps interactively, the easiest way to do it is to use pycapnp. We have decided not to add RPC functionality to the capnp command-line tool because pycapnp is better than anything we might provide.