Cap‘n Protocol简介

简介"/>

Cap‘n Protocol简介

Cap’n Protocol简介

Cap’n Proto is an insanely fast data interchange format and capability-based RPC system.

• cap’n protocol由protobuf的主要设计者kentonv主导，等价于Proto + RPC。提供序列化/反序列化、方法调用、异步Promise流水行等主要功能。
• 特点

1. 无encoding/decoding，基于内存布局的编码使得Cap’n Protocol的Structure可以直接写到磁盘上，以二进制文件的形式直接读出。这样在序列化/反序列化过程中，性能将大大提升。
2. 异步Promise PipeLine（如下图），传统RPC实现 foo + bar 调用，需要3个步骤： 调用foo，得到返回值x，调用bar(x)。Async Promise Pipelie，不需要返回X这个中间结果，而是一下将请求发送给Server端，server端只需要返回一个Promise即可。
   
3. Four Level RPC：作者将功能划分为4个Level，从低到高分别是Object references and promise pipelining、Persistent capabilities、Three-way interactions、Reference equality / joining，目前最新版0.9.1实现了前两个（Leve1和Leve2），作者给出的公告中说再1.0.0版本将实现 Three-way interactions（三向引用）

• Capnp组件图：capnp基于kj异步框架，使用promise、rpc功能。
  

使用Cap’n Protocol

• 官网提供了安装及Smaple
• Ubuntu 编译安装Capnp

curl -O ++-0.9.1.tar.gz
tar zxf capnproto-c++-0.9.1.tar.gz
cd capnproto-c++-0.9.1
./configure
make -j6 check
sudo make install

Sample（官网）

• 接口文件calculator.capnp

@0x85150b117366d14b;interface Calculator {# A "simple" mathematical calculator, callable via RPC.## But, to show off Cap'n Proto, we add some twists:## - You can use the result from one call as the input to the next#   without a network round trip.  To accomplish this, evaluate()#   returns a `Value` object wrapping the actual numeric value.#   This object may be used in a subsequent expression.  With#   promise pipelining, the Value can actually be used before#   the evaluate() call that creates it returns!## - You can define new functions, and then call them.  This again#   shows off pipelining, but it also gives the client the#   opportunity to define a function on the client side and have#   the server call back to it.## - The basic arithmetic operators are exposed as Functions, and#   you have to call getOperator() to obtain them from the server.#   This again demonstrates pipelining -- using getOperator() to#   get each operator and then using them in evaluate() still#   only takes one network round trip.evaluate @0 (expression :Expression) -> (value :Value);# Evaluate the given expression and return the result.  The# result is returned wrapped in a Value interface so that you# may pass it back to the server in a pipelined request.  To# actually get the numeric value, you must call read() on the# Value -- but again, this can be pipelined so that it incurs# no additional latency.struct Expression {# A numeric expression.union {literal @0 :Float64;# A literal numeric value.previousResult @1 :Value;# A value that was (or, will be) returned by a previous# evaluate().parameter @2 :UInt32;# A parameter to the function (only valid in function bodies;# see defFunction).call :group {# Call a function on a list of parameters.function @3 :Function;params @4 :List(Expression);}}}interface Value {# Wraps a numeric value in an RPC object.  This allows the value# to be used in subsequent evaluate() requests without the client# waiting for the evaluate() that returns the Value to finish.read @0 () -> (value :Float64);# Read back the raw numeric value.}defFunction @1 (paramCount :Int32, body :Expression)-> (func :Function);# Define a function that takes `paramCount` parameters and returns the# evaluation of `body` after substituting these parameters.interface Function {# An algebraic function.  Can be called directly, or can be used inside# an Expression.## A client can create a Function that runs on the server side using# `defFunction()` or `getOperator()`.  Alternatively, a client can# implement a Function on the client side and the server will call back# to it.  However, a function defined on the client side will require a# network round trip whenever the server needs to call it, whereas# functions defined on the server and then passed back to it are called# locally.call @0 (params :List(Float64)) -> (value :Float64);# Call the function on the given parameters.}getOperator @2 (op :Operator) -> (func :Function);# Get a Function representing an arithmetic operator, which can then be# used in Expressions.enum Operator {add @0;subtract @1;multiply @2;divide @3;}
}

• Clien端实现 calculator-client.c++

// Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
// Licensed under the MIT License:
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.#include "calculator.capnp.h"
#include <capnp/ez-rpc.h>
#include <kj/debug.h>
#include <math.h>
#include <iostream>class PowerFunction final: public Calculator::Function::Server {// An implementation of the Function interface wrapping pow().  Note that// we're implementing this on the client side and will pass a reference to// the server.  The server will then be able to make calls back to the client.public:kj::Promise<void> call(CallContext context) {auto params = context.getParams().getParams();KJ_REQUIRE(params.size() == 2, "Wrong number of parameters.");context.getResults().setValue(pow(params[0], params[1]));return kj::READY_NOW;}
};int main(int argc, const char* argv[]) {if (argc != 2) {std::cerr << "usage: " << argv[0] << " HOST:PORT\n""Connects to the Calculator server at the given address and ""does some RPCs." << std::endl;return 1;}capnp::EzRpcClient client(argv[1]);Calculator::Client calculator = client.getMain<Calculator>();// Keep an eye on `waitScope`.  Whenever you see it used is a place where we// stop and wait for the server to respond.  If a line of code does not use// `waitScope`, then it does not block!auto& waitScope = client.getWaitScope();{// Make a request that just evaluates the literal value 123.//// What's interesting here is that evaluate() returns a "Value", which is// another interface and therefore points back to an object living on the// server.  We then have to call read() on that object to read it.// However, even though we are making two RPC's, this block executes in// *one* network round trip because of promise pipelining:  we do not wait// for the first call to complete before we send the second call to the// server.std::cout << "Evaluating a literal... ";std::cout.flush();// Set up the request.auto request = calculator.evaluateRequest();request.getExpression().setLiteral(123);// Send it, which returns a promise for the result (without blocking).auto evalPromise = request.send();// Using the promise, create a pipelined request to call read() on the// returned object, and then send that.auto readPromise = evalPromise.getValue().readRequest().send();// Now that we've sent all the requests, wait for the response.  Until this// point, we haven't waited at all!auto response = readPromise.wait(waitScope);KJ_ASSERT(response.getValue() == 123);std::cout << "PASS" << std::endl;}{// Make a request to evaluate 123 + 45 - 67.//// The Calculator interface requires that we first call getOperator() to// get the addition and subtraction functions, then call evaluate() to use// them.  But, once again, we can get both functions, call evaluate(), and// then read() the result -- four RPCs -- in the time of *one* network// round trip, because of promise pipelining.std::cout << "Using add and subtract... ";std::cout.flush();Calculator::Function::Client add = nullptr;Calculator::Function::Client subtract = nullptr;{// Get the "add" function from the server.auto request = calculator.getOperatorRequest();request.setOp(Calculator::Operator::ADD);add = request.send().getFunc();}{// Get the "subtract" function from the server.auto request = calculator.getOperatorRequest();request.setOp(Calculator::Operator::SUBTRACT);subtract = request.send().getFunc();}// Build the request to evaluate 123 + 45 - 67.auto request = calculator.evaluateRequest();auto subtractCall = request.getExpression().initCall();subtractCall.setFunction(subtract);auto subtractParams = subtractCall.initParams(2);subtractParams[1].setLiteral(67);auto addCall = subtractParams[0].initCall();addCall.setFunction(add);auto addParams = addCall.initParams(2);addParams[0].setLiteral(123);addParams[1].setLiteral(45);// Send the evaluate() request, read() the result, and wait for read() to// finish.auto evalPromise = request.send();auto readPromise = evalPromise.getValue().readRequest().send();auto response = readPromise.wait(waitScope);KJ_ASSERT(response.getValue() == 101);std::cout << "PASS" << std::endl;}{// Make a request to evaluate 4 * 6, then use the result in two more// requests that add 3 and 5.//// Since evaluate() returns its result wrapped in a `Value`, we can pass// that `Value` back to the server in subsequent requests before the first// `evaluate()` has actually returned.  Thus, this example again does only// one network round trip.std::cout << "Pipelining eval() calls... ";std::cout.flush();Calculator::Function::Client add = nullptr;Calculator::Function::Client multiply = nullptr;{// Get the "add" function from the server.auto request = calculator.getOperatorRequest();request.setOp(Calculator::Operator::ADD);add = request.send().getFunc();}{// Get the "multiply" function from the server.auto request = calculator.getOperatorRequest();request.setOp(Calculator::Operator::MULTIPLY);multiply = request.send().getFunc();}// Build the request to evaluate 4 * 6auto request = calculator.evaluateRequest();auto multiplyCall = request.getExpression().initCall();multiplyCall.setFunction(multiply);auto multiplyParams = multiplyCall.initParams(2);multiplyParams[0].setLiteral(4);multiplyParams[1].setLiteral(6);auto multiplyResult = request.send().getValue();// Use the result in two calls that add 3 and add 5.auto add3Request = calculator.evaluateRequest();auto add3Call = add3Request.getExpression().initCall();add3Call.setFunction(add);auto add3Params = add3Call.initParams(2);add3Params[0].setPreviousResult(multiplyResult);add3Params[1].setLiteral(3);auto add3Promise = add3Request.send().getValue().readRequest().send();auto add5Request = calculator.evaluateRequest();auto add5Call = add5Request.getExpression().initCall();add5Call.setFunction(add);auto add5Params = add5Call.initParams(2);add5Params[0].setPreviousResult(multiplyResult);add5Params[1].setLiteral(5);auto add5Promise = add5Request.send().getValue().readRequest().send();// Now wait for the results.KJ_ASSERT(add3Promise.wait(waitScope).getValue() == 27);KJ_ASSERT(add5Promise.wait(waitScope).getValue() == 29);std::cout << "PASS" << std::endl;}{// Our calculator interface supports defining functions.  Here we use it// to define two functions and then make calls to them as follows:////   f(x, y) = x * 100 + y//   g(x) = f(x, x + 1) * 2;//   f(12, 34)//   g(21)//// Once again, the whole thing takes only one network round trip.std::cout << "Defining functions... ";std::cout.flush();Calculator::Function::Client add = nullptr;Calculator::Function::Client multiply = nullptr;Calculator::Function::Client f = nullptr;Calculator::Function::Client g = nullptr;{// Get the "add" function from the server.auto request = calculator.getOperatorRequest();request.setOp(Calculator::Operator::ADD);add = request.send().getFunc();}{// Get the "multiply" function from the server.auto request = calculator.getOperatorRequest();request.setOp(Calculator::Operator::MULTIPLY);multiply = request.send().getFunc();}{// Define f.auto request = calculator.defFunctionRequest();request.setParamCount(2);{// Build the function body.auto addCall = request.getBody().initCall();addCall.setFunction(add);auto addParams = addCall.initParams(2);addParams[1].setParameter(1);  // yauto multiplyCall = addParams[0].initCall();multiplyCall.setFunction(multiply);auto multiplyParams = multiplyCall.initParams(2);multiplyParams[0].setParameter(0);  // xmultiplyParams[1].setLiteral(100);}f = request.send().getFunc();}{// Define g.auto request = calculator.defFunctionRequest();request.setParamCount(1);{// Build the function body.auto multiplyCall = request.getBody().initCall();multiplyCall.setFunction(multiply);auto multiplyParams = multiplyCall.initParams(2);multiplyParams[1].setLiteral(2);auto fCall = multiplyParams[0].initCall();fCall.setFunction(f);auto fParams = fCall.initParams(2);fParams[0].setParameter(0);auto addCall = fParams[1].initCall();addCall.setFunction(add);auto addParams = addCall.initParams(2);addParams[0].setParameter(0);addParams[1].setLiteral(1);}g = request.send().getFunc();}// OK, we've defined all our functions.  Now create our eval requests.// f(12, 34)auto fEvalRequest = calculator.evaluateRequest();auto fCall = fEvalRequest.initExpression().initCall();fCall.setFunction(f);auto fParams = fCall.initParams(2);fParams[0].setLiteral(12);fParams[1].setLiteral(34);auto fEvalPromise = fEvalRequest.send().getValue().readRequest().send();// g(21)auto gEvalRequest = calculator.evaluateRequest();auto gCall = gEvalRequest.initExpression().initCall();gCall.setFunction(g);gCall.initParams(1)[0].setLiteral(21);auto gEvalPromise = gEvalRequest.send().getValue().readRequest().send();// Wait for the results.KJ_ASSERT(fEvalPromise.wait(waitScope).getValue() == 1234);KJ_ASSERT(gEvalPromise.wait(waitScope).getValue() == 4244);std::cout << "PASS" << std::endl;}{// Make a request that will call back to a function defined locally.//// Specifically, we will compute 2^(4 + 5).  However, exponent is not// defined by the Calculator server.  So, we'll implement the Function// interface locally and pass it to the server for it to use when// evaluating the expression.//// This example requires two network round trips to complete, because the// server calls back to the client once before finishing.  In this// particular case, this could potentially be optimized by using a tail// call on the server side -- see CallContext::tailCall().  However, to// keep the example simpler, we haven't implemented this optimization in// the sample server.std::cout << "Using a callback... ";std::cout.flush();Calculator::Function::Client add = nullptr;{// Get the "add" function from the server.auto request = calculator.getOperatorRequest();request.setOp(Calculator::Operator::ADD);add = request.send().getFunc();}// Build the eval request for 2^(4+5).auto request = calculator.evaluateRequest();auto powCall = request.getExpression().initCall();powCall.setFunction(kj::heap<PowerFunction>());auto powParams = powCall.initParams(2);powParams[0].setLiteral(2);auto addCall = powParams[1].initCall();addCall.setFunction(add);auto addParams = addCall.initParams(2);addParams[0].setLiteral(4);addParams[1].setLiteral(5);// Send the request and wait.auto response = request.send().getValue().readRequest().send().wait(waitScope);KJ_ASSERT(response.getValue() == 512);std::cout << "PASS" << std::endl;}return 0;
}

• Server端实现 calculator-server.c++

// Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
// Licensed under the MIT License:
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.#include "calculator.capnp.h"
#include <kj/debug.h>
#include <capnp/ez-rpc.h>
#include <capnp/message.h>
#include <iostream>typedef unsigned int uint;kj::Promise<double> readValue(Calculator::Value::Client value) {// Helper function to asynchronously call read() on a Calculator::Value and// return a promise for the result.  (In the future, the generated code might// include something like this automatically.)return value.readRequest().send().then([](capnp::Response<Calculator::Value::ReadResults> result) {return result.getValue();});
}kj::Promise<double> evaluateImpl(Calculator::Expression::Reader expression,capnp::List<double>::Reader params = capnp::List<double>::Reader()) {// Implementation of CalculatorImpl::evaluate(), also shared by// FunctionImpl::call().  In the latter case, `params` are the parameter// values passed to the function; in the former case, `params` is just an// empty list.switch (expression.which()) {case Calculator::Expression::LITERAL:return expression.getLiteral();case Calculator::Expression::PREVIOUS_RESULT:return readValue(expression.getPreviousResult());case Calculator::Expression::PARAMETER: {KJ_REQUIRE(expression.getParameter() < params.size(),"Parameter index out-of-range.");return params[expression.getParameter()];}case Calculator::Expression::CALL: {auto call = expression.getCall();auto func = call.getFunction();// Evaluate each parameter.kj::Array<kj::Promise<double>> paramPromises =KJ_MAP(param, call.getParams()) {return evaluateImpl(param, params);};// Join the array of promises into a promise for an array.kj::Promise<kj::Array<double>> joinedParams =kj::joinPromises(kj::mv(paramPromises));// When the parameters are complete, call the function.return joinedParams.then([KJ_CPCAP(func)](kj::Array<double>&& paramValues) mutable {auto request = func.callRequest();request.setParams(paramValues);return request.send().then([](capnp::Response<Calculator::Function::CallResults>&& result) {return result.getValue();});});}default:// Throw an exception.KJ_FAIL_REQUIRE("Unknown expression type.");}
}class ValueImpl final: public Calculator::Value::Server {// Simple implementation of the Calculator.Value Cap'n Proto interface.public:ValueImpl(double value): value(value) {}kj::Promise<void> read(ReadContext context) {context.getResults().setValue(value);return kj::READY_NOW;}private:double value;
};class FunctionImpl final: public Calculator::Function::Server {// Implementation of the Calculator.Function Cap'n Proto interface, where the// function is defined by a Calculator.Expression.public:FunctionImpl(uint paramCount, Calculator::Expression::Reader body): paramCount(paramCount) {this->body.setRoot(body);}kj::Promise<void> call(CallContext context) {auto params = context.getParams().getParams();KJ_REQUIRE(params.size() == paramCount, "Wrong number of parameters.");return evaluateImpl(body.getRoot<Calculator::Expression>(), params).then([KJ_CPCAP(context)](double value) mutable {context.getResults().setValue(value);});}private:uint paramCount;// The function's arity.capnp::MallocMessageBuilder body;// Stores a permanent copy of the function body.
};class OperatorImpl final: public Calculator::Function::Server {// Implementation of the Calculator.Function Cap'n Proto interface, wrapping// basic binary arithmetic operators.public:OperatorImpl(Calculator::Operator op): op(op) {}kj::Promise<void> call(CallContext context) {auto params = context.getParams().getParams();KJ_REQUIRE(params.size() == 2, "Wrong number of parameters.");double result;switch (op) {case Calculator::Operator::ADD:     result = params[0] + params[1]; break;case Calculator::Operator::SUBTRACT:result = params[0] - params[1]; break;case Calculator::Operator::MULTIPLY:result = params[0] * params[1]; break;case Calculator::Operator::DIVIDE:  result = params[0] / params[1]; break;default:KJ_FAIL_REQUIRE("Unknown operator.");}context.getResults().setValue(result);return kj::READY_NOW;}private:Calculator::Operator op;
};class CalculatorImpl final: public Calculator::Server {// Implementation of the Calculator Cap'n Proto interface.public:kj::Promise<void> evaluate(EvaluateContext context) override {return evaluateImpl(context.getParams().getExpression()).then([KJ_CPCAP(context)](double value) mutable {context.getResults().setValue(kj::heap<ValueImpl>(value));});}kj::Promise<void> defFunction(DefFunctionContext context) override {auto params = context.getParams();context.getResults().setFunc(kj::heap<FunctionImpl>(params.getParamCount(), params.getBody()));return kj::READY_NOW;}kj::Promise<void> getOperator(GetOperatorContext context) override {context.getResults().setFunc(kj::heap<OperatorImpl>(context.getParams().getOp()));return kj::READY_NOW;}
};int main(int argc, const char* argv[]) {if (argc != 2) {std::cerr << "usage: " << argv[0] << " ADDRESS[:PORT]\n""Runs the server bound to the given address/port.\n""ADDRESS may be '*' to bind to all local addresses.\n"":PORT may be omitted to choose a port automatically." << std::endl;return 1;}// Set up a server.capnp::EzRpcServer server(kj::heap<CalculatorImpl>(), argv[1]);// Write the port number to stdout, in case it was chosen automatically.auto& waitScope = server.getWaitScope();uint port = server.getPort().wait(waitScope);if (port == 0) {// The address format "unix:/path/to/socket" opens a unix domain socket,// in which case the port will be zero.std::cout << "Listening on Unix socket..." << std::endl;} else {std::cout << "Listening on port " << port << "..." << std::endl;}// Run forever, accepting connections and handling requests.kj::NEVER_DONE.wait(waitScope);
}

• 编译文件(CMakeLists.txt)

project("Cap'n Proto Samples" CXX)
cmake_minimum_required(VERSION 3.1)find_package(CapnProto CONFIG REQUIRED)# Don't build the rpc sample if find_package() found an installation of Cap'n Proto lite.
if(TARGET CapnProto::capnp-rpc)capnp_generate_cpp(calculatorSources calculatorHeaders calculator.capnp)add_executable(calculator-client calculator-client.c++ ${calculatorSources})add_executable(calculator-server calculator-server.c++ ${calculatorSources})target_link_libraries(calculator-client PRIVATE CapnProto::capnp-rpc)target_link_libraries(calculator-server PRIVATE CapnProto::capnp-rpc)target_include_directories(calculator-client PRIVATE ${CMAKE_CURRENT_BINARY_DIR})target_include_directories(calculator-server PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
endif()

• 执行，编译后会生成 calculator-client和calculator-server两个执行文件。

#先启动Server ip:port
./calculator-server 192.168.205.100:1234
# 另起终端启动Client
./calculator-client 192.168.205.100:1234

Cap’n proto的优缺点

• 优点：

1. 无encode和decode。
2. 异步Promise PipeLine。

• 缺点：

1. 不支持广播、组播。
2. 无服务动态方法。
3. 无Qos
4. 无加密传输
5. 没有E2E安全校验

总的来说，Capn proto实现了一套简单的Ez（easy promise base rpc），只适用于简单的点对点通信场景。但是复杂场景下的通信，比如系统状态广播这种，无法原生支持。目前版本，同系统内的进程间通信，仍然是socket通信，效率不高。并且没有服务发现功能，在跨域的通信场景下，与其他方式比如(someip)相比，目前的版本下无明显的优势。