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Cap'n Proto serialization/RPC system - core tools and C++ library

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Quick Overview

Cap'n Proto is a data serialization format and RPC system, designed to be faster and more efficient than traditional protocols like Protocol Buffers. It aims to provide near-zero overhead serialization and deserialization, making it ideal for high-performance applications and systems programming.

Pros

  • Extremely fast serialization and deserialization, with zero-copy reads
  • Supports schema evolution, allowing for backward and forward compatibility
  • Includes a powerful RPC system with capabilities-based security
  • Language-neutral, with implementations available for multiple programming languages

Cons

  • Less widely adopted compared to alternatives like Protocol Buffers or JSON
  • Learning curve can be steeper due to its unique concepts and design
  • Limited ecosystem and third-party tooling compared to more established formats
  • May be overkill for simple use cases or small projects

Code Examples

  1. Defining a schema:
@0xdbb9ad1f14bf0b36;  # Unique file ID

struct Person {
  name @0 :Text;
  age @1 :UInt32;
  email @2 :Text;
}
  1. Building and reading a message (C++):
#include <capnp/message.h>
#include <capnp/serialize.h>
#include "person.capnp.h"

// Building a message
capnp::MallocMessageBuilder message;
Person::Builder person = message.initRoot<Person>();
person.setName("John Doe");
person.setAge(30);
person.setEmail("john@example.com");

// Reading a message
capnp::ReaderOptions options;
capnp::SegmentArrayMessageReader reader(message.getSegmentsForOutput(), options);
Person::Reader personReader = reader.getRoot<Person>();
std::cout << personReader.getName().cStr() << std::endl;
  1. RPC example (server-side):
#include <capnp/ez-rpc.h>
#include "calculator.capnp.h"

class CalculatorImpl final : public Calculator::Server {
public:
  kj::Promise<void> add(AddContext context) override {
    int32_t result = context.getParams().getA() + context.getParams().getB();
    context.getResults().setResult(result);
    return kj::READY_NOW;
  }
};

int main() {
  capnp::EzRpcServer server("*:8000", kj::heap<CalculatorImpl>());
  kj::NEVER_DONE.wait(server.getWaitScope());
}

Getting Started

  1. Install Cap'n Proto:

    sudo apt-get install capnproto
    
  2. Create a schema file (e.g., example.capnp):

    @0xdbb9ad1f14bf0b36;
    struct Example {
      message @0 :Text;
    }
    
  3. Compile the schema:

    capnp compile -oc++ example.capnp
    
  4. Include the generated header in your C++ code and start using Cap'n Proto!

Competitor Comparisons

68,687

Protocol Buffers - Google's data interchange format

Pros of Protocol Buffers

  • Mature and widely adopted, with extensive language support
  • Robust backwards compatibility mechanisms
  • Efficient binary serialization format

Cons of Protocol Buffers

  • More verbose syntax compared to Cap'n Proto
  • Slower serialization/deserialization due to parsing overhead
  • Larger message sizes due to field identifiers

Code Comparison

Protocol Buffers:

message Person {
  required string name = 1;
  required int32 id = 2;
  optional string email = 3;
}

Cap'n Proto:

struct Person {
  name @0 :Text;
  id @1 :Int32;
  email @2 :Text;
}

Key Differences

  • Cap'n Proto offers zero-copy deserialization, resulting in faster processing
  • Protocol Buffers has a larger ecosystem and more third-party tools
  • Cap'n Proto's schema language is more concise and expressive
  • Protocol Buffers has better support for optional fields and default values
  • Cap'n Proto provides built-in RPC capabilities, while Protocol Buffers requires additional libraries

Both projects aim to provide efficient serialization formats, but Cap'n Proto focuses on performance and simplicity, while Protocol Buffers prioritizes compatibility and ecosystem support.

10,754

Apache Thrift

Pros of Thrift

  • Wider language support, including more legacy languages
  • Mature ecosystem with extensive documentation and community support
  • Built-in support for various transport and protocol layers

Cons of Thrift

  • Generally slower serialization and deserialization compared to Cap'n Proto
  • More complex setup and configuration required
  • Larger message sizes due to less efficient encoding

Code Comparison

Thrift IDL:

struct Person {
  1: string name
  2: i32 age
  3: bool is_active
}

Cap'n Proto schema:

struct Person {
  name @0 :Text;
  age @1 :UInt32;
  isActive @2 :Bool;
}

Both Thrift and Cap'n Proto use interface definition languages (IDL) to define data structures. However, Cap'n Proto's schema is generally more concise and readable. Thrift requires explicit field numbering, while Cap'n Proto uses the @ symbol followed by an ordinal number.

Cap'n Proto focuses on zero-copy deserialization and offers better performance in many scenarios. Thrift, on the other hand, provides a more comprehensive set of features and language support, making it suitable for a wider range of applications, especially those involving legacy systems or requiring specific transport protocols.

3,128

Apache Avro is a data serialization system.

Pros of Avro

  • Mature ecosystem with wide language support and integration with Hadoop
  • Schema evolution capabilities allow for easier backward and forward compatibility
  • Compact binary encoding for efficient data serialization

Cons of Avro

  • Generally slower serialization and deserialization compared to Cap'n Proto
  • Requires full schema parsing for each data record, potentially impacting performance
  • Less memory-efficient due to data copying during deserialization

Code Comparison

Avro schema definition:

{
  "type": "record",
  "name": "Person",
  "fields": [
    {"name": "name", "type": "string"},
    {"name": "age", "type": "int"}
  ]
}

Cap'n Proto schema definition:

struct Person {
  name @0 :Text;
  age @1 :Int32;
}

Both Avro and Cap'n Proto offer schema-based serialization, but Cap'n Proto's syntax is more concise and type-safe. Avro uses JSON for schema definition, while Cap'n Proto has its own domain-specific language. Cap'n Proto's approach allows for zero-copy deserialization and better performance, but Avro offers more flexibility in schema evolution and broader ecosystem support.

7,276

MessagePack is an extremely efficient object serialization library. It's like JSON, but very fast and small.

Pros of MessagePack

  • Simpler implementation and easier to integrate into existing projects
  • Wider language support and more mature ecosystem
  • Smaller message size for small data structures

Cons of MessagePack

  • Less efficient for larger data structures
  • Lacks built-in schema evolution and versioning support
  • No support for zero-copy deserialization

Code Comparison

MessagePack:

import msgpack

data = {"name": "John", "age": 30}
packed = msgpack.packb(data)
unpacked = msgpack.unpackb(packed)

Cap'n Proto:

#include <capnp/message.h>
#include <capnp/serialize.h>
#include "person.capnp.h"

capnp::MallocMessageBuilder message;
Person::Builder person = message.initRoot<Person>();
person.setName("John");
person.setAge(30);

Cap'n Proto offers a more strongly-typed approach with schema definitions, while MessagePack provides a simpler, more flexible serialization method. Cap'n Proto excels in performance and features for large-scale systems, whereas MessagePack is often preferred for its simplicity and broad language support in smaller projects or when working with dynamic data structures.

1,895

idiomatic codec and rpc lib for msgpack, cbor, json, etc. msgpack.org[Go]

Pros of go

  • Specifically designed for Go, offering seamless integration with Go projects
  • Supports a wide range of encoding formats, including JSON, MessagePack, and CBOR
  • Provides high-performance serialization and deserialization

Cons of go

  • Limited cross-language support compared to Cap'n Proto
  • Lacks the schema-based approach and forward/backward compatibility features of Cap'n Proto
  • May require more manual work for complex data structures

Code Comparison

Cap'n Proto schema:

struct Person {
  name @0 :Text;
  age @1 :UInt32;
}

go usage:

type Person struct {
  Name string `json:"name"`
  Age  uint32 `json:"age"`
}

Summary

While go excels in Go-specific environments and offers versatile encoding options, Cap'n Proto provides a more robust cross-language solution with advanced schema features. Cap'n Proto's approach is better suited for complex, evolving data structures and systems requiring strong compatibility guarantees. go, on the other hand, offers simplicity and high performance within the Go ecosystem, making it a solid choice for Go-centric projects with straightforward serialization needs.

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README

THIS IS THE V2 DEVELOPMENT BRANCH

On this branch, we may make breaking changes to the API at any time. Do not use this branch if you want stability. If you want "1.0 plus bug fixes", use the master branch.

For more, see the 1.0 release announcement:

https://capnproto.org/news/2023-07-28-capnproto-1.0.html


Cap'n Proto is an insanely fast data interchange format and capability-based RPC system. Think JSON, except binary. Or think Protocol Buffers, except faster. In fact, in benchmarks, Cap'n Proto is INFINITY TIMES faster than Protocol Buffers.

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