Cap’n Proto

Serialization & FormatsSerialization/NetworkingC++

What it is

Cap’n Proto is a fast data interchange format and RPC system. It provides schema-based serialization similar to Protocol Buffers, but with zero-copy design, enabling extremely fast message passing and storage.

Cap’n Proto provides a schema definition language (`.capnp` files) that describes structured data. It generates C++ code for serialization, deserialization, and RPC stubs. Its zero-copy design allows messages to be sent/received without heavy CPU overhead.

Installation

sudo apt install capnproto libcapnp-dev

Getting started

The smallest useful thing you can do with it, and what each part means.

Defining a schema

cpp

123456@0xbf5147cbbecf40a7;
struct Person {
  id @0 :UInt64;
  name @1 :Text;
  email @2 :Text;
}

Defines a simple schema for a `Person` object in a `.capnp` file.

Serializing and writing data

cpp

1234567891011121314#include <capnp/message.h>
#include <capnp/serialize.h>
#include "person.capnp.h"

int main() {
    ::capnp::MallocMessageBuilder message;
    Person::Builder person = message.initRoot<Person>();
    person.setId(123);
    person.setName("Alice");
    person.setEmail("alice@example.com");

    writeMessageToFd(1, message); // Write to stdout
    return 0;
}

Creates a `Person` object and serializes it using Cap’n Proto.

Advanced usage

Where the library earns its place over a simpler alternative.

Reading and deserializing data

cpp

::capnp::StreamFdMessageReader reader(0); // Read from stdin
Person::Reader person = reader.getRoot<Person>();
std::cout << person.getName().cStr() << std::endl;

Reads and deserializes a Cap’n Proto message from input.

Using lists

cpp

auto friends = person.initFriends(2);
friends[0].setName("Bob");
friends[1].setName("Charlie");

Cap’n Proto supports strongly-typed lists of structs and primitives.

Remote Procedure Calls (RPC)

cpp

12345// Cap’n Proto provides async RPC support.
// Example: defining an interface in .capnp:
interface Calculator {
  add @0 (a :Int32, b :Int32) -> (result :Int32);
}

Cap’n Proto includes a built-in RPC system with async method calls.

Errors and fixes

The failures you are most likely to hit, and what actually resolves them.

capnp::kj::Exception: message too large: Adjust message size limits when working with large data structures.
Schema mismatch: Ensure the `.capnp` schema matches the generated C++ headers used at runtime.
Serialization failure: Verify that required fields are initialized before writing the message.

Best practices

Use Cap’n Proto when low-latency, high-throughput serialization is required.
Prefer Cap’n Proto over Protobuf when zero-copy access matters.
Avoid unnecessary message copies; use memory-mapped files where possible.
Keep schemas stable—Cap’n Proto supports backward and forward compatibility.
For RPC, combine with secure transport (e.g., TLS) since Cap’n Proto itself doesn’t enforce encryption.

Background

Why it exists, and what it was reacting to.

Cap’n Proto was created by Kenton Varda, the original author of Google’s Protocol Buffers, to address performance bottlenecks in serialization. Unlike Protobuf, which requires encoding/decoding, Cap’n Proto messages are in-memory representations that can be directly written to disk or sent over the network. It is widely used in distributed systems and applications requiring low-latency communication.

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