# Cap'n Proto — Insanely Fast Data Serialization and RPC

> Cap'n Proto is a data interchange format and RPC system that operates on the wire format directly, eliminating encoding and decoding overhead. Created by the author of Protocol Buffers v2, it achieves zero-copy reads for maximum performance.

## Install

Save as a script file and run:

# Cap'n Proto — Insanely Fast Data Serialization and RPC

## Quick Use
```bash
# Install on Ubuntu/Debian
sudo apt install capnproto libcapnp-dev

# Define a schema
cat > addressbook.capnp << 'EOF2'
@0xdbb9ad1f14bf0b36;
struct Person {
  name @0 :Text;
  email @1 :Text;
  phones @2 :List(PhoneNumber);
  struct PhoneNumber {
    number @0 :Text;
    type @1 :Type;
    enum Type { mobile @0; home @1; work @2; }
  }
}
EOF2

# Generate code
capnpc -oc++ addressbook.capnp
```

## Introduction
Cap'n Proto was designed by Kenton Varda, who also created Protocol Buffers v2 at Google. The core insight is that the serialized format and the in-memory format can be identical, eliminating the encode/decode step entirely. This makes Cap'n Proto particularly effective for memory-mapped files, shared memory IPC, and high-throughput RPC where serialization overhead matters.

## What Cap'n Proto Does
- Serializes structured data with zero encoding/decoding overhead via direct memory layout
- Provides an RPC framework with promise pipelining to reduce network round-trips
- Generates type-safe code for C++, Rust, Go, Python, Java, and other languages
- Supports schema evolution with backward and forward compatibility rules
- Enables memory-mapped file access where data is read directly without parsing

## Architecture Overview
Cap'n Proto defines a canonical binary layout where every field has a fixed offset. A message in memory IS the serialized form; reading a field is a pointer dereference, not a parse. The RPC layer uses object-capability security and promise pipelining: a client can call a method on a result that hasn't arrived yet, and the server batches the calls to avoid extra round-trips. Schemas are compiled by `capnpc` into language-specific builders and readers.

## Self-Hosting & Configuration
- Install via system package manager or build from source with CMake
- Define schemas in `.capnp` files with a compact IDL syntax
- Generate code with `capnpc -o<language> schema.capnp`
- For Rust, use the `capnp` and `capnpc` crates; for Go, use `capnproto/go-capnp`
- RPC servers and clients use the generated interfaces with promise-based async I/O

## Key Features
- Zero-copy deserialization reads data directly from the wire format without allocation
- Promise pipelining reduces RPC latency by allowing chained calls before responses arrive
- Time-travel RPC enables calling methods on not-yet-returned objects
- Schema evolution supports adding/removing fields without breaking existing clients
- Canonical encoding ensures identical bytes for identical data, enabling content-addressable storage

## Comparison with Similar Tools
- **Protocol Buffers** — Protobuf requires encode/decode steps; Cap'n Proto operates directly on the wire format
- **FlatBuffers** — FlatBuffers also offers zero-copy reads but lacks Cap'n Proto's RPC system and promise pipelining
- **MessagePack** — MessagePack is schema-less and requires parsing; Cap'n Proto is schema-based with zero-copy access
- **Apache Avro** — Avro is optimized for big data with schema registries; Cap'n Proto targets low-latency RPC
- **gRPC** — gRPC uses Protobuf with encode/decode; Cap'n Proto RPC avoids serialization overhead entirely

## FAQ
**Q: When should I use Cap'n Proto over Protobuf?**
A: Choose Cap'n Proto when serialization performance is critical, when you need promise pipelining RPC, or when working with memory-mapped files.

**Q: Does Cap'n Proto support schema evolution?**
A: Yes. You can add new fields, deprecate old ones, and promote fields to groups without breaking existing readers.

**Q: What languages are supported?**
A: Official C++ and community-maintained bindings for Rust, Go, Python, Java, C#, JavaScript, Ruby, and others.

**Q: Is Cap'n Proto production-ready?**
A: Yes. Cap'n Proto is used in Cloudflare Workers, Sandstorm, and other production systems handling significant traffic.

## Sources
- https://github.com/capnproto/capnproto
- https://capnproto.org
- https://capnproto.org/rpc.html

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Source: https://tokrepo.com/en/workflows/asset-b9565c3f
Author: Script Depot