# Cap'n Proto — Insanely Fast Data Serialization

> A data interchange format and RPC system that uses zero-copy reads with no parsing or unpacking step, created by the author of Protocol Buffers v2.

## Install

Save as a script file and run:

# Cap'n Proto — Insanely Fast Data Serialization

## Quick Use
```bash
# Install on Debian/Ubuntu
sudo apt install capnproto libcapnp-dev
# Or build from source
git clone https://github.com/capnproto/capnproto.git
cd capnproto/c++ && cmake -B build && cmake --build build && sudo cmake --install build
```

```capnp
# addressbook.capnp
struct Person {
  name @0 :Text;
  age  @1 :UInt32;
}
```
```bash
capnp compile -oc++ addressbook.capnp
```

## Introduction
Cap'n Proto is a data serialization and RPC framework designed for extreme speed. Unlike Protocol Buffers or JSON, it uses a wire format that can be read directly from memory without a parsing step. This zero-copy approach means serialization is effectively free: the in-memory representation is the wire format.

## What Cap'n Proto Does
- Serializes structured data with zero-copy reads — no decode step needed
- Generates type-safe code for C++, Rust, Go, Python, Java, and other languages
- Provides a built-in RPC system with promise pipelining to reduce round trips
- Supports schema evolution with guaranteed backward and forward compatibility
- Handles large messages efficiently via memory-mapped files

## Architecture Overview
Cap'n Proto messages are laid out in memory as a tree of segments with pointer-based navigation. Reading a field is a pointer dereference, not a parse. The schema compiler (`capnp compile`) generates accessor classes that overlay the raw bytes. The RPC layer uses promise pipelining: when a call returns a reference, you can make follow-up calls on it before the first one resolves, and Cap'n Proto batches them over the wire.

## Self-Hosting & Configuration
- Build the C++ runtime and compiler with CMake (requires a C++17 compiler)
- Schema files (`.capnp`) define your data structures and RPC interfaces
- Compile schemas to target languages with `capnp compile -o<lang>`
- RPC server can run over TCP or Unix sockets with built-in TLS support
- Embed as a library or use the `capnp` CLI for serialization debugging

## Key Features
- Zero-copy design: reading a 1 MB message takes constant time regardless of schema complexity
- Promise pipelining eliminates multiple network round trips in RPC workflows
- Canonical encoding for deterministic hashing and comparison
- Schema evolution rules that prevent breaking changes at compile time
- MIT licensed with no runtime dependencies

## Comparison with Similar Tools
- **Protocol Buffers** — requires parsing/unpacking; Cap'n Proto is zero-copy
- **FlatBuffers** — also zero-copy but no built-in RPC and less mature schema evolution
- **MessagePack** — schema-less binary format; faster to get started but no type safety
- **Thrift** — includes RPC but serialization requires copying and is slower
- **gRPC** — HTTP/2-based RPC with Protobuf; Cap'n Proto RPC is lower-level and faster

## FAQ
**Q: How does zero-copy serialization work?**
A: The in-memory layout matches the wire format. When you build a message, the bytes you write are exactly what gets sent. The receiver reads fields directly from the buffer without deserialization.

**Q: Can I use Cap'n Proto with languages other than C++?**
A: Yes. There are official or community-maintained implementations for Rust, Go, Python, Java, C#, TypeScript, and others.

**Q: How does schema evolution compare to Protobuf?**
A: Cap'n Proto uses numbered fields (like Protobuf) and allows adding new fields, promoting fields to groups, and deprecating old ones. The compiler enforces compatibility rules.

**Q: Is Cap'n Proto suitable for storage, not just networking?**
A: Yes. The zero-copy format works well for memory-mapped files, making it effective for databases, caches, and on-disk indexes.

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

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Source: https://tokrepo.com/en/workflows/asset-a24c2585
Author: Script Depot