# Cap'n Proto — Zero-Copy Serialization and RPC System

> A data interchange format and RPC protocol designed for zero-copy reads, created by the author of Protocol Buffers v2.

## Install

Save in your project root:

# Cap'n Proto — Zero-Copy Serialization and RPC System

## Quick Use
```bash
# Install on Debian/Ubuntu
sudo apt-get install capnproto libcapnp-dev
# Compile a schema
capnp compile -oc++ myschema.capnp
```

## Introduction
Cap'n Proto is a serialization format and RPC system that eliminates the encode/decode step found in Protocol Buffers and similar systems. The wire format is the same as the in-memory format, enabling zero-copy reads that can significantly reduce latency and CPU overhead in performance-sensitive applications.

## What Cap'n Proto Does
- Serializes structured data with zero encoding or decoding overhead
- Provides a built-in RPC system with promise pipelining to reduce round trips
- Generates type-safe bindings for C++, Rust, Go, Python, Java, and other languages
- Supports schema evolution with backwards and forwards compatibility guarantees
- Handles mmap-friendly data files that can be read directly without parsing

## Architecture Overview
Cap'n Proto stores data in a pointer-based format that matches its in-memory layout. Messages are composed of segments containing structs, lists, and capabilities. The RPC layer uses an object-capability model where remote references are passed as capabilities, enabling promise pipelining: a caller can chain method calls on a not-yet-returned result without waiting for each round trip.

## Self-Hosting & Configuration
- Install the compiler and runtime from system packages or build from source
- Write .capnp schema files to define message types and RPC interfaces
- Run the capnp compiler with language-specific code generator plugins
- Link against the Cap'n Proto runtime library in your build system
- Use the KJ async framework (bundled) for non-blocking RPC servers in C++

## Key Features
- Zero-copy deserialization means reading a field is a pointer dereference, not a parse
- Promise pipelining lets you call methods on future results before they arrive
- Time-travel RPC reduces multiple sequential calls into a single round trip
- Schema evolution supports adding fields, renaming types, and reordering without breaking wire compatibility
- Canonical encoding mode ensures deterministic output for hashing and signing

## Comparison with Similar Tools
- **Protocol Buffers** — widely adopted but requires encode/decode steps; Cap'n Proto was created as its successor by the same author
- **FlatBuffers** — also zero-copy but lacks a built-in RPC system and promise pipelining
- **MessagePack** — schemaless binary format, compact but without type safety or zero-copy reads
- **Apache Avro** — schema-based with strong Hadoop integration, but requires full deserialization

## FAQ
**Q: Who created Cap'n Proto?**
A: Kenton Varda, who previously designed Protocol Buffers v2 at Google. He now works on Cloudflare Workers.

**Q: Is Cap'n Proto compatible with Protocol Buffers?**
A: No. They are separate formats with different wire encodings. However, schema concepts are similar, and migration tools exist.

**Q: What languages are supported?**
A: The reference implementation is C++. Community-maintained bindings exist for Rust, Go, Java, Python, TypeScript, C#, and others.

**Q: When should I use Cap'n Proto over Protobuf?**
A: When you need minimal serialization overhead, promise-pipelined RPC, or want to mmap data files directly without parsing.

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

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Author: AI Open Source