Cap'n Proto — Insanely Fast Serialization and RPC

Introduction

Cap'n Proto is a serialization framework where the in-memory representation IS the wire format. There is no encode/decode step — data is read directly from the serialized bytes via pointer arithmetic. This makes it faster than Protocol Buffers for many workloads, especially when only a subset of fields are accessed.

What Cap'n Proto Does

• Serializes structured data with zero-copy reads (no parsing or unpacking required)
• Provides a built-in RPC system with promise pipelining to reduce network round trips
• Defines schemas in a dedicated language with forward/backward compatibility guarantees
• Generates type-safe bindings for C++, Rust, Go, Python, Java, and JavaScript
• Supports memory-mapped files for instant random access to large datasets

Architecture Overview

Cap'n Proto uses a pointer-based layout where each struct and list is stored at a fixed position with inline pointers to child objects. Reading a field is a single pointer dereference with no deserialization. The RPC layer uses object capabilities and promise pipelining: a client can call a method on a not-yet-returned result, and the framework batches the calls into a single network round trip.

Self-Hosting & Configuration

• Install the capnp compiler from your package manager or build from source
• Add the runtime library as a dependency (libcapnp for C++, capnp crate for Rust)
• Define schemas in .capnp files and compile to target language bindings
• Use packed encoding for wire transfer to reduce size by 50-90% for sparse messages
• Integrate with CMake or Bazel using provided build rules

Key Features

• Zero-copy deserialization: read fields directly from the byte buffer with no allocation
• Promise pipelining in the RPC layer eliminates sequential round trips for chained calls
• Schema evolution supports adding/removing fields without breaking existing readers
• Time-travel RPC: call methods on future results before they arrive
• Canonical encoding enables content-addressable storage and deterministic hashing

Comparison with Similar Tools

• Protocol Buffers — requires encode/decode steps; Cap'n Proto skips this entirely for faster reads
• FlatBuffers — also zero-copy but lacks an RPC system and promise pipelining
• MessagePack — schema-less binary format; Cap'n Proto provides type safety and faster random access
• Apache Thrift — includes RPC but encoding is not zero-copy; Cap'n Proto is faster for read-heavy workloads
• gRPC — HTTP/2-based RPC on top of protobuf; Cap'n Proto RPC is lower-level with less overhead

FAQ

Q: When should I choose Cap'n Proto over Protocol Buffers? A: When read performance matters more than wire size, or when you need promise pipelining in your RPC layer. For simple request/response APIs, protobuf/gRPC has a larger ecosystem.

Q: Is Cap'n Proto suitable for network protocols? A: Yes. Use packed encoding for wire transfer and the built-in RPC framework for structured communication.

Q: Can I use Cap'n Proto without the RPC system? A: Absolutely. The serialization format works standalone for storage, IPC, or any data interchange scenario.

Q: How does schema evolution work? A: Fields are identified by number. New fields can be added with new numbers, and removed fields become unknown data that is preserved on read-modify-write.

Sources

• https://github.com/capnproto/capnproto
• https://capnproto.org