Cap'n Proto for Rust

documentation

For the latest news, see the capnproto-rust blog.

Introduction

Cap'n Proto is a type system for distributed systems.

With Cap'n Proto, you describe your data and interfaces in a schema file, like this:

@0x986b3393db1396c9;

struct Point {
    x @0 :Float32;
    y @1 :Float32;
}

interface PointTracker {
    addPoint @0 (p :Point) -> (totalPoints :UInt64);
}

You can then use the capnp tool to generate code in a variety of programming languages. The generated code lets you produce and consume values of the types you've defined in your schema.

Values are encoded in a format that is suitable not only for transmission over a network and persistence to disk, but also for zero-copy in-memory traversal. That is, you can completely skip serialization and deserialization! It's in this sense that Cap'n Proto is "infinity times faster" than alternatives like Protocol Buffers.

In Rust, the generated code for the example above includes a point::Reader<'a> struct with get_x() and get_y() methods, and a point::Builder<'a> struct with set_x() and set_y() methods. The lifetime parameter 'a is a formal reminder that point::Reader<'a> and point::Builder<'a> contain borrowed references to the raw buffers that contain the encoded messages. Those underlying buffers are never actually copied into separate data structures.

The generated code for the example above also includes a point_tracker::Server trait with an add_point() method, and a point_tracker::Client struct with an add_point_request() method. The former can be implemented to create a network-accessible object, and the latter can be used to invoke a possibly-remote instance of a PointTracker.

Features

• tagged unions
• generics
• protocol evolvability
• canonicalization
• Result-based error handling
• no_std support
• no-alloc support
• reflection

Crates

capnp	Runtime library for dealing with Cap'n Proto messages.
capnpc	Rust code generator plugin, including support for hooking into a build.rs file in a cargo build.
capnp-futures	Support for asynchronous reading and writing of Cap'n Proto messages.
capnp-rpc	Object-capability remote procedure call system with "level 1" features.

Examples

addressbook serialization, RPC

Who is using capnproto-rust?

• Sandstorm's raw API example app and collections app
• juice
• fractalide
• combustion-engine

Unimplemented / Future Work

• orphans