Cap'n Proto for Rust

documentation

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 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

related repositories

• capnproto-rust (the repo you're looking at right now): Runtime library for dealing with Cap'n Proto messages.
• capnpc-rust: The 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-rust: Object-capability remote procedure call system.

examples

addressbook serialization, RPC

who's using capnproto-rust?

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

unimplemented / future work

• orphans
• dynamic reflection
• default values for pointer fields