We’ve been resoundingly busy with our Raft implementation after a brief period of calm in the early summer! I’ll be posting a few bits over the next weeks to help people both learn Rust, and learn about Raft!

As a result of Raft development two new crates have been put together for general consumption! Both of these are macro-centric and lead mainly by James.

A Useful Error Pattern

Working with Raft we discovered a useful error pattern for making use of things like try!(). Let’s quickly recap on why error handling can be a pain sometimes.

Let’s say you’re off happily using .read() on some file descriptor (maybe a TcpStream) and than performing some action which may also result in an error. So you write a function like so:

use std::io::Read;
fn read_and_action<R: Read>(reader: R) -> Result<String, _> {
	let buf = Vec::with_capacity(10);
    try!(reader.read(&mut buf));
    try!(String::from_utf8(buf))
}

But wait! There is a problem here! The first try!() and the second try!() have different errors that they might return. This won’t work. Even worse, in a sufficiently complex library or application there can be many Error types in play!

We identified this very early in Raft and came up with a eloquent solution: Create an Error composed of the various possible errors! James went ahead and actually made a fantastic macro that you can use to do the same easily!

/// A simple convienence type.
pub type Result<T> = std::result::Result<T, Error>;

// From crate: https://github.com/james-darkfox/rs-wrapped_enum-macro
wrapped_enum!{
    #[derive(Debug)]
    pub enum Error {
    	// Cap'n Proto Errors
        CapnProto(capnp::Error),
        // Cap'n Proto schema errors
        SchemaError(capnp::NotInSchema),
        // std::io errors
        Io(io::Error),
        // Our very own!
        Raft(RaftError),
    }
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            Error::CapnProto(ref error) => fmt::Display::fmt(error, f),
            Error::SchemaError(ref error) => fmt::Display::fmt(error, f),
            Error::Io(ref error) => fmt::Display::fmt(error, f),
            Error::Raft(ref error) => fmt::Debug::fmt(error, f),
        }
    }
}

With this machinery in place our previous example starts to work, fantastic! This has saved us so much headache and complication in our code. Want it? Click one of the badges below!

Scoped Logging

Okay, let’s take a minute to remember how frigging awesome the log crate is. If you’ve got a project, large or small, this crate is super handy for helping trace, debug, and log whatever you need and to do it only when needed! The best part about the log crate is that when logs are off it’s a simple noop.

One problem we identified, however, was that sometimes it was very difficult to identify the source of a log message. Asking “Did it come from foo or bar?” is only fun so many times.

James and Dan put together the scoped_log crate to help with this. Check out the documentation there for a usage example.

When we use this crate with tests (which makes it all that much more awesome!) we use the following macro.

/// Prepares the environment testing. Should be called as the first line of every test with the
/// name of the test as the only argument.
#[cfg(test)]
macro_rules! setup_test {
    ($test_name:expr) => (
        let _ = env_logger::init();
        push_log_scope!($test_name);
    );
}

Now our logs look like this:

INFO:raft::replica: test_apply_client_message: ElectionTimeout
INFO:raft::replica: test_election_3: ElectionTimeout
INFO:raft::replica: test_election_2: ElectionTimeout
INFO:raft::replica: test_apply_client_message: transitioning to Leader
INFO:raft::replica: test_election_5: ElectionTimeout
INFO:raft::replica: test_election_5: transitioning to Candidate
DEBUG:raft::replica: test_election_5: Replica { id: 3, state: Follower, term: 0, index: 0 }: RequestVoteRequest from Replica { id: 0, term: 1, latest_log_term: 0, latest_log_index: 0 }

Much more context that we barely have to tool. Fantastic! This crate even maintains the same noop characteristic of the log crate which it heavily relies on. Want it?

Travis & Cargo!

If you’ve read any of my other articles you probably know I love tooling and automation. After my Rust, Travis, and Github Pages article Huon created the fantastic travis-cargo tool! It’s incredible!

With its help we’ve moved to a sudo-less build and overall things work much better. I’ll talk more about out tooling and infrastructure in a later post!

Aside: Contributors!!!

We now have 3 whole code contributors to Raft! This includes Dan Burkert, James McGlashan, and myself (Ana Hobden). I’d like to thank both James and Dan for their awesome work, interest, and mentorship!