Foca is a building block for your gossip-based cluster discovery. It's
a small no_std + alloc crate that implements the SWIM protocol along
with its useful extensions (SWIM+Inf.+Susp.).
Project:
The most notable thing about Foca is the fact that it does almost nothing. Out of the box, all it gives is a reliable and efficient implementation of the SWIM protocol that's transport and identity agnostic.
Knowledge of how SWIM works is helpful but not necessary to make use
of this library. Reading the documentation for the Message enum
should give you an idea of how the protocol works, but the paper is
a very accessible read.
Foca is designed to fit into any sort of transport: If your network
allows peers to talk to each other you can deploy Foca on it.
Not only the general bandwidth requirements are low, but you also
have full control of how members identify each other (see
./examples/identity_golf.rs) and how messages are encoded.
Please take a look at ./examples/foca_insecure_udp_agent.rs. It
showcases how a simple tokio-based agent could look like and lets
you actually run and see Foca swimming.
~~~ $ cargo run --features agent --example focainsecureudpagent -- --help focainsecureudpagent
USAGE:
focainsecureudp_agent [OPTIONS]
FLAGS: -h, --help Prints help information -V, --version Prints version information
OPTIONS:
-a, --announce
ARGS:
So you can start the agent in one terminal with
./foca_insecure_udp_agent 127.0.0.1:8000 and join it with as many others
as you want with using a different BIND_ADDR and --announce to a
running instance. Example:
./foca_insecure_udp_agent 127.0.0.1:8001 -a 127.0.0.1:8000.
The agent outputs some information to the console via tracing's
subscriber. It defaults to the INFO log level and can be customized
via the RUST_LOG environment variable using tracing_subscriber's
EnvFilter directives.
Every feature is optional. The default set will always be empty.
std: Adds std::error::Error support and implements foca::Identity
for std::net::SocketAddr*.tracing: Instruments Foca using the tracing crate.serde: Derives Serialize and Deserialize for Foca's public
types.bincode-codec: Provides BincodeCodec, a serde-based codec type
that uses bincode under the hood.postcard-codec: Provides PostcardCodec a serde-based, no_std
friendly codec that uses postcard under the hood.Only for examples:
identity-golf: For ./examples/identity_golf.rsagent: For ./examples/foca_insecure_udp_agent.rsWhen writing this library, the main goal was having a simple and small core that's easy to test, simulate and reason about; It was mostly about getting a better understanding of the protocol after reading the paper.
Sticking to these goals naturally led to an implementation that doesn't
rely on many operating system features like a hardware clock, atomics
and threads, so becoming a no_std crate (albeit still requiring heap
allocations) was kind of a nice accidental feature that I decided to
commit to.
I avoided looking at memberlist until I was satisfied with my own implementation. Since then I did take a non-thorough look at it:
memberlist supports custom broadcasts, which is a very cool feature
for complex service discovery scenarios, so now Foca has support
for disseminating user data too (see BroadcastHandler
documentation) :-)
It has a stream-based synchronization mechanism (push-pull) that's
used for joining and periodic merging state between members: It's
way beyond Foca's responsibilities, but it's a very interesting idea,
so I've exposed the Foca::apply_many method which enables code
using Foca to do a similar thing if desired.
Its configuration parameters change based on (current) cluster
size. It's super useful for a more plug-and-play experience, so
I want introduce something along those lines in the future, likely
by pulling Config into Foca as a trait implementation.
Foca is very focused on doing almost nothing, so it's likely that some things will end up on separate crates. But, in no particular order, I want to:
Provide a more plug-and-play experience, closer to what memberlist gives out of the box.
Make Foca run as a library for a higher level language. I'm not even sure I can take it that far, so sounds like fun!
Deliver a (re)usable simulator. Right now I've been yolo-coding one just to give me more confidence on what's implemented right now, but what I want is something that you can: Slap your own identity, codec and configuration; Set network parameters like TTL, loss rate, bandwidth; And then simulate production behavior (rolling restarts, partitions, etc) while watching convergence stats. This is a ridiculous amount of work.
Actually demonstrate a running Foca with no_std constraints; I
don't have access to devices to play with at the moment, so
it's been difficult to find motivation to pursue this.
Unless explicitly stated otherwise, all work is subject to the terms of the Mozilla Public License, version 2.0.
Files inside the examples/ directory are dedicated to the Public
Domain.