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#1 in #gossip
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Used in 2 crates
Foca: Cluster membership discovery on your terms
Foca is a building block for your gossip-based cluster discovery. It's
alloc crate that implements the SWIM protocol along
with its useful extensions (
- Git Repository: https://github.com/caio/foca
- Issue tracker: https://github.com/caio/foca/issues
- CI: https://github.com/caio/foca/actions/workflows/ci.yml
- Packages: https://crates.io/crates/foca
- Documentation: https://docs.rs/foca
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
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
showcases how a simple tokio-based agent could look like and lets
you actually run and see Foca swimming.
$ cargo run --features std,tracing,postcard-codec --example foca_insecure_udp_agent -- --help foca_insecure_udp_agent USAGE: foca_insecure_udp_agent [OPTIONS] <BIND_ADDR> FLAGS: -h, --help Prints help information -V, --version Prints version information OPTIONS: -a, --announce <announce> Address to another Foca instance to join with -f, --filename <filename> Name of the file that will contain all active members -i, --identity <identity> The address cluster members will use to talk to you. Defaults to BIND_ADDR ARGS: <BIND_ADDR> Socket address to bind to. Example: 127.0.0.1:8080
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
--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
RUST_LOG environment variable using tracing_subscriber's
Every feature is optional. The
default set will always be empty.
tracing: Instruments Foca using the tracing crate. High-level protocol interactions are emited as
DEBUGtraces, more details can be exposed with the
TRACElevel. No other levels are emitted.
Deserializefor Foca's public types.
BincodeCodec, a serde-based codec type that uses bincode under the hood.
no_stdfriendly codec that uses postcard under the hood.
When 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
Comparison to memberlist
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
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_manymethod 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
Configinto Foca as a trait implementation.
- The paper SWIM: Scalable Weakly-consistent Infection-style Process Group Membership Protocol
- HashiCorp's memberlist
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