#swim #gossip #service-discovery #memberlist

no-std foca

Gossip-based cluster membership discovery, based on SWIM

15 breaking releases

new 0.16.0 Oct 1, 2023
0.14.0 Sep 3, 2023
0.13.0 Jul 9, 2023
0.11.0 Mar 22, 2023
0.1.0 Nov 4, 2021

#1 in #gossip

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Used in 2 crates

MPL-2.0 license

4.5K SLoC

Foca: Cluster membership discovery on your terms

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.).



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 std,tracing,postcard-codec --example foca_insecure_udp_agent -- --help

    foca_insecure_udp_agent [OPTIONS] <BIND_ADDR>

    -h, --help       Prints help information
    -V, --version    Prints version information

    -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

    <BIND_ADDR>    Socket address to bind to. Example:

So you can start the agent in one terminal with ./foca_insecure_udp_agent 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 -a

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.

Cargo Features

Every feature is optional. The default set will always be empty.

  • std: Adds std::error::Error support, implements foca::Identity for std::net::SocketAddr* and exposes Config::new_lan and Config::new_wan
  • tracing: Instruments Foca using the tracing crate. High-level protocol interactions are emited as DEBUG traces, more details can be exposed with the TRACE level. No other levels are emitted.
  • 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.


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 commit 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 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.



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.


~17K SLoC