baf: big ass fan

A pub/sub system built on top of the /dev/fanout kernel module

What is /dev/fanout?

/dev/fanout is a kernel module that makes a linux character device act as a one to many ring buffer. This is a great building block for an efficient pub/sub mechanism.

How does baf work?

baf has 3 main components: nixos module, pubmsg, and submsg

nixos module

The nixos module sets up a oneshot systemd unit that statically sets up the system. You can import the module into your NixOS config and use it by enabling it and then setting a list of topics:

{
  baf.services.baf-setup = {
    enable = true;
    topics = [ "topic1" "topic2" ];
  };
}

For a more detailed example look at baf-test inside of the flake.nix in this repo.

NOTE: A baf-test topic will always be added for debugging

pubmsg

pubmsg <topic-name> is the simple program that a publisher can use. It will use the topic name to find the correct /dev/baf symlink and then will pipe any data on stdin to the /dev/baf device. If the topic is already in use or /dev/baf file doesn't exist it will exit.

Example usage:

service-x | pubmsg test1

This will take the stdout of service-x and send it to any program reading off of the /dev/baf character device.

For a more detailed example look at baf-test inside of the flake.nix in this repo.

submsg

submsg is the program a subscriber can use to get data. With no args, submsg will subscribe to every topic and feed data to a publisher via stdin. A user can provide a list of topics, submsg [TOPICS], to only get data from those /dev/baf character devices. A user can also use the -e flag to provide a list of topics to ignore.

Example

submsg topic1 topic2 | service-y

This will take data from topic1 and topic2 and send it to service-y on stdin.

Example

submsg -e topic1 | service-y

This will take data from every topic except for topic1 send it to service-y on stdin.

For a more detailed example look at baf-test inside of the flake.nix in this repo.

TODO

• remove the need for statically defining the system
  • create the /dev/baf devices dynamically when pubmsg or submsg gets ran
• improve submsg throughput
• add tests for correctness and perf