einhyrningsins(1) - graceful restarts for socket-based daemons

einhyrningsins(1)
einhyrningsins(1)

NAME

einhyrningsins - graceful restarts for socket-based daemons

SYNOPSIS

einhyrningsins [OPTIONS] [--] PROGRAM [PROGRAM_ARGS]

DESCRIPTION

einhyrningsins is a socket multiplexer featuring graceful restarts. It runs multiple copies of a child program, each of which are passed a shared socket (or multiple shared sockets) to bind(2) to and accept(2) connections from. Graceful, rolling restarts enable updates of the child program with zero downtime and no dropped connections.

This program requires special support in the child program to achive the graceful restarts (aka, exiting only after all connection close) and to be able to bind to inherited file descriptors (as indicated by environment variables). Child programs must also be able to run in parallel: for example, each copy must not try to write to the same output file without some form of locking.

OPTIONS

-n, --number COUNT: How many child processes to spawn.
-b, --bind ADDR:PORT[,OPT...]: Socket(s) to bind to. OPT specifies flags to be set on the socket. Options are n for non-blocking (O_NONBLOCK) and r for re-using addresses (SO_REUSEADDR). Eg, for both options, could pass -b 127.0.0.1:1234,r,n. This argument can be repeated.
-4, --ipv4-only: Only accept IPv4 connections
-6, --ipv6-only: Only accept IPv6 connections
-h, --help: Print a help menu
--version: Print program version
-v, --verbose: More verbose logging and output
--syslog: Enables logging via syslog(2) (for WARN and above).
-m, --manual: Enable manual (explicit) acknowledge mode, in which each child program must connect to the master's control socket and "ACK" within a graceperiod, or it will be considered unhealthy and get restarted.
--drop-env-var VAR: Clears the given variable from the child's environment. All other variables are passed through by default. This argument can be repeated.
-d, --socket-path PATH: Where to create the control socket (default: /tmp/einhorn.sock).
-r, --retries COUNT: How many times to attempt re-spawning before giving up.

CHILD API

einhyrningsins creates children by fork(1)-ing a new process and execve(1)-ing to run the proces itself. For every socket specified by a -b flag, a socket is bound in the main einhyrningsins process and then explicitly flagged to be passed on to the child processes. This means the child process will already have a valid file descriptor bound to each of the shared sockets. The file descriptor numbers are passed via the following environment variables:

EINHORN_FD_COUNT: How many sockets have been passed.
EINHORN_FD_<NUM>: One evironment for each socket with NUM from 0 to EINHORN_FD_COUNT-1.

When einhyrningsins is run in manual mode, each child process should connect to the control socket (at the UNIX path given by the EINHORN_SOCK_PATH variable) and write(2) a newline-terminated string like the following, containing the child's PID number:

{"command":"worker:ack", "pid":<PID>}

HISTORY

einhyrningsins is a partially-comparible re-implementation of einhorn(1) (a Ruby program) in Rust. Einhorn itself derived from Unicorn.

The word "einhyrningsins" is Icelandic for unicorn.

STATUS

einhyrningsins is a for-fun hobby project. It is not feature complete, fully documented, or tested.

COPYRIGHT

License GPLv3+: GNU GPL version 3 or later http://gnu.org/licenses/gpl.html. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.