einhyrningsins(1) -- 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.

 * `-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
Copyright 2016  Bryan Newbold
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.

## SEE ALSO
einhorn(1), einhyrningsinsctl(1), socket(7)