muinit
Lightweight subprocess supervisor:
- minimal 'init', e.g. for docker containers
- forwards signals to subprocesses
- reaps zombie subprocesses
- gracefully terminates all subprocesses after one exited
Though several sophisticated subprocess supervisors exist (e.g. supervisord, these are often heavy and based on scripting languages. More lightweight variants of miss some features useful for Docker containers. See the great blog article "Choosing an init process for multi-process containers" for what such a supervisor should implement.
Installation
Download the muinit binary from the releases
page or see below
to build yourself.
Building
Just build the muinit binary using
make
Usage
Just call the muinit binary with the subprocess commands and their
arguments separated by ---, e.g. muinit --- dnsmasq -k --- sshd -D.
Usage:
muinit [OPTIONS] --- COMMANDS
OPTIONS
-h show help message
-k SIGNALS signals to iterate over in subprocess termination
(comma-separated list of their numbers)
default: SIGTERM,SIGKILL
-s SIGNALS signals to forward to subprocesses (comma-separated numbers)
default: SIGINT
-t TIMEOUT set subprocess termination stage timeout in seconds
default: 2s
COMMANDS
Subprocesses to be spawned and their arguments are given after the
first '---' and are separated by '---' (do not include quotation marks).
Though muinit emulates an init session, try not to have subprocesses go
into background ('daemonize') if possible.
SIGNAL FORWARDING
Signals given via the `-s' option (and that can be caught) are forwarded
to subprocesses. Special cases are SIGALRM, which is used by muinit itself,
and SIGTERM, which resets the termination steps and is then forwarded.
The SIGNALS option values must be lists of comma-separated numbers of the
signals (run `kill -L' to see a list)
SUBPROCESS TERMINATION
Once a subprocess terminates (failing or successfully), muinit tries to
gracefully terminate the other subprocesses. This is done in several
successive steps until all children have terminated. The steps are defined
by the signal send in each respective step as given via the `-k' option
(default: SIGTERM,SIGKILL). The timeout to wait after each step before
trying the next one can be given via the `-t' option (default: 2s)
EXIT STATUS
Internal errors cause an exit status of 1. Otherwise the exit status equals
that of the first failed subprocess or 0 if all subprocesses succeed.
Example Dockerfile snippet
I use muinit as a lightweight supervisor in Docker containers to
logically boundly processes into the same container. For example, for
setting up a simple container with a static website and SFTP access to
edit it, I include in the respective Dockerfile:
ENTRYPOINT [ "/usr/sbin/muinit" ]
CMD [ \
"---", "/usr/sbin/lighttpd", "-D", "-f", "/etc/lighttpd/lighttpd.conf", \
"---", "/usr/sbin/dropbear", "-EFjksw", "-I", "120", "-G", "user", "-c", "/usr/lib/ssh/sftp-server" \
]