dinit/doc/COMPARISON

Comparison of Dinit with other supervision / init systems
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This is intended to be an objective description of the differences between Dinit
and several other similar software packages. I started writing this with the
intention of justifying Dinit's existence, but don't know feel that this
document achieves that goal. It does, however, serve as a short survey of
service supervision and init systems.


Systems without dependency management
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A variety of init/supervision packages do not perform proper dependency management
of supervisions. By this I mean that:
 - starting a service should automatically start any services that the former
   requires (and wait, if appropriate, until they have started)
 - likewise, stopping a service should automatically stop any dependent services.

Dinit (and various other packages) perform dependency management. The following
packages do not:

 * Daemontools (http://cr.yp.to/daemontools.html)
 * Epoch (http://universe2.us/epoch.html)
 * Finit (http://github.com/troglobit/finit)
 * Minit (http://www.fefe.de/minit)
 * Perp (http://b0llix.net/perp/)
 * Runit (http://smarden.org/runit/)


Cinit (http://www.nico.schottelius.org/software/cinit)
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An obscure init system which apparently offers portability and dependency
management, just as Dinit does.


InitNG (http://initng.org/trac)
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A highly modular init system which apparently offers dependency management
(as Dinit does). Portability status is unclear; may be Linux-only (Dinit
strives for portability).


Nosh (http://homepage.ntlworld.com/jonathan.deboynepollard/Softwares/nosh.html)
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Another seeminly modular init system offering dependency management and socket
activation, with BSD licensing. Service configuration is expressed through
directory structure (symbolic links represent service dependencies, etc). It
provides a simple shell-like scripting language with a fast interpreter, avoiding
the need for standard shell scripts. It was originally designed for BSD systems but
works on Linux too (i.e. is portable). It does not provide a D-Bus interface.
Seemingly, this package addresses most of the various complaints that have been
levelled against SystemD. Compared to Dinit, it favors chain-loading of smaller
utilities to provide even more basic functionality. By comparison Dinit tends to
avoid starting additional processes by design, expresses configuration in files
and tracks state internally.


OpenRC (Gentoo)
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The OpenRC system used in Gentoo Linux is a dependency-managing service supervision
system with funcionality similar in some respects to Dinit. It provides parallel
startup of services (like Dinit). Compared to Dinit:
 - OpenRC is configured using, and relies largely on, shell scripts
 - OpenRC cannot be used as a direct replacement for /sbin/init; instead it is
   intended that OpenRC be started by init.
 - Although the build system seems to have some support for BSD OSes, it did not
   build successfully on OpenBSD when tested (revision 33d3f33).


S6-RC (http://skarnet.org/software/s6-rc/s6-rc.html)
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S6-RC provides a dependency-management system over the top of the S6 supervision
system. Apparently it maintains most state on disk. S6-RC requires compiling the
complete set of service descriptions into a database. New services apparently
cannot be added once the system is in operation.

S6 in general follows a philosophy of breaking up functionality into smaller
parts and implementing these smaller parts as separate programs, wherever
practical. Thus, process supervision, log file management, and service management
are all separate programs.

In contrast, Dinit does not requires compiling service definitions, and allows
new services to be added after the system starts. Also, Dinit houses a much larger
body of functionality in a single executable program.


Systemd
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Systemd is probably the most widely used init system on Linux as of the last year
or two. Compared to Dinit:
 - Systemd provides a range of functionality not provided by Dinit, such as:
   - setting UID, priority, and other attributes of the service process. Dinit does
     not support this yet.
   - seat/session management
   - syslogd replacement (or at least, partial replacement)
   - ability to run tasks at certain times
   - inetd replacement (lazily launch services to handle connection to IP ports)
   - asynchronous filesystem check/mount
   - control group (cgroup) / container management
   - system time management
   Some of this functionality can be found in other daemons/packages which can be
   be used to supplement the functionality of Dinit.
 - Systemd running in "system" mode does not properly support running with a PID
   other than 1 [1]. That is, it must replace /sbin/init. Systemd can however be run in
   "user" mode where it (most likely) provides the same or greater level of functionality
   as Dinit, though in a more complex package.
 - Systemd exposes a D-Bus API. Dinit eschews D-Bus in favor of a compact custom protocol.
 - Systemd makes no attempt to be portable to operating system kernels other than
   Linux. The maintainers consider it infeasible to port to non-Linux-based OSes
   and will refuse patches designed to do so [2]. Dinit, by comparison, strives to be
   portable.

[1] http://freedesktop.org/software/systemd/man/systemd.html as at 18/11/2015
[2] http://freedesktop.org/wiki/Software/systemd/InterfacePortabilityAndStabilityChart/
    as at 18/11/2015


Upstart (Ubuntu; http://upstart.ubuntu.com)
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Upstart does not provide real dependency management; instead "events" (including services
starting or stopping) can be specified to trigger start/stop of other services. This is
backwards from the Dinit approach (and that taken by most dependency-managing supervision
systems) which allow the dependencies of a service to be specified declaratively. That is,
if service A depends on service B, Upstart is configured so as to start B whenever A starts
(and it's not possible, or at least not trival, to start A without also starting B).

Upstart apparently offers a D-Bus interface. Dinit eschews D-Bus in favour of a simple
custom control protocol.