Comparison of Dinit with other supervision / init systems
=========================================================

This is intended to be an objective description of the differences between
Dinit and several other similar software packages. Due to a myriad of details,
it is difficult to provide a very meaningful comparison without going into
great detail (which this document does not). It does, however, serve as a
short survey of service supervision and init systems, and provides a starting
point for understanding the unique features of Dinit.


Systems without dependency management
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

A variety of init/supervision packages do not perform proper dependency management
of supervisions. By this I mean that, broadly speaking:
 - 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/)

I've read arguments that suggest dependency management isn't really needed: if a
service really does require another, then it fail in some way when the dependency
goes down, and should then go down itself; supervision of the service may try to
restart it, but should use an increasing delay to avoid continuously bouncing the
service up and down. In my opinion this could create unnecessary load, unnecessary
delay, and noise in logs that might make it more difficult to pinpoint the cause
of problems, though I'll acknowledge that in simpler setups these are unlikely to
be real problems.

Not all services will necessarily behave as is required for this type of service
management to work properly. An argument could be made that this is a fault of the
particular service / daemon, but practical considerations may be in play. 

The basic problem of starting login sessions only after system initialisation has
(at least partially) completed is naturally solved with a dependency-managing
solution; you can have the tty sessions (getty) depend on some other service unit
which, in turn, depends on the basic system initialisation services. In non-
dependency-handling managers this must usually be special-cased (eg an "inittab"
which is processed once all services have started).

With all the above in mind, I feel that dependency management allows generally
greater flexibility and control in how services are managed. While this might not
always be useful, and comes at a certain cost of complexity, I argue that it is at
least sometimes useful, and that the cost is not so high. However, to be fair,
many systems have successfully taken the simpler approach.


Nosh suite (http://homepage.ntlworld.com/jonathan.deboynepollard/Softwares/nosh.html)
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Another seemingly 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; 
"start", "stop" and "run" commands are individual scripts within a service
directory bundle). It provides a simple shell-like scripting language which can be
used (via the "nosh" interpreter) to implement service commands without requiring
the user of a full shell. Several "chain loading" utilities are provided to
allow running processes in a different directory, with a different user id, with
resource limits, etc.

It was originally designed for BSD systems but works on Linux too (i.e. is
portable). It does not provide a D-Bus interface. 

Compared to Dinit, the two most significant differences appear to be use of a
directory structure for service configuration (Dinit uses a combination of
directory structure and ini-style configuration file), and use of small chain
loading utilities to implement service parameters (Dinit has a wider range of
direct configuration options via the service description file).

Nosh seems to be a quite mature system with a range of features that makes it
appear competitive, feature-wise, in terms of system/servive management, with
Systemd - though without a lot of the feature-creep extras that can easily be
implemented separately.


OpenRC (Gentoo)
-=-=-=-=-=-=-=-
The OpenRC system used in Gentoo Linux is a dependency-managing service supervision
system with functionality that may similar in some respects to Dinit. According to
Wikipedia, it provides parallel startup of services (like Dinit), though this is
disabled by default. It is designed to be used in conjunction with a primary init
which handles system management and which defers to openrc at boot or shutdown to
bring services up or down.

Unusually, OpenRC does not run as a daemon itself; it terminates once it has
established service states. It has some support for integration with service
supervisors such as S6.

Service configuration is specified via a shell script, with the 'openrc-run'
interpreter (which makes certain special shell functions available, and interprets
shell variables once the service script completes. For performance reasons
metatdata extracted from the service scripts is cached in an alternative format.

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)
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
S6-RC provides a dependency-management system over the top of the S6 supervision
system. S6-RC requires compiling the complete set of service descriptions into a
database. Services are are configured via a directory structure, with a
one-parameter-per-file style, rather than a single service description file.

As well as services, S6-RC has the concept of service "bundles", named groups
of services that can be started/stopped as a group.

New services cannot be added once the system is in operation, and service
definitions cannot be changed in general, except by re-compiling the database;
S6-RC will then start any new services as required (and stop any processes no longer
considered part of an active service).

S6-RC in general seems to follow the 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 to S6-RC, Dinit does not requires compiling service definitions, instead
loading and parsing them on-the-fly. Also, Dinit incorporates service
supervision and management into a single process (and does not require one
supervisory process per service). On the other hand, the Dinit process is
somewhat more complex than any of the individual S6-RC components.

S6-RC nicely manages chaining of service standard input/output, facilitating
setting up a logging chain where a logging process consumes the output of a
service, and either can be restarted while losing only minimal (if any)
output from the logs.

It appears that S6-RC supports only hard dependencies: that is if, a service depends
on another then that service must start, and stay running. Dinit supports a number
of dependency types including "soft" dependencies which allow the dependency to
stop or fail without necessarily stopping the dependent.


Systemd
-=-=-=-
Systemd is probably the most widely used init system on Linux as in recent years.
Compared to Dinit, Systemd provides a range of functionality such as:
   - setting priority and various other attributes of the service process that
     Dinit does not support [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
   - private tmp directories for services / login sessions
   - 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 or another service manager, and even in the
case of Systemd, some of the functionality is not part of the core process (the
actualy systemd binary).

In Systemd terminology, it manages "units" of which services are one type. In practice
this is an issue only of nomenclature; Dinit "services" and Systemd "units" are, I think,
essentially the same thing.

Systemd running in "system" mode does not properly support running with a PID other than
one [1]. That is, it must replace /sbin/init. Systemd can however be run in "user" mode
where it (most likely) provides roughly the same level of functionality as Dinit, though
in a more complex package.

The Systemd interdependence graph is more complex than for Dinit and most other
dependency-handling service managers: a service can conflict with another service, meaning
that starting one causes the other to stop and vice versa. Systemd implements shutdown
via a special "shutdown" unit which conflicts with other services so that they stop
when the shutdown is "started". Other service managers typically do not implement shutdown
as a service but as a special action; they then don't need to support conflicting
services.

The "shutdown" unit is just one example of a "special" service. Systemd has several such
services, for various purposes, including for tight integration with D-Bus (Systemd
exposes a D-Bus API, and contains its own implementation of the D-Bus protocol).

Systemd makes no attempt to be portable to operating system kernels other than Linux.
The maintainers have stated that they 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


Cinit (http://www.nico.schottelius.org/software/cinit; defunct)
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
An obscure init system which apparently offers portability and dependency
management, just as Dinit does. Development appears to have ceased some
time ago, unfortunately.


InitNG (http://initng.org/trac; defunct)
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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). Development may have ceased (website is now showing
Japanese text which I am unable to read) although there are what looks like
maintenance commits from 2017 in the Github repository at
https://github.com/initng/initng.


Upstart (Ubuntu; http://upstart.ubuntu.com)
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
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.
