user/app/_launch_intro.dox

/*
 * Copyright 2017 Haiku, Inc. All rights reserved.
 * Distributed under the terms of the MIT License.
 *
 * Authors:
 *		Axel Dörfler, axeld@pinc-software.de
 */


/*!
	\page launch_intro Introduction to the Launch Daemon

In general, you should name your jobs/services after the application signature
(if possible), and name the configuration files accordingly (in each case
without the "application/" prefix). Alternatively, you may use the name of your
package as file name. If you do so, you may want to include the version, too,
but do not add the version to the name of a job.

A "service" is re-started automatically by the launch_daemon as soon as it's
quit (or has crashed). Use a "job" instead, if you don't want this behavior.

Let's start with a simple example:
<code>
service x-vnd.MyGreatServer {
}
</code>
This will register a service named MyGreatServer, and assumes it uses a BServer
based application object. It will automatically launch the server either
during system boot (when you place your configuration file in
<code>/system/data/launch/</code>), or after user login (when it's in
<code>~/config/data/launch/</code>) via its signature (which it constructs
using the job's name). Furthermore, it will create a port for the server, so
that it can be immediately be talked to.

Unfortunately, BServer is private as of now, and you didn't want to make a
great effort to subclass it. In this case, you have to notify the launch_daemon
of this fact by adding a "legacy" to that configuration:
\code
service x-vnd.MyGreatServer {
	legacy
}
\endcode
If you want to save the cycles for querying for your server, you can also
directly specify the file that should be launched; in this case, the job name
is just a name. This could look like this:
\code
service x-vnd.MyGreatServer {
	launch /path/to/my/great/server
	legacy
}
\endcode
This method also allows you to add additional launch arguments like this:
\code
service x-vnd.MyGreatServer {
	launch /path/to/my/great/server --debug-mode
	legacy
}
\endcode
If you do not want to enable the service by default, but only provide a
template or basic configuration for the user, you can disable it like this:
\code
service x-vnd.MyGreatServer {
	launch /path/to/my/great/server --debug-mode
	legacy
	disabled
}
\endcode
You can then override this in the settings by redefining the service, and
overwrite the parts you want to change. In this case, it might just be:
\code
service x-vnd.MyGreatServer {
	disabled false
}
\endcode
The rest of the configuration will stay intact.

If you only want to launch your application depending on the current
environment, you can define conditions which must be met to launch your
application at all, and events which will trigger launching your application.

In the configuration file, this could look like this:
\code
service x-vnd.MyGreatServer {
	launch /path/to/my/great/server
	if {
		not safemode
		file_exists /path/to/license/file
	}
	on {
		demand
	}
}
\endcode
Alternatively, there are shortcuts for two of the above items, and you could
also write it like this:
\code
service x-vnd.MyGreatServer {
	launch /path/to/my/great/server
	if {
		file_exists /path/to/license/file
	}
	not_safemode
	on_demand
}
\endcode
If you have only single line conditions/events, you may also omit the curly
braces completely:
\code
service x-vnd.MyGreatServer {
	launch /path/to/my/great/server
	if file_exists /path/to/license/file
	not_safemode
	on demand
}
\endcode
Note, the "on demand" does not use the "on_demand" shortcut, but just saves the
curly braces of "on { demand }".

You can also use operators like "and", "or", and "not" in conditions. If you
put more than one condition into an "if" they must all be true in order to meet
the condition. Conditions will be evaluated every time the launch_daemon has a
reason to start your application -- the outcome of the condition may change
over time.

Likewise, if you put more than one event into an "on" only one of them needs to
be triggered in order to launch your job. While the event processing is already
prepared to allow for an "and" operator, this is not yet available.

You can also specify the environment variables for your application. They can
be either specified directly, or you can let a shell script define them for you:
\code
service  x-vnd.MyGreatServer {
	env {
		from_script /path/to/my/script.sh
		LC_CTYPE C.UTF-8
	}
	launch /path/to/my/great/server
}
\endcode
If you want to move the job into a specific target, you can write it like this:
\code
target my-target {
	service  x-vnd.MyGreatServer {
		launch /path/to/my/great/server
	}
}
\endcode
You will be able to move jobs into targets around in the configuration files,
but this has not been implemented at the time of this writing.

Like jobs, and services, targets can use conditions, events, and environment
variables. If you do not specify an event for your target, it will not launch
automatically unless it's requested by name. Furthermore, jobs within your
target will not be available unless the target has been launched either
manually or by event.

You can also let the launch_daemon create resources for your application, like
additional named ports. These ports will be available to other applications
without having to launch your application, and will belong to the launch_daemon
throughout their lifetime.

Finally, there is a "run" directive that you can use to launch targets
unconditionally. The "run" directive also allows to use conditions, but adds
the additional keywords "then", and "else" like this:
\code
run {
	if read_only
	then installer
	else desktop
}
\endcode
You can also use curly braces if you like or want to run more than one job.

It's a good idea to look at the existing configuration files to get an idea how
this is all supposed to work.
*/