Further explanation of...

From #drupal on IRC (edited for length):

christefano: can you tell me what the advantage to having daemon module run Drupal as a single process is? I'm looking at the project page and am scratching my head

aymerick: christefano: yeah, you are not the first one to ask me that :p   the daemon module goal is not to replace apache ... but you can use it (well, i hope it is usable) if you want a daemon process on your system, doing 'stuff', using the drupal APIs :p

aymerick: ok maybe I have to be more explicit on the project page about what this module is not

christefano: okay, so running Drupal as a daemon will list it in top?

aymerick: yep

christefano: hmmmm that sounds unbelievably useful. what happens if the daemon is killed?

oqp: could be useful for bot, cron and other bootstrap stuff

aymerick: I haven't finished yet to code all I want, but my goal is to be able to setup Timers that are fired every 'X seconds' (configurable) and that execute any PHP code your want (configurable too)

oqp: christefano you setup the OS to restart it, no?

aymerick: oqp: yeah, as any daemon on your system. you may use some monitoring engine that restart the daemon if it disappears

christefano: oqp: I mean, does killing the process affect anything other than the daemon

aymerick: christefano: well, not, killing the daemon only affects the daemon (normally). this is very experimental stuff right now... I have to figure out for example what happen with the cache stuff that is shared with a live drupal site

christefano: aymerick: with your permission, may I follow up on #216184 with your responses here?

aymerick: christefano: of course you cane :)

I have added more explanations on project's page.

I guess this question gets down to what I really am wondering - can this module enable Drupal to be a self contained 'app'. Meaning that someone wouldn't need apache, php, mysql? (sounds very far fetched, but what the heck)

Well, you still need php and mysql, but you don't need apache.

However it doesn't replace apache. The goal is not to allow a daemon to serve pages as apache do, even if it may be possible if you create a Listener that implements full HTTP server, but that's overkill, really, don't do that... except maybe for fun :-)

I still don't understand why running drupal as a daemon could be useful. Can you provide some examples?

I think it's quite good.
I've many useful things in my mind:

- It's good to make some validation tests, simpletests or jmeter tests for specified part of page, and the pages are loaded faster without theme.

- Sometimes the wrong theme hooks kills the Drupal core, so you can fix that without using theme()

- You can make some special debug messages when Drupal run as daemon

- I'll love idea of dumping the content to the shell and parse it, something like:
$ drush deamon --verbose menu node/1 > node1.txt
$ cd public_html && find . -name -exec drush deamon --verbose menu node/{}
$ for %i from 1 to 100; do drush deamon --verbose menu node/%i > my_exported_node_%i.txt
$ drush deamon -listen 21 ftp_module # Drupal handle ftp users and allow them log in using Drupal account details
$ drush deamon -listen 139 share_picture_gallery_in_LAN # LOL
$ drush deamon -listen 8080 drupal_proxy # 220 Drupal: Can I help you?

- Some scheduled cron tasks:
0 10 * * * drush deamon drupal_for_kids &
0 24 * * * killall drush && echo Children go sleep!

- used in some conditional server scripts (Drupal is more secured and firewalled, nobody knows that on this port there is some hidden service)
#!/bin/bash
if ( $user_authorized ) {
drush deamon waiting_for_SOAP_data & # you have 60 seconds for sent logs via SOAP
sleep 60 && killall drush &&
echo Sorry, time out! Thank you for your collecting data.
cdburn ~/public_html/default/all/files/logs/dump.txt
}

- additional security, random listen ports for some short times

- benchmarking server load test:
for %i from 1 to 100; do drush -listen 10%i deamon node; done

- benchmarking Drupal test:
time for %i from 1 to 100; do drush deamon node &; done

- Drupal reniced
renice +20 drush deamon cron.php # please, do that quick

- Drupal renice tests
time renice +10 drush deamon node
time renice -10 drush deamon node

- find the weakest node;)
for %i from 1 to 100; do time drush deamon node/%1; done | sort

etc. etc. ;)