Dependencies

[Jerry Haltom]

> IIRC, today you also need Cups to be running if you want it to detect
> networked printers. 
> I guess my view of the whole thing.. dependencies in particular... comes
> down to the fact that adding support for dependencies somewhat hides the
> problem of fixing things such as Cups. Sure, it's great we can start
> Cups when a printer is attached... but ideally Cups itself would only
> run when actual printing services are needed. 

I agree with this completely. It's long been on my "list" of things that
I'll never get done to remove the need for a local cupsd for network
printing. I very much think user space programs should connect to the
cupsd hosting the remote printers without a local queue being required.
Avahi plays a huge role in making this work nice. Then we'd end up with
~/.printers.conf, just being a file of IPP addresses.

Also would let you do cool things like using PROPER PER-USER
AUTHENTICATION to remote cups. Like SPNEGO!

> (and we'd use something more modern like mDNS / Avahi to discover
> printers from the desktop session so Cups don't need to be running for
> this simple task.)
> 
> One way to do this is to use D-Bus activation (though this is not yet
> available on the system message bus), e.g. when you call into the name
> org.Cups.Printing or whatever... the cups daemon is launched to handle
> the incoming request... when it's done servicing you... it may exit
> again (standard feature of IPC systems that support activation).

I also agree with this. DBus activation is a great way for a desktop or
program to explicitly start an access cups. Abstract "device events"
pushed through upstart are a good way to launch Cups in response to a
local hardware event. Cups should turn itself off if it has no devices.
All of these access points start Cups when it's needed, and it turns
itself off when it's been idle for awhile.
> 
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