[cups.general] Re: query about the cups class scheduling algorithm

[Helge Blischke]

Anonymous wrote:
> 
> Ashley GC wrote:
> 
> > I would like the cups scheduler to know what printers are available,
> > either by pinging the printer regularly, or by assuming the printer
> > is available and if the tcp connection times out, moving the job to
> > a different queue in the same class.
> > And if all this could be on the webpage(http://server:631/classes/cl)
> > then that would be nice.
> 
> These features are essential. Reading the CUPS documentation one sees;
> 
>  "In addition, CUPS can automatically merge multiple identical
>   network  printers into "implicit classes". This allows clients
>   to send jobs to the implicit class and have them print on the first
>   available printer or server"
> 
> The trouble is, as Ashley and others have pointed out, detecting the availablity of a 
> printer in CUPS is touch and go. A printer can be offline, out-of-paper or thrown out
> the window and CUPS will happily report that the printer is ready. So is there
> back-channel support or not ? The FAQ says not until version 1.2 which seems
> at odds with the documentation. Could someone give a quick appraisal of the situation,
> please or tell me what we are doing wrong.

As CUPS 1.1 is designed, I think there is no general easy solution. I'd
guess CUPS 1.2 will
implement something like this making use of the IPP notify objects for
real IPP printers.
But in general, it depends on the capabilities of the printer's
interface(s) itself.
For instance, the socket interface (better known as HP JetDirect) only
provides backchannel
information - to a certain extent - if the driving backend uses specific
features of HP's
PJL, and many parallel interfaces only provide sparse information on the
printer's state.

For the time being, I don't think pinging/querying lots of distributed
printers in short intervals
is a suitable solution.

What we currently do if we really need up to date status information on
printers, is to
operate small collections of printers by a dedicated CUPS server on a
dedicated box
so we can afford the overhead to query the printer's state as often as
needed (by wrapping
the original bqackend(s) with suitable scripts).

Helge
 

-- 
H.Blischke at srz-berlin.de
H.Blischke at srz-berlin.com
H.Blischke at acm.org