[cups-devel] Life time of CUPS subscriptions

[Michael Sweet]

Till,

> On Jan 10, 2016, at 4:25 PM, Till Kamppeter <till.kamppeter at gmail.com> wrote:
>> ...
>> Please file a bug - we should not be doing this for push notifications (e.g. dbus and rss as shipped in CUPS). For pull (ippget) notifications we will want to keep running since the client doing the pull will cause cupsd to restart... :/
>> 
> 
> How can CUPS distinguish whether a subscription is for push or pull notifications?

Pull subscriptions use the notify-pull-method Subscription Template attribute, while push subscriptions use the notify-recipient-uri Subscription Template attribute.  Currently the only pull method that has been defined for IPP is "ippget" (RFC 3996).

> ...
> So subscriptions are generally designed to survive the termination of the application and to be re-used when the application is started again?

Yes.

> Are they also supposed to survive a reboot of the system?

They can, per RFC 3995, but whether they do is implementation-defined behavior.  In the case of cupsd, we chose to preserve subscriptions across reboots/restarts since otherwise you need to do a lot of extra work to get persistent notifications going - think RSS and other kinds of "tell me when something goes wrong" notification schemes...

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Michael Sweet, Senior Printing System Engineer