In the new world, how do you bolt into CUPS handling for a unique type of printer?

[HattonLe]

Have to admit I'm struggling to get my head around the CUPS subsystem at the moment for a scenario of using a unique type of printer. For example say I have a HP printer with a special widget that needs to be activated before printing via a mains relay.

In the old world you would have a dedicated print driver for that printer that would invoke an app that turns on the relay when the print job starts.

I found this useful diagram on the internet (not sure how up to date it is)

I assume "Printer Applications" are the 2nd row from the bottom in the diagram?

How is a "Printer Application" associated with a specific printer type? (is it via "usr/share/cups/drv/cupsfilters.drv"), e.g. how does CUPS know to invoke the HP printer application when printing a pdf.

In my scenario I can't see in CUPS 3.0 (or even in 2.4.19) how you could associate a relay control app with a specific printer.

With my experiments in 2.4.19, at the moment the only way I seem to be able to get a specific app to run is to define a unique input mime type which causes the cupsfilter to invoke a custom filter app during the print job processing.
(But that method isn't the one to use as you might not want a particular document type to trigger a specific printers actions).

[michaelrsweet]

See the most recent CUPS Plenary and Printer Applications slides from this May's OpenPrinting summit for more up-to-date information (ESP Print Pro in the above diagram hasn't been sold in a couple decades now...)

Printer Applications replace printer drivers and backends. Essentially they emulate an IPP printer so that the client-side software/spooler can focus on securely generating/converting print documents and displaying status information. A Printer Application can accept printer-specific print files (ESC/POS, PCL, ZPL, etc.) as well as print files using standard formats, and the client spooler relays these printer-ready files directly to the printer/Printer Application rather than running a local filter to convert it.

The association between Printer Application and printer isn't automatic - I know Till Kamppeter had been trying to get something setup with the Snap store (so a Printer Application snap could register which printers it supports and the OS could automatically download/install/configure the Printer Application when needed) but I don't know whether that went anywhere. Most Printer Applications can recognize the printers they support and auto-add if connected via USB or at least automatically choose the right internal "driver" to use for printing if you explicitly add it.