The central component of CUPS is the cupsd daemon, which acts as the scheduler and main communication hub.
It listens for client requests and manages the print queues, directing jobs to the appropriate destinations.
Configuration files, job queues, and other administrative tasks are managed through this daemon.
Backend Modules:
Backend modules are responsible for communicating with printers or print servers.
Each backend module corresponds to a specific printing protocol or method (e.g., IPP, LPD, SMB, USB).
When a print job is sent, the cupsd communicates with the appropriate backend to handle the actual printing process.
Filter Chains:
Filters are software components that transform print jobs from one format to another. For example, converting a PostScript file to a format that the printer can understand.
CUPS uses a filter chain mechanism where multiple filters can be chained together to process a print job sequentially.
Filters are typically located in the /usr/lib/cups/filter/ directory and are invoked based on the MIME type and printer capabilities.
PPD Files:
PostScript Printer Description (PPD) files provide detailed information about the capabilities and features of a printer.
These files contain metadata such as supported resolutions, color options, paper sizes, and more.
CUPS uses PPD files to generate printer-specific options in the print dialog and to configure the appropriate filters and backends for each printer.
Workflow:
Client Request:
A user or application initiates a print job by sending a request to the cupsd daemon, usually via the ipp:// or http:// protocol.
Job Processing:
The cupsd receives the print job request and processes it based on the destination printer's configuration.
It consults the PPD file associated with the printer to determine the supported options and default settings.
Filtering and Transformation:
If necessary, the cupsd invokes the appropriate filters to transform the print job into a format compatible with the printer.
This may involve converting file formats, adjusting print settings, or applying specific transformations.
Backend Communication:
Once the job is processed and transformed, the cupsd communicates with the appropriate backend module based on the printer's connection method (e.g., network, USB).
The backend module handles the low-level communication with the printer or print server, sending the print data and managing the printing process.
Job Completion and Status:
After sending the print data, the backend module monitors the printing progress and updates the job status in the CUPS system.
Users can check the status of their print jobs, manage print queues, and perform administrative tasks through the CUPS web interface (http://localhost:631) or command-line utilities (lpstat, lpadmin).
Extensibility and Integration:
CUPS supports a wide range of printers and print services through its modular architecture.
New backends, filters, and PPD files can be added to support custom printing solutions, legacy devices, or specialized requirements.
Integration with various authentication mechanisms, job control policies, and network protocols is facilitated through plugins, configuration settings, and API interfaces.
