Editing CUPS (section)

=== Scheduler ===
The CUPS scheduler implements [[Internet Printing Protocol]] (IPP) over HTTP/1.1. A helper application (cups-lpd) converts [[Line Printer Daemon protocol]] (LPD) requests to IPP. The scheduler also provides a web-based interface for managing print jobs, the configuration of the server, and for documentation about CUPS itself.<ref name="scheduler" />

An ''authorization'' module controls which IPP and HTTP messages can pass through the system.<ref name="auth">{{cite web|work=[[Easy Software Products]]|publisher=CUPS Software Design|url=http://www.cups.org/doc-1.1/sdd.html#3_8_1|title=Authorization|access-date=January 9, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070110180805/http://www.cups.org/doc-1.1/sdd.html#3_8_1|archive-date=January 10, 2007}}</ref> Once the IPP/HTTP packets are authorized they are sent to the ''client'' module, which listens for and processes incoming connections. The client module is also responsible for executing external [[Common Gateway Interface|CGI]] programs as needed to support web-based printers, classes, and job status monitoring and administration.<ref name="clientmodule">{{cite web|work=[[Easy Software Products]]|publisher=CUPS Software Administrators Manual|url=http://www.cups.org/doc-1.1/sdd.html#3_8_3|title=Authorisation|access-date=January 9, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070110180805/http://www.cups.org/doc-1.1/sdd.html#3_8_3|archive-date=January 10, 2007}}</ref> Once this module has processed its requests, it sends them to the ''IPP'' module which performs [[Uniform Resource Identifier]] (URI) validation to prevent a client from sidestepping any [[access control]]s or [[authentication]] on the HTTP server.<ref name="ippmodule">{{cite web|work=[[Easy Software Products]]|publisher=CUPS Software Design|url=http://www.cups.org/doc-1.1/sdd.html#3_8_7|title=IPP|access-date=January 9, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070110180805/http://www.cups.org/doc-1.1/sdd.html#3_8_7|archive-date=January 10, 2007}}</ref> The URI is a text [[String (computer science)|string]] that indicates a name or address that can be used to refer to an abstract or physical resource on a network.

The scheduler allows for classes of printers. Applications can send requests to groups of printers in a class, allowing the scheduler to direct the job to the first available printer in that class.<ref name="classes">{{cite web|work=[[Easy Software Products]]|publisher=CUPS Software Administrators Manual|url=http://www.cups.org/doc-1.1/sam.html#2_4|title=Classes|access-date=January 9, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070106145359/http://www.cups.org/doc-1.1/sam.html#2_4|archive-date=January 6, 2007}}</ref> A ''jobs'' module manages print jobs, sending them to the filter and backend processes for final conversion and printing, and monitoring the status messages from those processes.<ref name="jobs">{{cite web|work=[[Easy Software Products]]|publisher=CUPS Software Administrators Manual|url=http://www.cups.org/doc-1.1/sam.html#2_3|title=Jobs|access-date=January 9, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070106145359/http://www.cups.org/doc-1.1/sam.html#2_3|archive-date=January 6, 2007}}</ref>

The CUPS scheduler utilizes a ''configuration'' module, which parses configuration files, initializes CUPS [[data structure]]s, and starts and stops the CUPS program. The configuration module will stop CUPS services during configuration file processing and then restart the service when processing is complete.<ref name="configurationmodule">{{cite web|work=[[Easy Software Products]]|publisher=CUPS Software Design|url=http://www.cups.org/doc-1.1/sdd.html#3_8_4|title=Configuration|access-date=January 9, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070110180805/http://www.cups.org/doc-1.1/sdd.html#3_8_4|archive-date=January 10, 2007}}</ref>

A ''logging'' module handles the logging of scheduler events for access, error, and page [[Computer data logging|log files]]. The ''main'' module handles timeouts and dispatch of I/O requests for client connections, watching for [[Signal (computing)|signals]], handling child process errors and exits, and reloading the server configuration files as needed.<ref name="loggingmodule">{{cite web|work=[[Easy Software Products]]|publisher=CUPS Software Design|url=http://www.cups.org/doc-1.1/sdd.html#3_8_9|title=Logging|access-date=January 9, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070110180805/http://www.cups.org/doc-1.1/sdd.html#3_8_9|archive-date=January 10, 2007}}</ref>

Other modules used by the scheduler include:

* the ''MIME'' module, which handles a [[Multipurpose Internet Mail Extensions]] (MIME) type and conversion database used in the filtering process that converts print data to a format suitable for a print device;<ref name="MIMEmodule">{{cite web|work=[[Easy Software Products]]|publisher=CUPS Software Design|url=http://www.cups.org/doc-1.1/sdd.html#3_8_11|title=MIME|access-date=January 9, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070110180805/http://www.cups.org/doc-1.1/sdd.html#3_8_11|archive-date=January 10, 2007}}</ref>
* a ''PPD'' module that handles a list of [[Postscript Printer Description]] (PPD) files;<ref name="ppdmodule">{{cite web|work=[[Easy Software Products]]|publisher=CUPS Software Design|url=http://www.cups.org/doc-1.1/sdd.html#3_8_12|title=PPD|access-date=January 9, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070110180805/http://www.cups.org/doc-1.1/sdd.html#3_8_12|archive-date=January 10, 2007}}</ref>
* a ''devices'' module that manages a list of devices that are available in the system;<ref name="devicesmodule">{{cite web|work=[[Easy Software Products]]|publisher=CUPS Software Design|url=http://www.cups.org/doc-1.1/sdd.html#3_8_5|title=Devices|access-date=January 9, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070110180805/http://www.cups.org/doc-1.1/sdd.html#3_8_5|archive-date=January 10, 2007}}</ref>
* a ''printers'' module that handles printers and PPDs within CUPS.<ref name="printersmodule">{{cite web|work=[[Easy Software Products]]|publisher=CUPS Software Design|url=http://www.cups.org/doc-1.1/sdd.html#3_8_13|title=Printers|access-date=January 9, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070110180805/http://www.cups.org/doc-1.1/sdd.html#3_8_13|archive-date=January 10, 2007}}</ref>