doc/help/spec-design.html

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   4         <TITLE>CUPS Design Description</TITLE>
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   7
   8 <P>Like most printing systems, CUPS is designed around a central
   9 print scheduling process that dispatches print jobs, processes
  10 administrative commands, provides printer status information to
  11 local and remote programs, and informs users as needed. Figure 1
  12 shows the basic organization of CUPS.</P>
  13
  14 <H3>Scheduler</H3>
  15
  16 <P>The scheduler is a HTTP/1.1 server application that handles
  17 HTTP requests. Besides handling printer requests via IPP POST
  18 requests, the scheduler also acts as a full-featured web server
  19 for documentation, status monitoring, and administration.</P>
  20
  21 <P>The scheduler also manages a list of available printers on
  22 the LAN and dispatches print jobs as needed using the
  23 appropriate filters and backends.</P>
  24
  25 <H3>Configuration Files</H3>
  26
  27 <P>The configuration files consist of:</P>
  28
  29 <ul>
  30
  31         <LI>The HTTP server configuration file.</LI>
  32
  33         <LI>Printer and class definition files.</LI>
  34
  35         <LI>MIME type and conversion rule files.</LI>
  36
  37         <LI>PostScript Printer Description (PPD) files.</LI>
  38
  39 </ul>
  40
  41 <P>The HTTP server configuration file is purposely similar to
  42 the Apache server configuration file and defines all of the
  43 access control properties for the server.</P>
  44
  45 <P>The printer and class definition files list the available
  46 printer queues and classes. Printer classes are collections of
  47 printers. Jobs sent to a class are forwarded to the first
  48 available printer in the class, round-robin fashion.</P>
  49
  50 <P>The MIME type files list the supported MIME types
  51 (text/plain, application/postscript, etc.) and "magic' rules for
  52 automatically detecting the format of a file.  These are used by
  53 the HTTP server to determine the <tt>Content-Type</tt> field for
  54 <tt>GET</tt> and <tt>HEAD</tt> requests and by the IPP request
  55 handler to determine the file type when a <tt>Print-Job</tt> or
  56 <tt>Send-File</tt> request is received with a
  57 <tt>document-format</tt> of
  58 <tt>application/octet-stream</tt>.</P>
  59
  60 <P>The MIME conversion rule files list the available filters.
  61 The filters are used when a job is dispatched so that an
  62 application can send a convenient file format to the printing
  63 system which then converts the document into a printable format
  64 as needed. Each filter has a relative cost associated with it,
  65 and the filtering algorithm chooses the set of filters that will
  66 convert the file to the needed format with the lowest total
  67 "cost".</P>
  68
  69 <P>The PPD files describe the capabilities of all printers, not
  70 just PostScript printers. There is one PPD file for each
  71 printer. PPD files for non-PostScript printers define additional
  72 filters through <tt>cupsFilter</tt> attributes to support
  73 printer drivers.</P>
  74
  75 <P ALIGN="CENTER"><IMG SRC="../images/cups-block-diagram.gif"
  76 WIDTH="768" HEIGHT="768" ALT="CUPS Block Diagram"/><BR>
  77 <I>Figure 1 - CUPS Block Diagram</I></P>
  78
  79 <H3>CUPS API</H3>
  80
  81 <P>The CUPS API contains CUPS-specific convenience functions for
  82 queuing print jobs, getting printer information, accessing
  83 resources via HTTP and IPP, and manipulating PPD files. Unlike
  84 the rest of CUPS, the CUPS API is provided under the terms of
  85 the GNU LGPL so it may be used by non-GPL applications.</P>
  86
  87 <H3>Berkeley and System V Commands</H3>
  88
  89 <P>CUPS provides the System V and Berkeley command-line
  90 interfaces for submitting jobs and checking the printer status.
  91 The <tt>lpstat</tt> and <tt>lpc status</tt> commands
  92 also show network printers ("printer@server") when printer
  93 browsing is enabled.</P>
  94
  95 <P>The System V administation commands are supplied for managing
  96 printers and classes. The Berkeley printer administration tool
  97 (<tt>lpc</tt>) is only supported in a "read-only' mode to
  98 check the current status of the printer queues and
  99 scheduler.</P>
 100
 101 <H3>Filters</H3>
 102
 103 <P>A filter program reads from the standard input or from a file
 104 if a filename is supplied. All filters must support a common set
 105 of options including printer name, job ID, username, job title,
 106 number of copies, and job options. All output is sent to the
 107 standard output.</P>
 108
 109 <P>Filters are provided for many file formats and include image
 110 file and PostScript raster filters that support non-PostScript
 111 printers. Multiple filters are run in parallel to produce the
 112 required output format.</P>
 113
 114 <P>The PostScript raster filter is based on the ESP Ghostscript
 115 core. Instead of using the Ghostscript printer drivers, the CUPS
 116 filter uses a generic CUPS raster printer driver and
 117 CUPS-compliant front-end to support any kind of raster printer.
 118 This allows the same printer driver filter to be used for
 119 printing raster data from any filter.</P>
 120
 121 <pre>Talk about Apple's use of CUPS...</pre>
 122
 123 <H3>CUPS Imaging</H3>
 124
 125 <P>The CUPS Imaging library provides functions for managing
 126 large images, doing colorspace conversion and color management,
 127 scaling images for printing, and managing raster page streams.
 128 It is used by the CUPS image file filters, the PostScript RIP,
 129 and all raster printers drivers.</P>
 130
 131 <H3>Backends</H3>
 132
 133 <P>A backend program is a special filter that sends print data
 134 to a device or network connection. Backends for parallel,
 135 serial, USB, LPD, IPP, and AppSocket (JetDirect) connections are
 136 provided in CUPS 1.2.</P>
 137
 138 <P>SAMBA version 2.0.6 and higher includes a SMB backend
 139 (<tt>smbspool(1)</tt>) that can be used for printing to
 140 Windows.</P>
 141
 142 <H2 CLASS="title">Network Printing</H2>
 143
 144 <P>Traditionally, network printing has been one of the hardest
 145 things to get working under UNIX. One reason is because each
 146 vendor added their own extensions to the LPD protocol (the
 147 previous standard for network printing), making cross-platform
 148 printing difficult if not impossible.</P>
 149
 150 <P>Another reason is that you have to administer every network
 151 printer on every client machine. In some cases you can "clone'
 152 the printer configuration from a "master' client to each of the
 153 others, but even that can be time-consuming and error-prone.
 154 Something better is needed.</P>
 155
 156 <P>CUPS provides "printer browsing", which allows clients to
 157 automatically see and use printers from any server on a LAN.
 158 This means that you only need to configure the server and the
 159 clients will automatically see the printers and classes on
 160 it.</P>
 161
 162 <P>In addition, CUPS can automatically merge multiple identical
 163 network printers into "implicit classes". This allows clients to
 164 send jobs to the implicit class and have them print on the first
 165 available printer or server. In addition, failsafe and
 166 load-balancing functions are enabled simply by defining the same
 167 printer on multiple servers!</P>
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