7.6.5. Accounting for Printer Usage

So, you need to charge for printouts. And why not? Paper and ink cost money. And then there are maintenance costs---printers are loaded with moving parts and tend to break down. You have examined your printers, usage patterns, and maintenance fees and have come up with a per-page (or per-foot, per-meter, or per-whatever) cost. Now, how do you actually start accounting for printouts?

Well, the bad news is the LPD spooling system does not provide much help in this department. Accounting is highly dependent on the kind of printer in use, the formats being printed, and your requirements in charging for printer usage.

To implement accounting, you have to modify a printer's text filter (to charge for plain text jobs) and the conversion filters (to charge for other file formats), to count pages or query the printer for pages printed. You cannot get away with using the simple output filter, since it cannot do accounting. See section Filters.

Generally, there are two ways to do accounting:

• Periodic accounting is the more common way, possibly because it is easier. Whenever someone prints a job, the filter logs the user, host, and number of pages to an accounting file. Every month, semester, year, or whatever time period you prefer, you collect the accounting files for the various printers, tally up the pages printed by users, and charge for usage. Then you truncate all the logging files, starting with a clean slate for the next period.
• Timely accounting is less common, probably because it is more difficult. This method has the filters charge users for printouts as soon as they use the printers. Like disk quotas, the accounting is immediate. You can prevent users from printing when their account goes in the red, and might provide a way for users to check and adjust their ``print quotas.'' But this method requires some database code to track users and their quotas.

The LPD spooling system supports both methods easily: since you have to provide the filters (well, most of the time), you also have to provide the accounting code. But there is a bright side: you have enormous flexibility in your accounting methods. For example, you choose whether to use periodic or timely accounting. You choose what information to log: user names, host names, job types, pages printed, square footage of paper used, how long the job took to print, and so forth. And you do so by modifying the filters to save this information.

7.6.5.1. Quick and Dirty Printer Accounting

FreeBSD comes with two programs that can get you set up with simple periodic accounting right away. They are the text filter lpf, described in section lpf: a Text Filter, and

pac, a program to gather and total entries from printer accounting files.

As mentioned in the section on filters ( Filters), LPD starts the text and the conversion filters with the name of the accounting file to use on the filter command line. The filters can use this argument to know where to write an accounting file entry. The name of this file comes from the af capability in /etc/printcap, and if not specified as an absolute path, is relative to the spooling directory.

LPD starts lpf with page width and length arguments (from the pw and pl capabilities). lpf uses these arguments to determine how much paper will be used. After sending the file to the printer, it then writes an accounting entry in the accounting file. The entries look like this:

   2.00 rose:andy
   3.00 rose:kelly
   3.00 orchid:mary
   5.00 orchid:mary
   2.00 orchid:zhang

When you are ready to charge users for printouts, run the

pac program. Just change to the spooling directory for the printer you want to collect on and type pac. You will get a dollar-centric summary like the following:

  Login               pages/feet   runs    price
orchid:kelly                5.00    1   $  0.10
orchid:mary                31.00    3   $  0.62
orchid:zhang                9.00    1   $  0.18
rose:andy                   2.00    1   $  0.04
rose:kelly                177.00  104   $  3.54
rose:mary                  87.00   32   $  1.74
rose:root                  26.00   12   $  0.52

total                     337.00  154   $  6.74

-Pprinter

Which printer to summarize. This option works only if there is an absolute path in the af capability in /etc/printcap.

-c

Sort the output by cost instead of alphabetically by user name.

-m

Ignore host name in the accounting files. With this option, user smith on host alpha is the same user smith on host gamma. Without, they are different users.

-pprice

Compute charges with price dollars per page or per foot instead of the price from the pc capability in /etc/printcap, or two cents (the default). You can specify price as a floating point number.

-r

Reverse the sort order.

-s

Make an accounting summary file and truncate the accounting file.

names...

Print accounting information for the given user names only.

In the default summary that pac produces, you see the number of pages printed by each user from various hosts. If, at your site, host does not matter (because users can use any host), run pac -m, to produce the following summary:

  Login               pages/feet   runs    price
andy                        2.00    1   $  0.04
kelly                     182.00  105   $  3.64
mary                      118.00   35   $  2.36
root                       26.00   12   $  0.52
zhang                       9.00    1   $  0.18

total                     337.00  154   $  6.74

pac -p1.50

Finally, running pac -s will save the summary information in a summary accounting file, which is named the same as the printer's accounting file, but with _sum appended to the name. It then truncates the accounting file. When you run pac again, it rereads the summary file to get starting totals, then adds information from the regular accounting file.

7.6.5.2. How Can You Count Pages Printed?

In order to perform even remotely accurate accounting, you need to be able to determine how much paper a job uses. This is the essential problem of printer accounting.

For plain text jobs, the problem's not that hard to solve: you count how many lines are in a job and compare it to how many lines per page your printer supports. Do not forget to take into account backspaces in the file which overprint lines, or long logical lines that wrap onto one or more additional physical lines.

The text filter lpf (introduced in lpf: a Text Filter) takes into account these things when it does accounting. If you are writing a text filter which needs to do accounting, you might want to examine lpf's source code.

How do you handle other file formats, though?

Well, for DVI-to-LaserJet or DVI-to-PostScript conversion, you can have your filter parse the diagnostic output of

dvilj or dvips and look to see how many pages were converted. You might be able to do similar things with other file formats and conversion programs.

But these methods suffer from the fact that the printer may not actually print all those pages. For example, it could jam, run out of toner, or explode---and the user would still get charged.

So, what can you do?

There is only one sure way to do accurate accounting. Get a printer that can tell you how much paper it uses, and attach it via a serial line or a network connection. Nearly all PostScript printers support this notion. Other makes and models do as well (networked Imagen laser printers, for example). Modify the filters for these printers to get the page usage after they print each job and have them log accounting information based on that value only. There is no line counting nor error-prone file examination required.

Of course, you can always be generous and make all printouts free.