Linux Print System at Cisco Systems, Inc.

At your office, can you print to the nearest printer, or do you have to use a printer set up just for your desktop machine? If you wanted to send a job to a printer in another department or in another office, could you do it as easily as with your local printer? In most large companies, the answer would be no.

System Administrators seldom want to take care of printing problems, and there is rarely anyone else to turn to for help. Out of desperation, you're determined to fix a problem yourself, only to be told, “My colleague tried that, and broke printing for a week. Don't touch it.” In short, printing has become a Pandora's box no one wants to open. It is surrounded by more folklore and black magic than any other area in modern computing.

In this article, I will describe the general problems with printing in a large corporate environment and the general methods for solving these problems. I will then detail how I solved the particular problems at Cisco. Using software almost entirely downloadable from the Internet, I produced a highly visible, mission-critical, fault-tolerant print system used every day by over 10,000 people worldwide.

In a nutshell, the solution depends on multiple Linux servers which, by communicating with each other, effectively work as a single “distributed machine”. This approach offers one solution to many problems inherent not only in print systems but, more generally, any network resource (e.g., mail, disk space, etc.) in a large corporate network.

A distributed machine may sound complex, but very little magic is involved. As we shall see, the “magic” comes from applying the traditional UNIX method of combining many little pieces into a whole significantly greater than its parts.

First, let's talk about some of the general problems that people face when printing in a corporate environment. Printers are based on mechanical parts that are slower and less reliable than the computers sending jobs to them; thus, queues and frequent status updates are required.

Although vendors are trying to create standards (for example, the work of the Printer Working Group [PWG]), the current standardization is poor, and printer manufacturers are programming their machines to talk to as many different standards (or emulations) as possible. Few of these emulations work well or allow good user control of the printer.

Essentially, there are two main strategies for organizing printing: direct client-to-printer or via a central print server.

The client connects directly to a printer and takes complete control as it sends its print job. Any printer status is sent directly back to the client. Once the printer has finished, the client disconnects and the printer is then available for another client.

This method is simpler for use in small offices. Each person's machine is set up in isolation of everyone else's—no need to think about any larger issues. Since each user is isolated, problems usually affect only a single user. Provided his printer does not fail, an individual user can carry on printing, regardless of problems others may be having.

Each user controls his own queue directly. He can cancel a job on his own machine using whatever tools the operating system provides.

With a direct client-to-printer system, performing any global changes is difficult. If the IP address of the printer changes, the engineers have many client machines to track down and reconfigure.

Each client machine has to compete for the printer. If the printer is already busy when a client tries to print, the client has to keep retrying until the printer is free.

Providing an orderly queue is difficult. Since all clients get control of the printer in a random order, there is no guarantee of when any particular client's job will start printing, or that jobs will be printed in any particular order. It is almost impossible for a client to know that other clients are waiting for the printer.

A client can print only to the printers that support its protocol. For example, an Apple Macintosh can talk only to the printers that support AppleTalk.

Tracking down and canceling unknown jobs is also not easy. For example, if a printer is busy printing a 2,000-page document, the sending client is not apparent, and to actually stop the print job, the system administrator has to get appropriate permissions on that client machine.

A fix to one protocol can break others. Service technicians, who tend to be experts in their own field, can easily break things for other protocols. For example, an engineer may reset a printer to fix a Novell printing problem, and in the process break the TCP/IP setup (which he doesn't understand anyway).

Certain printers cannot switch well between different protocols. Some printers, particularly the older ones, have even been known to crash completely (requiring a reboot) when switching between TCP/IP and AppleTalk.