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In computing, spooling is a specialized form of multi-programming for the purpose of copying data between different devices. In contemporary systems, it is usually used for mediating between a computer application and a slow peripheral, such as a printer. Spooling allows programs to "hand off" work to be done by the peripheral and then proceed to other tasks, or to not begin until input has been transcribed. A dedicated program, the spooler, maintains an orderly sequence of jobs for the peripheral and feeds it data at its own rate. Conversely, for slow input peripherals, such as a punched card, a spooler can maintain a sequence of computational jobs waiting for data, starting each job when all of the relevant input is available; see batch processing. The spool itself refers to the sequence of jobs, or the storage area where they are held. In many cases, the spooler is able to drive devices at their full rated speed with minimal impact on other processing.
Spooling is a combination of Data buffer and queueing.
For example, when a large organization prepares payroll cheques, the computation takes only a few minutes or even seconds, but the printing process might take hours. If the payroll program printed cheques directly, it would be unable to proceed to other computations until all the cheques were printed. Similarly, before spooling was added to PC operating systems, were unable to do anything else, including interact with the user, while printing.
Spooler or print management software often includes a variety of related features, such as allowing priorities to be assigned to print jobs, notifying users when their documents have been printed, distributing print jobs among several printers, selecting appropriate paper for each document, etc.
A print server applies spooling techniques to allow many computers to share the same printer or group of printers.
On printers using fanfold continuous forms a leading banner page would often be printed twice, so that one copy would always be face-up when the jobs were separated. The page might include lines printed over the fold, which would be visible along the edge of a stack of printed output, allowing the operator to easily separate the jobs. Some systems would also print a banner page at the end of each job, assuring users that they had collected all of their printout.
A batch processing system uses spooling to maintain a queue of ready-to-run tasks, which can be started as soon as the system has the resources to process them.
Some store and forward messaging systems, such as uucp, used "spool" to refer to their inbound and outbound message queues, and this terminology is still found in the documentation for email and Usenet software.
Because the unit record equipment on IBM mainframes of the early 1960s was slow, it was common for larger systems to use a small offline computer such as an IBM 1401 instead of spooling.
The term "spool" may originate with the Simultaneous Peripheral Operations On-Line (SPOOL) software; this derivation is uncertain however, as it may be a backronym.Tanenbaum, Andrew S. Modern Operating Systems. 3rd Ed. Pearson Education, Inc., 2008. Another explanation is that it refers to "spools" or of magnetic tape, although “spool” is an uncommon usage.
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