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Network Algorithmics by George Varghese

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Chapter 7

Maintaining Timers

That was, is, and shall be: Time’s wheel runs back or stops.

— ROBERT BROWNING

A timer module in a system is analogous to a secretary who keeps track of all the appointments of a busy executive. The executive tells the secretary to schedule appointments and sometimes to cancel appointments before they occur. It is the secretary’s job to interrupt the executive with a warning just before the scheduled time of an appointment. Many secretaries actually do this using a so-called tickler file, which is a moving window over the next N days. When the day’s appointments are done, the tickler file is rolled to bypass the current day. We will find a strong analogy between a tickler file and a timing wheel, the main data structure ...

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