A number of Drupal features and maintenance tasks are designed to be done at regular intervals on an ongoing basis. These are tasks that need to be executed periodically and include updating the search index, updating aggregator feeds, closing polls, and discarding old logging information. As you can see, these are important and vital functions for most Drupal web sites.
Drupal is programmed in PHI, and PHP running on a web server like Apache offers no solution to the problem of scheduling tasks to be run at repeating intervals—that just isn't what Apache and PHP were designed to do. Therefore, you need to use other tools to trigger the tasks.
The cron program is the most common tool used for scheduling these tasks for Drupal. The cron program is used on UNIX-based machines to launch programs and scripts on a scheduled timer.
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