Richard Pendergast, director of Travelocity systems for Sabre, says that about every six months, Sabre has to do maintenance that requires taking down the entire site for several hours. By notifying customers, Sabre can minimize ill will. Likewise, eBay performs backups and does maintenance while the system is running, but it also needs to take the site down occasionally and schedules its outages for Friday mornings before 4 a.m. Our users know it's a regularly scheduled outage. They expect it.
Demand can be an enemy of response time and uptime.
At DoubleClick, predicting demand isn't a problem. With thousands of sites, increased demand on one site isn't likely to affect the overall system. Demand fluctuates seasonally, during the Christmas rush, for example. Sabre finds some elements of demand to be predictable, others surprising. The company has daily and weekly demand crests, as well as surprises when airlines post sudden fare sales.