Evan Schuman's StorefrontBacktalk

Think cloud computing will solve the problems of overloaded E-Commerce sites? Not necessarily. The Web site of U.K. grocery giant Tesco on December 5 ground to a halt after a surge of customers tried to take advantage of a loyalty-card promotion. But that surge wasn’t unanticipated.

Just days earlier, Tesco had said that cloud services provider Akamai would be offloading 90 percent of the load—to make sure nothing would go wrong. That didn’t exactly work: When the Web site crashed, customers turned to the call center and clogged it, too.

The ability to quickly scale up processing power is one of the chief attractions of cloud computing, and there’s no doubt it works in at least some cases. For example, Amazon’s ability to ramp up capacity via the cloud doesn’t just keep the E-tailer rock solid during the holiday peak; it also probably keeps it going in the face of denial-of-service attacks. Still, getting everything in the cloud working correctly to handle a sudden surge isn’t as easy as cloud boosters make it sound—and Tesco is Exhibit A.

In fact, Tesco expected a big response to its holiday loyalty-card promotion, which started in early November and let customers convert points into vouchers for twice the usual value. That’s why the $100 billion chain said it brought in Akamai, whose cloud services were expected to handle 90 percent of the Web site workload during peak times of the promotion.

“It took just eight hours for the services to be put in place and we have been delighted with the results,” Tesco IT Executive Ed Camp was quoted as saying in a news release. “It has given us the peace of mind that customers taking advantage of this promotion can do so whilst enjoying great online service and security.”

That peace of mind only lasted until the final weekend of the promotion. Along with the usual customer foot-dragging, an earlier-than-usual major snowfall kept the crowds down until early December. That resulted in long lines on December 4 and 5, as customers waited for cashiers to manually convert points to vouchers. When cashiers began telling customers that they could do the conversion themselves on the Web site, the load slowed the site to a crawl, and then to a complete halt.

Neither Tesco nor Akamai has detailed how that happened. Akamai would only say that the failures happened on parts of the Tesco site that weren’t being supported by Akamai’s cloud services.

That sounds like buck-passing. But like many cloud providers, Akamai offers an à la carte menu of cloud-service offerings. The most complicated (and pricey) filter all traffic through Akamai’s servers. That’s clearly not what Tesco went for, with its eight-hour set-up time and just a little too much peace of mind.

To get to the cloud, everything still had to go through Tesco’s own systems. Even if the cloud performed perfectly—and that’s still an if—the number of transactions Tesco customers could do was limited by that bottleneck. Simply shifting the back-end load to the cloud couldn’t relieve that limit.

That puts a finer point on the standard advice that E-tailers should use the cloud for major marketing events like this. There’s almost always part of a retailer’s E-Commerce site that remains out of the cloud. That part has to be retooled to support the big wave of customers, too.

Even if you beef up the number of virtual servers in the cloud, you’ve still got to get traffic to those servers. If there’s limited bandwidth, or limited capacity in the transaction systems that contain loyalty-points data or in the systems that let customers log in—those will be bottlenecks. Unless those bottlenecks are cleared so they can pass huge amounts of work up to the cloud, they remain the spots that will crack when a crunch comes.