Amazon has not yet said exactly what caused one of its cloud storage services to go down for several hours Tuesday, but businesses who were affected can learn from the headache. The Seattle-based tech giant dominates the cloud services market, far outpacing its next three competitors. So when an issue in a cluster of data centers in Northern Virginia took down one of the company's most popular storage services, the outage rippled across the internet. The problem may appear to be online, but the