About a month ago, Microsoft touted in a news release what appeared to be a sales victory: Symantec would use Microsoft's cloud-computing services to help power its Norton-branded cybersecurity products.

On Wednesday, Amazon Web Services returned fire. The Amazon unit said Symantec had chosen AWS for "the vast majority of its cloud workloads," appearing to trump its biggest rival in the emerging market for on-demand computing infrastructure.

Businesses are increasingly running their software programs and storing their data on servers rented from Amazon, Microsoft or Google, rather than using their own corporate data centers. That's set up a marketing war, as cloud-computing providers seek to demonstrate their expertise by trumpeting the business they get from well-known customers.

Sometimes, as in Symantec's split business, the news can seem contradictory. That's partly because sprawling corporate customers use hundreds or thousands of software services built by many vendors. Coca-Cola units for instance, use AWS, Microsoft Azure, Google Cloud Platform, and IBM's cloud arm. The cloud-computing rivals are sometimes vague on what customers are actually using their services for.

Wednesday brought the start of what's likely to be a flood of similar announcements from Amazon in the next week. Monday is the kickoff of re:Invent, the AWS conference in Las Vegas that draws tens of thousands of developers, customers and technology partners. The trade show features a set of AWS announcements of new features and customers, and the Amazon unit typically releases a few bits of news in the days and weeks ahead of the show.

In another announcement Wednesday afternoon in the hours before Corporate America decamped for the Thanksgiving holiday, AWS said it would offer customers training in how to use its advanced, artificial-intelligence-like software services. The new ML (short for machine learning) Solutions Lab will offer customers one- to six-month boot camps -- training available to AWS customers who use the company's business customer support services.

As to Amazon's Symantec announcement, a Microsoft spokesman said the company is "proud to be a strategic cloud provider for Symantec and to partner closely with the company for its hybrid cloud strategy."

If history is a guide, that probably won't be the last word.