Amazon and Oracle don't much like each other.

On Wednesday, two months after Oracle co-CEO Mark Hurd called Amazon's cloud infrastructure "old" and claimed his company was gaining share, Amazon Web Services chief Andy Jassy slammed Oracle for locking customers into painfully long and expensive contracts.

"People are very sensitive about being locked in given the experience they've had the last 10 to 15 years," Jassy said on Wednesday on stage at Amazon's AWS Summit in San Francisco. "When you look at cloud , it's nothing like being locked into Oracle."

Jassy was addressing a cultural shift in the way technology is bought and sold. No longer does the process involve the purchase of heavy proprietary software with multi-year contracts that include annual maintenance fees. Now, Jassy says, it's about choice and ease of use, including letting clients turn things off if they're not working.

He specifically went after Oracle's core database business, saying that "over the last few decades, it has been a lonely place for customers" because of the high prices and vendor lock-in.

"Customers are sick of it," he said.

The companies, founded by two of the world's wealthiest and most outspoken businessmen, have become unlikely rivals since Amazon jumped into the computing services market and Oracle later started migrating to the cloud.

AWS has emerged as the dominant supplier of cloud technology, with a bigger share of the market than about the next dozen players combined, according to Synergy Research Group.

It's become a particularly sensitive battle to Oracle because when businesses turn to databases in Amazon's cloud, they to tend to favor products that are much cheaper and more flexible than Oracle's. Amazon's homegrown Aurora database "is the fastest-growing service in the history of AWS," Jassy said.