Open-source darling MySQL is facing a new uprising within its customer base over plans disclosed this week to reserve some key upcoming features, and their source code, for paying users of its namesake database.

Officials at Sun Microsystems, which acquired MySQL in February, confirmed that new online backup capabilities now under development will be offered only to MySQL Enterprise customers -- not to the much larger number of users of the free MySQL Community edition.

The plan was detailed during meetings at MySQL's annual user conference in the US, during which Sun also delayed until late June the release of a MySQL 5.1 upgrade in order to iron out some remaining bugs.

This is the second dust-up between MySQL and its users in the past eight months. Last August, an earlier decision to stop making the MySQL Enterprise source code openly available to users without paid subscriptions drew criticism from some members of the MySQL community.

Red Hat and many other open-source vendors test new features by first offering them to nonpaying users, who also get access to the source code for those features.

MySQL's software reportedly is used by tens of millions of individual users, and its corporate customers include Google, Yahoo and some of the most popular Web 2.0 sites. Cheerfully acknowledging in an interview last year that only one in a thousand MySQL users paid for the software, then-CEO Marten Mickos said that the company had no plans to make some of its products and source code proprietary.

"We've had that debate many times," said Mickos, who now is senior vice president of Sun's database group. "I think we might win a few new customers, but we would lose 2 million users. We're not ready for that kind of compromise." He added that other vendors that had built closed-source products on top of open-source software "don't seem successful.

The decision to now withhold some features from the community version caught MySQL loyalists by surprise, and some are accusing MySQL of betraying the community that helped build it up.

"Does not MySQL believe in open source? Or just partially believe?" asked Vadim Tkachenko in a blog post. Tkachenko, a former MySQL employee who now works as a consultant at Percona, said that while Sun itself is releasing open-source versions of previously proprietary products, MySQL seems to be trying to hide features from the open-source community. "I understand this all is about money, and anyone is free to do with his product anything, but MySQL does not seem [to be] playing [an] open game here," he wrote.

"As before MySQL AB illustrates that they have stop believing in open source when it becomes time to make money," wrote Lukas Kahwe Smith, another MySQL user, on his blog.