Yahoo! doesn't plan to release all its software code under an open source licence because it's unpractical and carries limited benefits, Yahoo! developer Jeremy Zawodny said on his blog.

"Being good open source citizens means contributing where it makes sense, allowing our employees to be a part of the open source world, and helping to evangelize the benefits of open source software," Zawodny wrote.

Much of the internally developed code is specifically geared towards a search engine that serves up billions of pages per day, he explained. This requires the code to be tightly knit together and to include references to trade secrets that won't be published under an open source licence.

"There'd be places in the code where magic voodoo functions are called but we couldn't really talk about what they do or how they might work. That's called our secret sauce or 'business logic' if you prefer. A good deal of that is kept under wraps for very legitimate reasons."

Zawodny is a developer for the MySQL open source database and works for Yahoo!'s platform engineering group. He's one of the search engine's best known bloggers.

The blog posting was written in response to a critical note from Matt Asay, vice president of business development for Alfrasco, developer of an open source content management system.

He accused Yahoo! and Google of being bad open source citizens because they refrain from opening the source code of all their internally developed applications. This is in part the result of a lack of modularity in their software, which makes it easier for multiple developers to work on a single application.

"It is disappointing that they are such heavy users of open source and have architected themselves into a corner that makes giving back impossible or problematic," Asay concluded.

Zawodny countered that modularity impacts performance. Given Yahoo's size, minor performance bottlenecks can add up to major headaches.

While he welcomed the possibility of receiving bug fixes, he argued that the internet portal didn't have much to gain from outside developers adding features to existing Yahoo! services.

"Then we'd be beat up for not integrating it fast enough. It's tricky to introduce new features to a product that tens of millions of people are using."

Yahoo! has been contributing to open source applications including Linux, FreeBSD, Perl and MySQL. The company also publishes a series of application programming interfaces (APIs) that allow third party developers to create applications that interact with Yahoo! services, allowing them to add search to applications or develop plug-ins for its messenger software.