Google's application-hosting service suffered an outage on Tuesday, highlighting one of the downsides of the new cloud computing services.

Between 9 a.m. and 11:30 a.m. West Coast time and again later in the day, a significant percentage of users trying to access the service were unable to do so, according to a post on the Google App Engine forum.

"This outage was the result of a bug in our datastore servers and was triggered by a particular class of queries," wrote a member of the App Engine Team who called himself "Pete."

At around 1:40 p.m. Google was able to isolate the problem and restore the service, he wrote. Google is still working on a fix for the bug, however.

App Engine is a service that lets developers host their Web applications for free for up to 5 million monthly page views, and includes up to 500M bytes of persistent storage. It is aimed largely at developers rather than companies looking to host commercial services, so the outage may have had less impact than similar recent outages at Amazon's hosted services, said Phil Shih, an analyst with Tier 1 Research.

Google's App Engine "is very much a service targeted at developers who are kind of experimenting with new projects," he said. "It's a very limited and unique audience with a specific set of needs who may be able to arguably tolerate a little more unreliability."

Google only launched App Engine earlier this year, and Shih suspects that the company may have quickly pulled together the offering in hopes of catching up with the momentum around cloud computing. "Google is far, far behind Amazon," he said.

Still, even Amazon, which has been offering its Web Services since 2002, has its share of problems. Amazon has had outages on both its hosted computing and hosted storage services this year.

The troubles both companies are having reflects how new these types of offerings are, said Shih, who emphasizes his advice that companies shouldn't host any mission-critical applications on these services. "This is not something you can trust for anything that you depend on having online all the time," he said.