Google described Tuesday's Gmail outage as a "Big Deal" and stated the actions the company took to make sure it won't happen again. The Mountain View, Calif. company pointed the finger at a miscalculation during a "routine upgrade" to its Web servers, in a blog post.

Tuesday's two-hour outage follows interruptions of service for Gmail in February, and then again in March. The last major technical problem occurred in May -- with people unable to use its search engine, as well as Google News.

Knowing how many people depend on Google Gmail for both personal and business communications, Ben Treynor, vice president of engineering and site reliability czar, wrote: "I'd like to apologize to all of you."

It appears that Google "slightly underestimated the load" that some recent changes designed to improve service availability placed on the request routers and servers that direct Web queries to the appropriate Gmail server for response, Treynor explains.

"As a result, people couldn't access Gmail via the Web interface because their requests couldn't be routed to a Gmail server," he wrote. "IMAP/POP access and mail processing continued to work normally because these requests don't use the same routers."

Google says since the outage it has already "increased request router capacity well beyond peak demand to provide headroom," and determined that "request routers don't have sufficient failure isolation" -- so if there's a problem in one datacenter, it shouldn't affect servers in another datacenter.

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