Gmail service experienced a widespread outage Tuesday due to an overload router. The disruption of the service led Gmail users to get an “Unable to reach Gmail” error message as their computers tried repeatedly to reconnect to the service.

So what happened to Gmail yesterday? They took a “small fraction” of Gmail’s servers offline to perform routine upgrades as usual. Also here’s a part of the Google press release.

However, as we now know, we had slightly underestimated the load which some recent changes (ironically, some designed to improve service availability) placed on the request routers — servers which direct web queries to the appropriate Gmail server for response. At about 12:30 pm Pacific a few of the request routers became overloaded and in effect told the rest of the system “stop sending us traffic, we’re too slow!”. This transferred the load onto the remaining request routers, causing a few more of them to also become overloaded, and within minutes nearly all of the request routers were overloaded. As a result, people couldn’t access Gmail via the web interface because their requests couldn’t be routed to a Gmail server. IMAP/POP access and mail processing continued to work normally because these requests don’t use the same routers.

Google said Tuesday afternoon that it fixed the problem. And they have turned their full attention to preventing this kind of thing in the future. Though occasional disruptions are common, widespread outages involving Google’s services are rare.

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