Actually Gmail was not completely down,but its web interface.Many Gmail users always are familiar to check or send emails through visiting the web interface instead of using IMAP/POP access.Thus,the downtime of the web interface causes a lot of users pending on their workings.
Google posted the explanation on official blog with a long article written by by Ben Treynor, VP Engineering and Site Reliability Czar.Cutting it into short,the issue was an underestimation during offline maintenance.The details are as follow:
This morning (Pacific Time) we took a small fraction of Gmail’s servers offline to perform routine upgrades. This isn’t in itself a problem — we do this all the time, and Gmail’s web interface runs in many locations and just sends traffic to other locations when one is offline.
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.
The Gmail engineering team was alerted to the failures within seconds (we take monitoring very seriously). After establishing that the core problem was insufficient available capacity, the team brought a LOT of additional request routers online (flexible capacity is one of the advantages of Google’s architecture), distributed the traffic across the request routers, and the Gmail web interface came back online.
Gmail team also promises that they will enhance the service by adding more capacities of routers and optimize the system policy to avoid this kind of matter happen again.
As a loyal user,I hope Gmail won’t make me disappointed again.