Gmail, Google’s free web mail service that turns five this year but is still in beta, went down for a couple of hours yesterday. The outage even affected Google Apps customers and Google is paying them a 15 day credit to cover any lost productivity due to the Gmail downtime.

Why did Gmail go down for such a long period?

The official explanation provided by Gmail puts the blame on a routine software update. They had to shut down one datacenter for a software update but that overloaded other data centers and the problem just cascaded.

This morning, there was a routine maintenance event in one of our European data centers. Unexpected side effects of some new code that tries to keep data geographically close to its owner caused another data center in Europe to become overloaded, and that caused cascading problems from one data center to another.

When the Gmail service was back up and running, users were asked to complete a CAPTCHA in order to prove that they are “humans and not robots.” This has prompted some experts to believe that hackers and not software bugs may have been responsible for the Gmail outage.

This may have been due to an Distributed Denial Of Services Attack led by thousands of PCs controlled by a few people.. As all these PCs may have accessed Gmail simultaneously in a matter of a few seconds, the email giant, unable to handle the large traffic, could have crashed.

While this can be a possible reason, I would go with the explanation offered by Gmail’s site reliability manager. Here’s why - there are chances that people may be logging into their Google Accounts repeatedly just to make sure that the Gmail service is up but since this action can be incorrectly classified as that of a robot, Google decided to place a CAPTCHA on the login screen of Gmail.

Whatever may be the actual reason behind Gmail outage, it does highlight how important it is to have an offline copy of your emails. You don’t want to land in a situation where your flight e-tickets are in the online inbox but you can’t print them as the Gmail service is down. On the positive side, some people are saying they had more focused meetings during the downtime as employees weren’t checking emails.

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