Amazon's Elastic Compute Cloud (EC2) service suffered a networking outage yesterday that caused instances to become inaccessible from outside Amazon's network. This meant that connections from the Internet failed to reach some instances, though these instances continued running and were accessible from other machines within EC2. The outage lasted for up to an hour, according to comments in the discussion forums.
Amazon staff are investigating the issue and as of now there is no official word on the cause, but what is likely to be of most interest to EC2 users is whether all of the availability zones (locations) were affected. If the outage was localized to one or two zones (out of three) it would demonstrate the benefit of distributing your instances between multiple zones using the new Availability Zone feature.
If all the zones were affected, this would demonstrate a central point of failure that partially nullifies the strategy of distributing your instances across zones. In this case, Amazon would need to modify the architecture to remove the centralized failure point.
The forum discussion thread contains conflicting reports from users about whether the fault was localized or general. I will update this post when more information is available.
An Amazon staff member has posted a post mortem comment stating that this networking issue did indeed affect multiple Availability Zones. It looks like this happened because the failure was caused by misconfiguration rather than by a hardware or connection failure -- basically one of those completely unexpected events for which there are no contingency plans.
This was an embarrassing glitch, and one which cannot be repeated if developers are to believe that different availability zones give true isolation from failures.
On the bright side, the issue was fixed quickly despite the cause being difficult to find, and the full and frank explanation from Amazon gives grounds for hope that the necessary monitoring and process improvements will be put in place to prevent a repeat.