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Amazon.com’s U.S. retail site became unavailable around 10:25 AM PST, and now appears to be back up. Amazon’s not naming names — all that director of strategic communications Craig Berman would say was that: “Amazon’s systems are very complex and on rare occasions, despite our best efforts, they may experience problems.”

Berman did confirm, however, that neither Amazon Web Services nor international sites were affected.

So what happened? Let’s look at the facts.

Traffic to http://www.amazon.com was getting there. So DNS was configured properly to send traffic to Amazon’s data centers. Global server load balancing (GSLB) is the first line of defense when a data center goes off the air. Either GSLB didn’t detect that the main data center was down, or there was no spare to which it could send visitors.

When traffic hit the data center, the load balancer wasn’t redirecting it. This is the second line of defense, designed to catch visitors who weren’t sent elsewhere by GSLB.

If some of the servers died, the load balancer should have taken them out of rotation. Either it didn’t detect the error, or all the servers were out. This is the third line of defense.

Most companies have an “apology page” that the load balancer serves when all servers are down. This is the fourth line of defense, and it didn’t work either.

The HTTP 1.1 message users saw shows something that “speaks” HTTP was on the other end. So this probably wasn’t a router or firewall.

This sort of thing is usually caused by a misconfigured HTTP service on the load balancer. But that would happen late at night, be detected, and rolled back. It could also happen from a content delivery network (CDN) not retrieving the home page properly.

So my money’s on an AFE or CDN problem. But as Berman notes, Amazon’s store is a complex application and much of their infrastructure doesn’t follow “normal” data center design. So only time (and hopefully Amazon) will tell.

Site operators can learn from this: Look into GSLB, and make sure you have geographically distributed data centers (possibly through AWS Availability Zones.) It’s another sign we can’t take operations for granted, even in the cloud.