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How an Amazon engineer's slip-up started a 20-hour Netflix cock-up

Xmas cloud outage sparked by fumbled delete op

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An Amazon engineer hit the wrong button on Christmas Eve, deleting critical data in its load balancers and ultimately knackering vid streaming biz Netflix for 20 hours.

The Netflix outage hit customers in the US, Canada and Latin America on 24 December, particularly those using games consoles and mobiles to watch films, while desktop Mac and PC users suffered time-outs and delays. A mistake at Netflix's hoster - Amazon Web Services - caused the downtime and Amazon's developer team worked through the night into Christmas morning to get the problems fixed by 8am 25 December.

Statements from Netflix and Amazon explained that the problem started when a developer accidentally wiped important data on how video traffic should be directed by the Elastic Load Balancers (ELBs) at Amazon Web Services.

These load balancers juggle requests from devices to servers and spread the workload across systems. The ELBs rely on information referred to as state data to control their operation and track what requests are going where.

On Christmas Eve an engineer running a maintenance process accidentally directed it at this vital state data and, as a result, deleted the information on the production ELBs in the East USA region. The unlucky bod didn't realise his or her mistake and it took the Amazon team a while to work out exactly what had gone wrong since many requests for media were still being processed as usual. At the peak of the cock-up 6.8 per cent of the AWS balancers were out of action.

After realising the problem, Amazon engineers worked through the night to restore the missing ELB state data from a backup taken just before midday on Christmas Eve and merge new data from the past 20 hours with the historic snapshot.

Netflix uses hundreds of Elastic Load Balancers grouped together by the type of device supported by the service. That was why console and mobile users were affected to a greater extent than users on other machines.

Amazon has slapped a number of protections on the Elastic Load Balancers to prevent this happening again. Access to the state data has been restricted. Amazon added that it has streamlined its data recovery process following a few bungles in the attempt to get the service back up and running.

Netflix said the breakdown revealed the challenges of maintaining a globally available cloud service. Both companies have apologised to customers whose festive film-watching was disrupted. ®

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