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  • Unfortunate handling of an error condition caused Facebook Outage:



    Unfortunate handling of an error condition caused Facebook Outage:

    2.5 hrs of outage world wide, the worst outage facebook has ever faced in four years. Even now fake accounts has been blocked.. Rumors spread like wild fire that facebook has been hacked and will be permanently shut down . In this mere 2.5 hrs people have comeup with several reasons why the facebook was down ,but none of them was even closer to the real one.

    Apologizing for such worst outage an engineer from the company explained exactly what went wrong. The cause of the issue sounds relatively complicated, however the conclusion was that the company had to restart the entire site.

    The key flaw that caused this outage to be so severe was an unfortunate handling of an error condition. An automated system for verifying configuration values ended up causing much more damage than it fixed.

    The intent of the automated system is to check for configuration values that are invalid in the cache and replace them with updated values from the persistent store. This works well for a transient problem with the cache, but it doesnít work when the persistent store is invalid.

    Today we made a change to the persistent copy of a configuration value that was interpreted as invalid. This meant that every single client saw the invalid value and attempted to fix it. Because the fix involves making a query to a cluster of databases, that cluster was quickly overwhelmed by hundreds of thousands of queries a second.
    To make matters worse, every time a client got an error attempting to query one of the databases it interpreted it as an invalid value, and deleted the corresponding cache key. This meant that even after the original problem had been fixed, the stream of queries continued. As long as the databases failed to service some of the requests, they were causing even more requests to themselves.

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