Around an hour ago, the Stack Overflow and other Stack Exchange sites were in read-only mode, which as far as I have seen wouldn't let me log in, so I was unable to post any questions, answers, or comments. However somehow I saw some new questions suddenly appearing (I noticed this on both Stack Overflow and Meta Stack Overflow). How is that possible?

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    It was probably unstable, i.e. read-only for X minutes, then back online, then read-only again. So in those "windows" of uptime users could do everything. – Shadow The Vaccinated Wizard May 6 '14 at 22:07
  • @ShadowWizard oh, well I saw no indication of something like that so I found it weird, mostly because I saw many different questions and never stopped seeing the This site is currently in read-only mode; we’ll return with full functionality soon. message, but I guess that's a very logical explanation. – DarkAjax May 6 '14 at 22:09
  • I suspect this message is being set by the team when things are unstable... unstable doesn't always mean totally offline so if someone started to write a question when it was online and submitted when it was read-only, the server did accept it. Need a dev to confirm of course. :) – Shadow The Vaccinated Wizard May 6 '14 at 22:13

From the @StackStatus post:

The Stack Exchange read-only test is starting now, DNS failover will happen over the next 5 minutes.

That would lead me to believe that they tinker with DNS records to make the failover to the secondary read only data center in Oregon happen. Stack Exchange has set the time to live on their DNS records to 300 seconds. Meaning the information that translates stackexchange.com into an IP address should only be cached for 300 seconds before asking Stack Exchange for the information again. If your computer or ISP decides it won't obey that 300 second cache limit, that user would have seen the New York version of Stack Exchange which was not in read only mode.

Obviously, Nick (or another Stack Exchange employee) may have a different account of the events. But I've had a similar situation happen in the past.

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    If this is true, then of course the opposite behaviour happens when the fall-back occurs; people's DNS caches expire at different times; those that expire first find themselves pointed to the writable datacenter earlier than others. The "others" still see a read-only version of SE, but with the new posts (being posted on writable datacenter) being propagated to the read-only via replication. – Matt May 7 '14 at 10:19

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