NoSQL at Twitter

Curious about this in snowflake (mentioned in slides):

https://github.com/twitter/snowflake/blob/master/src/main/sc...

  def nextId(): Long = synchronized {
    var timestamp = timeGen()

    if (lastTimestamp > timestamp) {
        log.warning("clock is moving backwards. Rejecting requests until %d.", lastTimestamp);
        throw new InvalidSystemClock("Clock moved backwards. Refusing to generate id for %d milliseconds".format(lastTimestamp - timestamp));
    }

Does that mean that the inability to tweet grinds to a halt if the system clock gets set back? That could be a problem whenever the server time is being sync'd with the time server and is running ahead (or if time server goes down/connection goes down- it happens), etc. I know there would be other issues too, but this seems a little fishy.