[Hadoop Wiki] Update of "HedWig" by mahadevkonar

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The "HedWig" page has been changed by mahadevkonar.
http://wiki.apache.org/hadoop/HedWig?action=diff&rev1=17&rev2=18

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  === Architecture ===
  At the highest level, Hedwig is a collection of *regions* spread across the internet. Any region may publish on a topic, and those messages must be delivered to any subscriber in any region that has subscribed to that topic.
  
- {{attachment:hedwig.png}}
+ {{attachment:hedwig_overview.png}}
  
  Now digging down into a region, it consists of a collection of '''hub servers'''. Hub servers aggregate messages published in a region and persist them. They also subscribe to hubs in other regions to listen for messages that their clients are subscribed to. Clients always subscribe only to  local hub servers. Hedwig plans to use [[http://hadoop.apache.org/zookeeper/|Zookeeper]] for persistence of metadata, and [[http://hadoop.apache.org/zookeeper/docs/r3.3.2/bookkeeperOverview.html|Bookkeeper]] for persistence of actual messages.
  
- {{attachment:region_1.jpg}}
+ {{attachment:region_main1.jpg}}
  
  Topics are randomly split over hubs. When the hub responsible for a topic fails, another hub should take over responsibility of the topic.