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http://redis.io/topics/twitter-clone
http://www.elasticsearch.org/guide/reference/api/
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1.mysql index innodb（B+-tree）http://en.wikipedia.org/wiki/B-tree
     http://en.wikipedia.org/wiki/B%2B_tree
Figure 3-9 is an abstract diagram of how InnoDB and MyISAM arrange the table.
This illustration makes it easier to see how differently InnoDB and MyISAM store
data and indexes.