大型网站的架构演进以及对mysql与nosql的使用
随着数据量和访问量的增长，网站构架大致有这么几个发展阶段(以PHP+MySQL+Memcached为例)：
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1: PHP + MySQL
2: PHP + MySQL (Master + Slaves)
3: PHP + MySQL (Master + Slaves) + Memcached (Middleware)
4: PHP + MySQL (Sharding + Master + Slaves) + Memcached (Middleware)
5: PHP + MySQL (Sharding + Master + Slaves) + Memcached (Middleware) + NoSQL
