Memcache入门介绍

zhouandtao

什么是Memcached？
http://memcached.org/ 写道
Free & open source, high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating（缓解） database load.

Memcached is an in-memory key-value store for small chunks（数据块） of arbitrary （任意的）data (strings, objects) from results of database calls, API calls, or page rendering.

Memcached is simple yet powerful. Its simple design promotes quick deployment, ease of development, and solves many problems facing large data caches. Its API is available for most popular languages.
　　 
Memcached能做什么？

　　memcached allows you to take memory from parts of your system where you have more than you need and make it accessible to areas where you have less than you need.
　　

　　memcached also allows you to make better use of your memory. If you consider the diagram to the right, you can see two deployment scenarios:
　　

　　deployment strategy 1 : Each node is completely independent (top).
　　deployment strategy 2 : Each node can make use of memory from other nodes (bottom).
　　The first scenario illustrates the classic deployment strategy, however you'll find that it's both wasteful in the sense that the total cache size is a fraction(一部分) of the actual capacity of your web farm, but also in the amount of effort required tokeep the cache consistent across all of those nodes.
　　

　　With memcached, you can see that all of the servers are looking into the same virtual pool of memory. This means that a given item is always stored and always retrieved from the same location in your entire web cluster.
　　-----应用需求的增长，数据的增长-----
　　Also, as the demand for your application grows to the point where you need to have more servers, it generally also grows in terms of the data that must be regularly accessed. A deployment strategy where these two aspects of your system scale together just makes sense.
　　

　　The illustration to the right only shows two web servers for simplicity, but the property remains the same as the number increases. If you had fifty web servers, you'd still have a usable cache size of 64MB in the first example, but in the second, you'd have 3.2GB of usable cache.
　　

　　Of course, you aren't required to use your web server's memory for cache. Many memcached users have dedicated machines（专用的） that are built to only be memcached servers.
Memcached有什么特点？
若干个疑问
　　1. 在oschina（http://www.oschina.net/p/memcached/）介绍Memcached时说是集中式缓存系统，那么分布式和集中式有何区别？Memcached是分布式的还是集中式的？
其他
　　memcached缺乏认证以及安全管制，这代表应该将memcached服务器放置在防火墙后。
　　memcached的API使用三十二位元的循环冗余校验（CRC-32）计算键值后，将资料分散在不同的机器上。当表格满了以后，接下来新增的资料会以LRU机制替换掉。由于memcached通常只是当作快取系统使用，所以使用memcached的应用程式在写回较慢的系统时（像是后端的数据库）需要额外的程式码更新memcached内的资料。