關於 Memcached
memcached 是一個高性能、分散式記憶體物件快取系統，本質上是通用的，但最初是用來透過減輕資料庫負載來加速動態 Web 應用程式。
你可以把它想成應用程式的短期記憶體。
它的功能
memcached 允許你從系統中記憶體較多的地方取得記憶體，並讓記憶體較少的地方可以使用。
memcached 也能讓你更有效地使用記憶體。如果你考慮右側的圖表，你可以看到兩種部署情境
- 每個節點完全獨立（頂部）。
- 每個節點都可以使用其他節點的記憶體（底部）。
第一個情境說明了傳統的部署策略，然而你會發現它既浪費，因為快取總大小只是 Web 農場實際容量的一小部分，而且在所有這些節點中保持快取一致所需的努力也很大。
使用 memcached，你可以看到所有伺服器都在查看同一個虛擬記憶體池。這表示特定項目始終儲存在整個 Web 群集中的同一個位置，並始終從該位置擷取。
此外，隨著應用程式需求增加到需要更多伺服器時，通常在必須定期存取的資料方面也會增加。一種讓系統的這兩個面向一起擴充的部署策略很有道理。
右側的說明為簡潔起見只顯示兩個 Web 伺服器，但隨著數量增加，屬性保持不變。如果你有五十個 Web 伺服器，在第一個範例中，你仍然會有 64MB 的可用快取大小，但在第二個範例中，你會有 3.2GB 的可用快取。
當然，你不需要使用 Web 伺服器的記憶體作為快取。許多 memcached 使用者有專用的機器，專門用來當作 memcached 伺服器。
起源
Memcached 最初是由 Brad Fitzpatrick 於 2003 年為 LiveJournal 開發。
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