部分置換儲存式快取代理伺服器之效能分析與設計實作

Abstract

部分儲存式代理快取或稱分段儲存式快取，是將物件分成許多獨立的檔案片段加以儲存或移除。當快取伺服器收到使用者的要求時，會先將儲存在快取中的檔案片段傳給使用者以減低使用者感受到的回應的延遲。在這篇論文中我們檢視了將這個理念實現在快取上的可能性。首先，我們提出了一個可以利用現有的HTTP來對多媒體檔案進行快取的架構。在這個架構下，可以明顯地降低使用者所感受到的傳輸延遲，互動式VCR多媒體的存取也可實現，最重要的是我們可以簡化現有代理快取伺服器在快取多媒體檔案的設計。同時我們也提到部分儲存式代理快取伺服器所面臨到的重要問題：我們稱它為「過期開頭片段問題」，也就是說快取中儲存的片段和原始伺服器端的不一樣。如果發生這樣的問題將導致使用者前後所得到的檔案不連貫。如此，使用者會先收到在快取上的過期資料，並和後來從伺服端得到的新資料不能連貫。要解決這樣的問題，我們提出了一個創新的演算法稱為「部分移除確認」，這個演算法將取得伺服器端物件一致性的確認，物件的分段以及移除放在一起做。結果顯示我們所提的方法除了有效的降低了上述不一致性的發生機率，且可以簡單地實作在現有的快取軟體架構之下並明顯的降低在多媒體檔案傳輸上的起始延遲。

Proxy partial caching, e.g. segment-based proxy caching or proxy prefix caching, partitions the object into non-overlapping pieces. The proxy cache then treats each piece as an individual file when performing caching and replacing. Upon receipt of request to the object, the proxy cache delivers the cached portion to the client immediately to mask the start-up delay. In this thesis, we examine the viability of applying this technique to web proxy cache. First, we propose a architecture of caching multimedia streams via HTTP. With in this proposed architecture, the start-up delay of video playout is easily masked, the interactive VCR function can be realized, and caching the multimedia streams in the web proxy cache can be greatly simplified. Second, proxy partial caching introduces a problem called dirty-first-segment problem, i.e. cached portion in the proxy cache is inconsistent with the original in the server. If it happens, cached portion cannot be concatenated with the latter portion from the server. As a result, user will receive cached portion first and then perceive an undesirable situation, such as a discontinuous video playout. To relieve this problem, we propose a novel algorithm called validating upon partial replacement, which synchronizes the operation of proactive consistency validation, partition, and replacement performed upon an object. We show that our proposed algorithm bears the merits of controllable chance to disturb users, easily embedding into the existing web proxy cache software with minimum modification, and significant improvement of cache performance in the aspect of start-up delay reduction for multimedia stream accesses.