January 2013
Intermediate to advanced
332 pages
5h 23m
Chinese
Content preview from JAVASCRIPT網頁應用程式設計
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第八章
即時網頁（Real-time web）
即時網頁為什麼會變得這麼重要？因為既然我們所處的真實世界，可讓我們隨時隨地、
即時的取得各式各樣的新資訊，人們自然也會期許網頁應用程式能比照辦理。使用者渴
求著能透過網頁應用程式滿足即時對談、即時獲取新知，以及即時搜尋資訊等需求。時
至今日，人們對於網際網路傳遞資訊的速度要求，早已和以往大不相同（晚幾分鐘才看
到插播新聞快報的情形，已經不能被接受了）。像 Google、Facebook，以及 Twitter 之類
的大型公司，皆已迅速的趕上這股潮流，為他們所提供的服務加上了即時互動的功能。
這是目前最火紅的潮流，而且相信這股熱潮還會持續延燒下去。
即時網頁技術的演進
過去，網頁是以 HTTP 的請求 / 回應模型建構：用戶端發出檢視網頁的請求，伺服器端
便針對該請求提供回應，除非用戶端再次發出檢視另一張網頁的請求，否則什麼事都不
會發生。隨後問世的 Ajax，讓網頁運作感覺更動態一些（因為 Ajax 會在背景發送請求
給伺服器端）。不過，如果伺服器端在提供回應之後，還有更多更新的資訊想提供給用
戶端，仍舊沒辦法在用戶端已經載入完網頁之後，再通知用戶端；也就是說，伺服器端
無法主動將最新資訊推送至用戶端。
有鑑於此，眾人紛紛集思廣益，構思出各種解決方案。一種最簡單的做法是輪詢
（polling），它的概念很簡單：不斷地詢問伺服器，確認是否有更新的資訊。這麼做的
確能讓使用者感受到即時的資訊更新。但在實務上，這種做法會降低效能，因為它迫使
伺服器每秒都得處理大量的連線，每組連線都會佔用一定程度的 ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.