什么是透明缓存?

这是我们的系列文章的另一部分 “什么是...?" 文章, 旨在提供定义, 历史, 以及围绕在线视频行业的重要术语和问题的背景. 

With the explosive growth of video content on the web—including user-generated content that goes viral, resulting in a massive number of views in fairly short timeframes—the old way of doing the business of video has shown signs of faltering. 根据预计数字, some of which place video content at over ninety per cent of all internet traffic by 2013, service providers have begun to look in earnest at a number of solutions that allow video content viewing without tying up precious backhaul data pipes.

一个备受关注的解决方案是透明缓存的概念. The idea has merit, for two key reasons, and we expect it to grow in use over the next few years.

透明缓存:概述

缓存这个术语指的是将对象存储在一个组中, 最理想的是接近那个物体的需要, in much the same way that an Amazon warehouse would store hundreds of Kindle Fire tablets in each of several warehouses around the country—anticipating sales of the tablet would occur within a few hours' delivery of each warehouse.

在计算机世界里, 缓存也是类似的, 但有一个变化:文件的副本存储在本地, 或者至少更接近终端用户设备, 这样就可以重复使用了. 重用部分是理解透明缓存的关键. 在边缘缓存所有视频内容当然是可行的, 非常接近每个用户, 这是至少一家大型公司的商业模式 内容分发网络(CDN). 它适用于优质内容, 比如电视节目或电影, but the idea of edge caching all video content on the web—on the off chance that all of the content will be viewed in equal percentages—is neither practical or financially viable.

仍然, there's a need to balance between caching just a small amount of content at the edge and caching it all. 这就是透明部分的用武之地. The idea is to set business rules that automate the process of moving content from the network core to the network edge, 无需人为干预任何特定的视频文件. 这样做允许边缘缓存自我刷新, based on changes in viewing preferences on a much more granular level than could be accomplished by even a large group of human operators. 对最终用户来说, 谁的视频开始得快, 网络运营商, 谁没有热门视频在网络上不断传播呢, 透明缓存的想法带来了希望.

那么透明缓存优点的两个关键原因是什么? First, it's been around for quite some time; second, 流媒体 正在朝着增强透明缓存好处的模型发展吗.

透明缓存的历史

在早期，缓存是网站托管和服务的唯一领域. 内容可以缓存在本地计算机上, 以饼干或图像的形式, and most users understand the idea of "clearing the cache" to make sure the most recent content is available for their browsers. ISP网络上的缓存设备也保存了更受欢迎的网站, 比如新闻网站, 但这些缓存也必须经常清理，以保持头条新闻的新鲜.

缓存不能很好地处理的一个领域是动态内容. 想象一下这样的网页 Kayak.com, 它从众多航空公司的定价数据库中收集数据:每时每刻, 座位可用性和价格变化, 因此，缓存大部分内容并不实际.

这对流媒体视频世界来说是幸运的, 哪里95%以上的视频都是点播和静态的, 缓存的工作方式与网站缓存的工作方式相同. 然而，这些文件要大得多, which means caching most often occurs on the network rather than at the local device.

透明缓存也进入了主流计算领域, 不仅仅是网站, 甚至集成到最新的操作系统中. Let's use an non-video computing example to illustrate the point: a standard office server, 远程用户通过瘦连接登录.

Everyone who's a road warrior knows the pain of retrieving a large PowerPoint document from the office server to a remote laptop, 进行更改，然后再次上传演示文稿. And it doesn't get any better if all you want to do is view the content: If the laptop leaves the VPN or network, 即使是很短的一段时间, 整个过程必须从头再来. 这个过程浪费了宝贵的时间和百家乐软件，即使是最近查看的文件.

当微软发布Windows 7时, 它增加的特性之一是透明缓存的概念. 据微软称, 使用Windows 7, 客户端计算机更积极地缓存远程文件, reducing the number of times a client computer might have to retrieve the same data from a server computer".

微软 handles its process in much the same way that a YouTube video is temporarily cached to a local desktop: The first time a user opens a file from a shared folder, the Windows 7 machine reads the file from the server computer and then stores it in a cache on the local disk. 在随后访问内容的尝试中, a Windows 7 local machine will retrieve the file from the local disk instead of reading it from the server computer.

当我们讨论流的透明缓存时, 这个过程非常相似, although the caching occurs at a device near the edge of the service provider's network rather than on the local hard disk.

One area that's key to making transparent caching work is the area of data integrity. 动态内容在前面提到过, and it's interesting to see how 微软 balances transparent caching and data integrity in Windows 7.

"Windows 7 always contacts the server computer to ensure the cached copy is up-to-date,该公司网站上写道. "如果服务器计算机不可用，则永远不会访问缓存, 对文件的更新总是直接写入服务器计算机."

速度(和交付协议)很重要

以Windows 7为例, 以上, 微软指出，在高速网络上，透明缓存默认情况下是不启用的. 这适用于小于10MB的文件, 说, 因为服务器可以在几秒钟内下载这些文件, but it doesn't really work for video files of several hundred megabytes—or multiple gigabytes—in size.

以视频为中心的透明缓存需要工作, 无论内容交付给最终用户的速度如何. 为 自适应比特率内容, this means transparently caching not just the initially requested bitrate of a particular video file but all of the available bitrates for the file.

除了, 一个健壮的透明缓存系统还具有改进缓存的能力, including configuration of the amount of disk space the cache uses and the ability to block particular types of video files from being cached, 无论是通过格式或文件大小或流行.

除了速度， 交货协议 也很重要, and this is an area where 流媒体 is trending more towards traditional website caching, 尤其是点播内容.

批准 MPEG破折号 (dynamic adaptive 流媒体 over HTTP) as a derivative of both Adobe and 微软 adaptive 流媒体 technologies, 苹果也是如此 HTTP直播(HLS), are solid steps in moving 流媒体 delivery away from specialized protocols and back towards the granddaddy of web-serving protocols: HTTP, 或超文本传输协议.

考虑到HTTP缓存已有近20年的历史, the move to stream content in small fragments / segments that are delivered via HTTP servers rather than specialized video servers will only increase the benefits of transparent caching.

透明缓存的其他好处

Beyond the two benefits pointed out 以上, transparent caching has two additional benefits.

简单地说，一个额外的好处是加速原始缓存. 一些公司声称原始缓存的性能提高了10倍, 具有透明缓存收集音乐, video, and web content into a dedicated origin caching box that then negotiates with both edge servers and edge-based transparent proxy caches.

第二个额外的好处是将透明缓存移到中间, 而不是在边缘或网络核心. This allows mezzanine content to be offloaded to a dedicated media server for subsequent conversion to adaptive bitrates for adaptive delivery. 当然，有些编码系统有自己的分割, 然后可以存储为标准的HTTP代理缓存, 但其他人只是将其编码为夹层文件, 那么谁会从网络中间透明缓存中受益呢.

结论

The primary beneficiaries of transparent caching are the end user and the internet service provider. 从财务角度来看, 即使服务提供商使用CDN来提供内容, 内容仍然必须通过ISP主干的一部分, 为每个视频服务增加额外的运输费用. 在透明缓存场景中, 然而, ISP的特定用户群感兴趣的大部分内容都是可用的.

随着越来越多的内容变得可用, we expect to see transparent caches across larger ISP's networks take on a geo-targeted flavor—not unlike the way that local affiliate television broadcasters cater to a local market—that will allow the ISP to partner with video ad networks to monetize video traversing the ISP network, 扭转从业务成本中心到利润中心的趋势.