什么是透明缓存?
This is another installment in our series of “什么是...?" 文章, 旨在提供定义, 历史, and context around significant terms and issues in the online video industry.
随着网络视频内容的爆炸式增长，包括病毒式传播的用户生成内容, 在相当短的时间内产生了大量的观看量——传统的视频商业模式已经显示出摇摇欲坠的迹象. 根据预计数字, 到2013年，其中一些视频内容将占所有互联网流量的90%以上, 服务提供商已经开始认真考虑一些解决方案，这些解决方案允许在不占用宝贵的回程数据管道的情况下观看视频内容.
One proposed solution that's finding traction is the concept of transparent caching. 这个想法有两个主要原因，我们预计它将在未来几年内得到广泛应用.
透明缓存:概述
The term caching refers to storing objects in a group, ideally close to the need for that object, 就像亚马逊仓库在全国各地的几个仓库中分别存放数百台Kindle Fire平板电脑一样，预计平板电脑在每个仓库发货后的几个小时内就会销售出去.
在计算机世界里, 缓存也是类似的, but with a twist: A copy of a file is stored locally, or at least closer to the end-user device, 这样就可以重复使用了. The re-use part is key to understanding transparent caching. It's certainly feasible to cache all video content at the edge, 非常接近每个用户, and that is the business model for at least one large 内容分发网络(CDN). 它适用于优质内容, 比如电视节目或电影, 但是，边缘缓存网络上所有视频内容的想法——所有内容以相同的百分比被观看的可能性很小——既不实际，也不经济可行.
仍然, 需要在缓存边缘的少量内容和缓存所有内容之间取得平衡. That's where the transparent part comes in. 其想法是设置业务规则，使内容从网络核心移动到网络边缘的过程自动化, without requiring human intervention for any specific video file. Doing so allows the edge cache to refresh itself, 基于观看偏好的变化，在更细粒度的层面上，甚至比一大群人工操作人员所能完成的还要多. 对最终用户来说, 谁的视频开始得快, 网络运营商, who doesn't have popular video constantly traversing the network, the idea of transparent caching holds promise.
So what are the two key reasons that transparent caching holds merit? First, it's been around for quite some time; second, 流媒体 is moving towards a model that enhances the benefits of transparent caching.
透明缓存的历史
In the early days, caching was the sole domain of website hosting and serving. Content could be cached at a local computer, 以饼干或图像的形式, 大多数用户都理解“清除缓存”的概念，以确保他们的浏览器可以使用最新的内容. Caching devices on the ISP's network also held the more popular sites, 比如新闻网站, but those caches also had to be frequently cleared to keep headline stories fresh.
The one area that caching didn't really work well with was dynamic content. 想象一下这样的网页 Kayak.com, which aggregates data from numerous airlines' pricing databases: moment by moment, 座位可用性和价格变化, so it's not really practical to cache the majority of this content.
为tunately for the world of 流媒体 video, where more than ninety-five per cent of all video is on-demand and static, caching works in the same way that it worked for website caching. Yet the files are significantly bigger, 这意味着缓存最常发生在网络上而不是本地设备上.
Transparent caching has also moved in to mainstream computing, 不仅仅是网站, and is even integrated in to recent operating systems. 让我们使用一个非视频计算示例来说明这一点:一个标准的办公服务器, with a remote user logging in across a thin connection.
每一个经常出差的人都知道从办公服务器将一份大的PowerPoint文档检索到远程笔记本电脑的痛苦, making changes and then uploading the presentation again. 如果你想做的只是查看内容，如果笔记本电脑离开了VPN或网络，情况也不会更好, 即使是很短的一段时间, the process has to begin all over again. The process wastes precious time and resources, even for a recently viewed file.
当微软发布Windows 7时, one of the features it added was the concept of transparent caching. 据微软称, 使用Windows 7, "client computers cache remote files more aggressively, 减少客户端计算机从服务器计算机检索相同数据的次数。.
微软处理这一过程的方式与YouTube视频暂时缓存到本地桌面的方式大致相同:用户第一次从共享文件夹打开文件, Windows 7机器从服务器计算机读取文件，然后将其存储在本地磁盘的缓存中. On subsequent attempts to access the content, Windows 7本地机将从本地磁盘检索文件，而不是从服务器计算机读取.
When we talk about transparent caching for 流媒体, 这个过程非常相似, 尽管缓存发生在靠近服务提供商网络边缘的设备上，而不是在本地硬盘上.
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, 看看微软如何在Windows 7中平衡透明缓存和数据完整性是很有趣的.
“Windows 7总是与服务器计算机联系，以确保缓存副本是最新的,该公司网站上写道. "The cache is never accessed if the server computer is unavailable, and updates to the file are always written directly to the server computer."
Speed (and the Delivery Protocol) Matters
以Windows 7为例, 以上, Microsoft notes that transparent caching is not enabled by default on fast networks. This works for files that are less than 10MB, 说, since the server can download these files in just a few seconds, 但对于几百兆或几千兆大小的视频文件，它实际上并不适用.
Videocentric transparent caching needs to work, regardless of the speed at which the content is delivered to the end user. 为 自适应比特率内容, 这意味着不仅可以透明地缓存特定视频文件的初始请求比特率，还可以缓存该文件的所有可用比特率.
除了, a robust transparent caching system also has the ability for improved caching, 包括缓存使用的磁盘空间数量的配置以及阻止特定类型的视频文件被缓存的能力, whether by format or file size or popularity.
除了速度， 交货协议 也很重要, 这是一个流媒体更倾向于传统网站缓存的领域, 尤其是点播内容.
批准 MPEG破折号 (基于HTTP的动态自适应流)作为Adobe和Microsoft自适应流技术的衍生产品, 苹果也是如此 HTTP直播(HLS), 在将流传输从专门的协议转移到web服务协议的鼻祖:HTTP上是否有坚实的步骤, 或超文本传输协议.
Given the almost twenty-year 历史 of HTTP caching, 通过HTTP服务器而不是专门的视频服务器传输小片段/片段的流媒体内容只会增加透明缓存的好处.
透明缓存的其他好处
除了上面指出的两个好处之外，透明缓存还有两个额外的好处.
Briefly, one of the extra benefits is acceleration of origin caching. Some companies claim increases of up to 10 times performance for the origin cache, with transparent caching gathering music, video, 并将web内容放入专用的原始缓存盒中，然后与边缘服务器和基于边缘的透明代理缓存进行协商.
The second additional benefit is to move the transparent caching to the middle, rather than at the edge or network core. 这允许将夹层内容卸载到专用媒体服务器，以便随后转换为自适应比特率以进行自适应交付. Granted some encoding systems do their own segmentation, which can then be stored as a standard HTTP proxy cache, but others simply encode to a mezzanine file, which then benefits from mid-network transparent caching.
结论
透明缓存的主要受益者是终端用户和互联网服务提供商. 从财务角度来看, even if a service provider uses a CDN to serve up content, the content still must traverse a portion of the ISP's backbone, adding additional transport costs for every video served. 在透明缓存场景中, 然而, the majority of content of interest to the ISP's specific user base is available.
As more and more content becomes available, 我们期望在大型ISP的网络中看到透明的缓存呈现出一种地理定位的味道——就像当地附属电视广播公司迎合当地市场的方式一样——这将允许ISP与视频广告网络合作，通过ISP网络将视频货币化, reversing the trend from a business cost to a profit center.