A while ago youtube changed its cache system. Now when you watch a video, if you click backward on timeline, the video will be downloaded again, and youtube does not read it from cache. It is very slow on low band-width connection. I want to know why youtube decide to do that? It's obviously the opposite of UX.
closed as off topic by Mervin, Benny Skogberg, JohnGB, Matt Obee, JonW♦ Feb 17 at 8:24
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Your question is wrong. Youtube DOES cache videos.
So, you are wrong in asking the question. Secondly, they have messed up the buffer system, not the cache system of Youtube. If you pause a video on a slow connection, allowing it to buffer before you watch it, that doesn't happen i.e. the buffer also stops on pausing the video. There have also been reports of you can reload the video from start i.e. get the cached copy but as soon as you hit it somewhere from the middle of the timeline (skip to some part), that loses the whole buffer uptil then and/or the cached video.
However, there have been some proposed workarounds like clearing your cookies to reset the buffer mechanism and I have tried them, and they do work - however not a good option.
So yes, the cache is still pretty much there the buffer is not.
And I believe the new Youtube is a reason for that. If you use an old YOutube extension of Chrome which still offers the same old Youtube interface, you wouldn't face any such problem which indicates that they haven't changed the functionality at a server level. Its most likely the case of front/user-end youtube that we see; the new implementation is messing up the buffer bar!
Hope that clears it up.
P.S: messy buffer also equals bad UX.
Often the most common reasons for video re-loading when you move timeline are:
So for example, a video may start playing at 360P, but when the system detects that your bandwidth can handle additional 720p, it will transition over to 720p. When that happens, it has to write a new cache file, because you cannot have a video file with two different resolutions.
So when you try to go back to time period before the resolution-transition took place, your existing local cache gets wiped, and new cache starts.
Another common reason is, people often fast-forward when they watch videos, which also interrupts local caching. Let's say you are 5 minutes into a video, but you've skipped the first 30 seconds. At this point, Youtube has cached 0:30 - 5:00, so you are free to drag your timeline within that spot. But if you go to 0:00 - 0:30, your player has to download that stream, and 0:30 - 5:00 cache will be lost.
With that said, you can avoid the cache interruption problem by doing the followings:
As soon as video begins, change the Video quality setting to your desired resolution (360p, 480p, 720p, etc.) and turn off the Auto. And don't fast-forward.
By taking these steps, you'll ensure that the video is cached locally in its entirety, and you'll be able to move back and forth within the timeline without reloading.