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0 pointsponker5y ago0 comments

You mean all those Twitch streamers see their chats More than 4 seconds after the relevant video has elapsed? And can you explain what you mean by “stateful”? Thanks

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mmcclure5y ago

> all those Twitch streamers see their chats More than 4 seconds after the relevant video has elapsed

Yep! I think Twitch can sometimes be at 4 seconds or less for what it's worth, but yes, that delay is real. It's generally not really noticeable because the communication is totally async; the streamer is doing other stuff, finishing other thoughts, etc, then can get to messages as they see them.

> can you explain what you mean by “stateful”?

Sure! I was talking about what kind of connection is necessary to deliver the video. A lot of real-time video solutions require stateful connections to the client, which means that once the client connects, the connection is kept open, and video data is streamed via that connection. Common examples on the web are things like WebSockets and WebRTC, but it gets really expensive because you basically have to maintain a persistent connection with every single viewer and it makes it impossible (or extremely difficult) to do any kind of meaningful caching.

Stateless connections on the other hand are most of your common HTTP requests. The client asks for a resource, and gets it back. No prior connection setup is required, servers, networks, whatever can change between each request and everything will merrily chug along, which makes it much easier to scale.

ponkerOP5y ago

Huh, thanks, I think I’m starting to get it. Is it that the connection-based model needs the data “copied” into each user’s stream on the server-side while the “stash the files in a bin” stateless model allows the networking hardware to cache this data somewhere in memory and just copy it on the fly onto different network links?

sudhirj5y ago

Stashing files–let's say that each file is one second long and is just numbered with the unix timestamp – makes things cost effective because now the server is just dropping files no 1,2,3,4... into a directory, and everyone is pulling them out in sequence the way an other file would be downloaded. This also allows exploiting the HTTP architecture – if you set the Cache-Control: public headers on the files (which you can do happily because they'll never change) they'll be cached at lots of places along the way, like CDNs, the local ISP, the office network, etc. HTTPS blew out most of these caching benefits, but at least the CDNs can cache the files at edges all over the world.

donavanm5y ago

Its not just that it can be cached, but that it can use very standard existing infrastructure like HTTP CDNs, mobile browsers, etc. The limitation is that the audio/video is encoded as segments, each a few seconds long. Because of this it looks kind of like serial batch processing with latency constraints based on the batch size (segment duration). This is in contrast to say webrtc or rtmp thats a lot closer to a multiplexed stream of data.

ec1096855y ago

HTTP connections are stateful too. Clients aren’t doing tls handshakes every request, for instance.

jorams5y ago

It's important to note here that the 4 seconds of latency here is a pretty recent development. 30 seconds used te be the norm.

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