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by aeonik
556 days ago
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As long as the speed of light remains constant for all observers, who cares if everyone agrees on simultaneity? Distributed systems don't need to know what time it is, just what happened. Well known systems are implemented this way, and the UI is great, people barely even notice. |
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Between the posters in this group, we've got some ideals we'd like to meet
a) messages should be displayed as soon as possible when they arrive (this one isn't written much, but I think it's generally agreed)
b) messages should be displayed in a globally consistent order
c) message order should not change after display / newly arrived messages must be at the bottom/top
Unfortunately, we can't meet all these ideals unless instant messaging becomes actually instant instead of just pretty fast messaging subject to the speed of light and other sundry delays.
You can get all the properties if you serialize messages through some single queue somewhere, but of course that means additional delay and a spof.
You can most likely get b and c if you compromise a, and just don't display messages for long enough that you probably have everything and can order it according to the gloablly consistent ordering algorithm.
If you compromise b, you can definitely do a and c. Messages go to the end when received. Easy peasy. You can't leave placeholders for messages that will be sorted earlier but haven't arrived yet, because you generally won't know they exist until they arrive; although there are some cases where you could know. If a user receives message X and sends message Y, but due to delays and what not, you receive Y before X ... Y could indicate the presence of X, and a reciever who gets Y first could reserve a place for X... but that doesn't work for simultaneous messages.
If you compromise c, you can do a and b, messages are inserted into the ordered list on arrival based on the consensus ordering algorithm. Easy enough.
Of course, there's not widespread agreement on b and c. So half of the thread is people saying b is clearly non-negotiable, another half is saying c is clearly non-negotiable, and the other half is saying why can't we just have everything we want.