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A couple of posts back we spent a while inside XOR distance, then used it to build Kademlia, the DHT algorithm that lets a network find anything without a directory.
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We're taking BitTorrent apart, piece by piece, and Kademlia is going to walk right back in through the side door.
Also, fun fact before we start: a suspicious number of people on Reddit think Bram Cohen, the guy who wrote BitTorrent alone in Python in 2001, is secretly Satoshi Nakamoto.
I'm saying that by the end of this post you'll understand why people keep saying it.
In 2004, a measurement firm called CacheLogic reported that BitTorrent alone was responsible for roughly 35% of all internet traffic.
One protocol. Written by one guy. No company. No datacenter. No servers anywhere with "BitTorrent Inc" on the rack.
Every "normal" system you've ever worked on scales by throwing money at it: bigger box, more replicas, a CDN in front.
BitTorrent had nobody to throw money at anything, so every hard problem, capacity, trust, scheduling, incentives, discovery, had to get solved inside the protocol itself.
Distributing a file in 2001 meant one server, one uplink, and every download eating a slice of that uplink.
Get linked from somewhere popular and you get what people at the time literally called the Slashdot effect: your server saturates at the exact moment the most people want the thing it's serving.
You're stuck picking a poison: over-provision and burn money on capacity nobody uses most days, or under-provision and eat an outage on the one day it actually mattered.
Here's BitTorrent's actual trick, and it's almost insultingly simple once you say it out loud: every peer that downloads a file also becomes a source for it.
A flash crowd that would take down a normal server literally makes a BitTorrent swarm faster.
It's the one system I know of where the traffic spike is the fix, not the incident.
Slicing the file also buys you parallelism for free: you're pulling different pieces from different peers concurrently instead of waiting on one slow pipe, and the moment a piece is verified you can start re-serving it.
You become a source within seconds of joining, not after the whole download finishes.