𝗛𝗼𝘄 𝗱𝗼𝗲𝘀 𝗗𝗶𝘀𝗰𝗼𝗿𝗱 𝗵𝗮𝗻𝗱𝗹𝗲 𝗮 𝗺𝗶𝗹𝗹𝗶𝗼𝗻 𝗼𝗻𝗹𝗶𝗻𝗲 𝘂𝘀𝗲𝗿𝘀 𝗶𝗻 𝗮 𝘀𝗶𝗻𝗴𝗹𝗲 𝘀𝗲𝗿𝘃𝗲𝗿?

As time passed, the overall size of Discord's user base, including its most prominent communities, has grown massively. This affected servers that started to slow down and hit their throughput limits. So, they needed to scale individual Discord servers from tens of thousands to millions of concurrent users.

Whenever someone sends a message on Discord or joins a channel, they need to update the date UI of everyone online on that server. They call that server a "𝗴𝘂𝗶𝗹𝗱," which runs in a 𝘀𝗶𝗻𝗴𝗹𝗲 𝗘𝗹𝗶𝘅𝗶𝗿 𝗽𝗿𝗼𝗰𝗲𝘀𝘀, while there is another process (a "𝘀𝗲𝘀𝘀𝗶𝗼𝗻") for each connected client. The guild process tracks sessions of users who are members of that guild and are responsible for actions to those sessions. When sessions get updates, forward them to the web socket socket to the client.

The main issue is that 𝗮 𝘀𝗶𝗻𝗴𝗹𝗲 𝗺𝗲𝘀𝘀𝗮𝗴𝗲 𝗻𝗲𝗲𝗱𝘀 𝘁𝗼 𝗴𝗼 𝘁𝗼 𝘁𝗵𝗲 𝗻𝘂𝗺𝗯𝗲𝗿 𝗼𝗳 𝗽𝗲𝗼𝗽𝗹𝗲 𝗼𝗻𝗹𝗶𝗻𝗲 on that server, which means if a server has 1000 people online and they all send a message once, that's 1 million notifications.