This is in the context of a websocket (which is what the original story is about). Presumably, websocket is being used because HTTP isn't enough, namely, you want to receive pushes from the server. This _often_ comes in the form of data that multiple connections are interested in: game state, chat, collaborative editing. At scale, this data, or a copy of it, often stays in memory. E.g. a chat system might keep a list of room + brief chat history + user list in memory. This memory is being mutated by concurrent connections.