What's actually on the wire when a dot moves across a map

The moving dot in a delivery app is one of those things you stop noticing. I wanted to know what it costs: does the app ask “where is the driver now?” every few seconds, or does the server push positions down a connection that never closes — and how often is often enough?

To keep it honest and small, there’s no separate driver app: any browser that opens the page shares its own location and sees everyone else’s. An Express server serves the page and runs a Socket.io server alongside it on the same HTTP server.

1. 1
   The browser asks for continuous location with watchPosition — high accuracy, no cached fixes, a five-second timeout.
2. 2
   Each new fix is emitted over the open socket as a send-location event with latitude and longitude.
3. 3
   The server adds the sender’s socket id and broadcasts a receive-location event to every connected client.
4. 4
   Each client keeps a marker per socket id: an existing marker moves with setLatLng, a new id gets a new marker.
5. 5
   When a socket disconnects, the server broadcasts it and every map removes that marker.

io.on("connection", (socket) => {
  socket.on("send-location", (data) => {
    io.emit("receive-location", { id: socket.id, ...data });
  });
  socket.on("disconnect", () => {
    io.emit("user-disconnected", socket.id);
  });
});

1. 1
   Updates are driven by the device, not a timer. The phone decides when it has a new fix. That’s efficient when you’re still and chatty when you’re moving — and too rare an update makes the dot teleport.
2. 2
   Broadcast to everyone doesn’t scale. Every update goes to every client, so traffic grows with the square of the number of users. A real app sends a driver’s position only to the people waiting on that trip.
3. 3
   Location is sensitive by default. Broadcasting raw coordinates to every connection is fine for a demo and unacceptable anywhere else.

1. 1
   Rooms per trip, so a position only reaches the people who should see it.
2. 2
   Throttle on the sender and interpolate on the viewer, so the marker glides instead of stepping.
3. 3
   ETA, which needs routing on a road network rather than just a position.