I'm not really sure this is ever really desirable: it's much easier to add more web frontends (especially after T4209, basically just spin up as many as you want) than scale out the notification infrastructure, and adding authentication to the node server probably creates a great deal of ongoing complexity (the code needs to be kept in sync forever, and the node sever needs to get way more complicated).

Beyond authentication concerns, interpreting data from the node server as HTML also creates a risk that we don't have right now: if an attacker finds an XSS hole in notifications, they can hypothetically push notifications out and XSS all connected users. This risk is small, but such an attack could be severe. Right now, we never treat anything on the notification channel as HTML, and all the stuff from the webserver goes through the standard rendering stack.

Basically, I'd worry that this optimizes for a local minimum, where users approximately fit on one web frontend. If you had many more users and several web frontends, scaling pressure on the frontends would become much easier to deal with than scaling pressure on the notification server.

(In the very long term, I imagine scaling the notification server by spreading it across multiple machines and subscribing it to other instances of itself -- that is, "child" servers would connect to a "parent" server over the same API that Flash uses, and just subscribe to all PHIDs. This also gets far more complicated if it has to do auth.)

One thing that may be worth doing is making the master do the Ajax call, then passing the data onto the other local browser clients. However, we're limited in our ability to do this for some notification types (e.g., Conpherence messages need to make a call that depends on per-window state) and I'm not sure how common it is for users to have enough tabs open for this to matter much.

We could also do something like pause updates of notifications when windows are in the background, and then just update once (to get the complete current state) when the window is focused.

T6559 may obsolete this.