longer lived sessions slow down

[felixroos]

longer lived flok sessions steadily accumulate history / memory, which causes the browser to slow down more and more over time.
here's a write up: https://www.pastagang.cc/blog/performance/#long-lived-flok-sessions

i'm not completely sure where the history is stored, but it is always sent to people joining the room from a browser that hasn't downloaded the history yet. this also happens if the room is empty.

1. open "about:blank" an anonymous browser tab
2. open the developer tools network tab and filter by "WS" (WebSockets)
3. go to https://flok.cc/s/pastagang
4. when loading has finished, click on "pastagang"
5. you can see there's a message of 3.6MB size

doing the same on other, more fresh rooms doesn't load as much from the server, so i assume this must be the history.
the history is then stored in the browsers IndexDB, so I assume when you come to the page with a populated IndexDB, the server will only give you the diff (which is why the 3.6MB can only be observed from an anonymous tab)
those 3.6MB are a heavily compressed binary format, which gets even larger when converted to JSON. as shown in the post, you can run this in the developer console:

indexedDB.open("pastagang").onsuccess = function (event) {
  const db = event.target.result;
  const transaction = db.transaction("updates", "readonly");
  const store = transaction.objectStore("updates");
  const request = store.getAll();
  request.onsuccess = () => console.log(JSON.stringify(request.result).length);
  db.close();
};

for me, this logs 47937119, which is the number of characters in the (minified) JSON.
1 character is typically 1Byte, so this JSON is around 48MB.
i guess when someone moves the cursor, this data is touched in some way, so it takes a while till the browser can respond again, delaying any callbacks that happen in between (like a strudel clock callback, or anything going on in codemirror).

it would be good if there was a way to prune / delete a session, so the name can remain the same, but the history would vanish.
@munshkr do you know where the history is stored? is it handled by @flok-editor/pubsub or by @flok-editor/session ? is there something we can do about it? even needing to manually prune the session would be fine, as this only has to be done once a week or even fewer times (the pastagang session is over 2 months old).

the idea with adding a timestamp feels more like a workaround, because it would probably cause weird bugs in the time of transition + we'd still accumulate a lot of garbage on the server