How to do actions when MongoDB Realm Web SDK change stream closes or times out? - mongodb

Question

I want to delete all of a user's inserts in a collection when they stop watching a change stream from a React client. I'm using the Realm Web SDK for this.
Here's a summary of my code with what I want to do at the end of it:
import * as Realm from "realm-web";
const realmApp: Realm.App = new Realm.App({ id: realmAppId });
const credentials = Realm.Credentials.anonymous();
const user: Realm.User = await realmApp.logIn(credentials);
const mongodb = realmApp?.currentUser?.mongoClient("mongodb-atlas");
const users = mongodb?.db("users").collection("users");
const changeStream = users.watch();
for await (const change of changeStream) {
switch (change.operationType) {
case "insert": {
...
break;
}
case ...
}
}
// This pseudo-code shows what I want to do
changeStream.on("close", () => // delete all user's inserts)
changeStream.on("timeout", () => // delete all user's inserts)
changeStream.on("user closes app thus also closing stream", () => ... )
Realm Web SDK patterns seem rather different from the NodeJS ones and do not seem to include a method for closing a stream or for running a callback when it closes. In any case, they don't fit my use case.
These MongoDB Realm Web docs lead to more docs about Realm. Unless I'm missing it, both sets don't talk about how to monitor for closing and timing out of a change stream watcher instantiated from the Realm Web SDK, and how to do something when it happens.
I thought another way to do this would be in Realm's Triggers. But it doesn't seem likely from their docs.
Can this even be done from a front end client? Is there a way to do this on MongoDB itself in a "serverless" way?

Answer 1

If you want to delete the inserts specifically when a (client-)listener of a change-stream stops listening you have to implement some logic on client side. There is currently no way to get notified of such even within Mongodb Realm.
Sice a watcher could be closed because the app / browser is closed I would recommend against running the deletion logic on your client. Instead notify a server (or call a Mongodb Realm function / http endpoint) to make the deletions.
You can use the Beacon API to reliably send a request to trigger the delete, even when the window unloads.
Client side
const inserts = [];
for await (const change of changeStream) {
switch (change.operationType) {
case 'insert': inserts.push(change);
}
}
// This point is only reached if the generator returns / stream closes
navigator.sendBeacon('url/to/endpoint', JSON.stringify(inserts));
// Might also add a handler to catch users closing the app.
window.addEventListener('unload', sendBeacon);
Note that the unload event is not reliable MDN. But there are some alternatives which maybe be good enough for your use case.
Inside a realm function you could delete the documents.
That being said, maybe there is a better way to do what you want to achieve. Is it really the timeout of the change stream listener that has to trigger the delete or some other userevent?