watch only wallet or get balance of other wallet

[skyrunner]

If I need to monitor thousands of wallets, how can I do it?

I want to be able to notify a user immediately of a payment

looks like bitcoin core can tell the balance only of local wallet. Kinda strange

[cp1]

You can monitor transactions as they come in bitcoind.  Read the mempool or the blocks as they come in.

[skyrunner]

what does it mean?
how can i do it via API?

[flatfly]

You can find an example here:

https://bitcointalk.org/index.php?topic=373490.80

https://gist.github.com/flatfly/9151814

Or you can also get live data by connecting to public Electrum servers.

Hope this helps.