I have an app running with:

* one instance of nginx as the frontend (serving static file)
* a cluster of node.js application for the backend (using cluster and expressjs modules)
* one instance of Postgres as the DB

Is this architecture sufficient if the application needs scalability (this is only for HTTP / REST requests) for:

* 500 request per seconds (each requests only fetches data from the DB, those data could be several ko, and with no big computation needed after the fetch).
* 20000 users connected at the same time

Where could be the bottlenecks ?