Mendix has assisted the customer to address the question above. The app has successfully been able to handle all traffic required. For anybody looking for an answer to a comparable situation, here’s some info.
1000+ concurrent users is not uncommon for Mendix applications. The complexity for this app is the peak caused by the push notifications, but can be caused by other traffic generating activities. In this app, all users use the same functionality at the same moment, accessing the same data for all concurrent users.
To handle the peak in user requests, additional Mendix runtime instances are started. In addition, the database has been resized, to support a higher number of concurrent database actions. Finally, it has been identified that for this use case, handling the functionality asynchronously through a queue would reduce the sizing requirements.
After improving the app, it has successfully been used to handle 2000+ concurrent users in production.