How to exchange signals between applications?

I have two unconnected applications. One is the main app that performs the business logic and CRUD on database.

A 2nd app periodically rebuilds a database cache (long running taks). I want to send a signal to the main app when the rebuild starts, and when it's finished, as the main app should take specific actions while rebuilding takes place.

How could I achive this best using spring-boot?