How to implement realtime sync with rest API?

I am making a appointment booking app that's syncs with my clients software (let's call the client's software softX ).

SoftX has a local Mariadb installation that stores all the appointments and bookings of my client.

Now my booking app should only show slots that's available in my clients softX on a particular day. SoftX has a rest api that I can make calls to to get available slots.

Here's the interesting part. The rest api calls are generally slow and every time someone tries to book an appointment through my app, I have to make new api calls.

My question is is there a way I can store the available slots in cache and instead of making repeat api calls, get an update when there is a change in the database. Like websockets or long polling.

Hope i was able to explain it. Thanks a lot in advance.