Site is recalling builder even with TTL value

Do you have more information how TTL is working except this title ?
We have next builder (see screenshot) with TTL = 900 seconds. I added counter variable so I expect that we update this variable once per 900 seconds as builder should be recalled by TTL time. But unfortunately it happens randomly.
Without ttl it’s also not cached forever (until next deploy).
Could you help me what I am doing wrong?

Are you using /.netlify/functions or /.netlify/builders? The former will not be cached, the latter will be.

Thanks, builders solution is working.
Could you also clarify, does Netlify offer an API to purge a single page from cache, instead of using a static TTL?

Hey @super12431,

So far, no (stable) way to do that. This is being worked on though.