Is this Hasura GraphQL Engine open source? What's the business model? Are there any features missing if you don't pay anything?
How does it get notified by PostgreSQL when the result of a query changes, since AFAIK PostgreSQL has no such built-in functionality? (it works if other apps change the database, right?)
Hasura's homepage[0] links to their Github[1], so it's definitely open source. The "Enterprise" section of the website seems a bit lite and all the pricing information is on the "Support" tab, so the business model seems to be support-based. I only skimmed so I can't speak to what missing features there might be if you roll-your-own.
[0]: https://hasura.io/