Ask HN: What database is used in modern web stack?

The one that fits your need. If you keep the persistence layer properly separated, you can switch DBs as your needs evolve.

Recently I've seen a shift back to SQL, especially Postresql. Couchbase seems to be popular too.

I'm assuming "modern" means "bleeding edge" otherwise, whatever you've worked with in the past probably fits.

I see that a lot of people are interested in MongoDB, PostgreSQL, Apache Cassandra, InfluxDB.