Can Redis write out to a database like PostgreSQL?

Redis is increasingly used as a caching layer, much like a more sophisticated memcached, and is very useful in this role. You usually use Redis as a write-through cache for data you want to be durable, and write-back for data you might want to accumulate then batch write (where you can afford to lose recent data).

PostgreSQL's LISTEN and NOTIFY system is very useful for doing selective cache invalidation, letting you purge records from Redis when they're updated in PostgreSQL.

For combining it with PostgreSQL, you will find the Redis foreign data wrapper provider that Andrew Dunstain and Dave Page are working on very interesting.

I'm not aware of any tool that makes Redis into a transparent write-back cache for PostgreSQL. Their data models are probably too different for this to work well. Usually you write changes to PostgreSQL and invalidate their Redis cache entries using listen/notify to a cache manager worker, or you queue changes in Redis then have your app read them out and write them into Pg in chunks.

Redis is persistent if configured to be so, both through snapshots and a kind of WAL called AOF. Loads of people use it as a primary datastore. https://redis.io/topics/persistence

If one is referring to the greater world of Redis compatible (resp protocol) datastores, many are not limited to in-memory storage: https://keydb.dev/ http://ssdb.io/ and many more...

---