ID generation in the client instead of the database is the only option for distributed apps. But generating unique IDs in such apps is hard. And it's essential to generate them properly because JPA will use IDs to define entity states. The safest option is to use UUIDs and Hibernate's generators, but there are more options starting from custom generators to dedicated ID generation servers.
In the previous article, we discussed server-generated IDs for JPA entities. All the ID generation strategies described in the article are based on one fundamental principle: there is a single point that is responsible for generating IDs: a database. This principle might become a challenge: we depend on a particular storage system, so switching to another (e.g., from PostgreSQL to Cassandra) might be a problem. Also, this approach does not work for distributed applications where we can have several DB instances deployed on several data centers in several time zones.