Should I use one PostgreSQL instance or separate ones for each application?

A good middle ground is one PostgreSQL server with a separate database and user for each application. It avoids maintaining several database installations while still keeping schemas, permissions, backups, and credentials separated. I’d only split an app into its own PostgreSQL instance if it needs a different PostgreSQL version, has unusual configuration requirements, or becomes resource-intensive enough to affect the other services.

There’s also a strong case for one PostgreSQL instance per service, especially when using application-provided deployment examples. Different applications may require different PostgreSQL versions, extensions, or upgrade schedules, and sharing an instance can create compatibility or schema conflicts. The resource overhead is usually modest, but the operational overhead is higher.

For a small home server, I’d probably start with one PostgreSQL instance and isolate each application with its own database, role, and credentials. Make sure each service has its own schema or database rather than sharing tables or the default schema. If one application later causes compatibility, performance, or backup problems, move it to a dedicated instance then.

The important distinction is between a PostgreSQL server and the databases inside it. Multiple databases in one PostgreSQL installation are relatively lightweight, while separate installations provide stronger isolation but require more maintenance. Whichever layout you choose, keep backups and restore testing per application, and check each app’s supported PostgreSQL versions before combining everything.