Switch PostgreSQL Streams on Rocky Linux 9 with DNF Modules

Desired Outcome

Enable a Rocky Linux 9 administrator to install PostgreSQL 13 or 15 from the AppStream repository, switch between them, and verify the active version—all without reinstalling the OS.

Prerequisites

• Rocky Linux 9 system with a recent kernel (≥ 4.18) and fully updated packages.
• Root or sudo access.
• AppStream repository enabled (default on fresh installs).
• Backup of any pre‑existing PostgreSQL data (e.g., pg_dumpall or a filesystem snapshot).
• SELinux in enforcing mode (default) – be prepared to adjust contexts if moving $PGDATA.

Procedure

1. List Available PostgreSQL Streams

   # dnf module list postgresql
   

   Look for the postgresql module and note the stream numbers (e.g., 13, 15). The current stream will have a [e] next to it.

2. Enable the Desired Stream

   # sudo dnf module enable postgresql:13
   

   Replace 13 with the target version. The command removes any conflicting packages from other streams.

3. Install PostgreSQL Server

   # sudo dnf install -y postgresql-server
   

   DNF pulls the packages that belong to the enabled stream.

4. Initialize the Database Cluster

   # sudo postgresql-setup --initdb
   

   This creates /var/lib/pgsql/data (or $PGDATA) and sets up system tables.

5. Start and Enable the Service

   # sudo systemctl enable --now postgresql
   

   Service name is postgresql regardless of stream; the binary path is chosen by the module.

6. Verify the Active Version

   # psql --version
   # psql -U postgres -c "SELECT version();"
   

   The output should include the stream’s major version (e.g., PostgreSQL 13).

Switching Streams

1. Disable Current Stream

   # sudo dnf module disable postgresql
   

2. Enable New Stream

   # sudo dnf module enable postgresql:15
   

3. Re‑install and Re‑initialize

   # sudo dnf install -y postgresql-server
   # sudo postgresql-setup --initdb
   

   Note: This creates a fresh cluster; migrate data from the old cluster before dropping it.

4. Restart Service

   # sudo systemctl restart postgresql
   

5. Confirm New Version

   # psql -U postgres -c "SELECT version();"
   

Validation

• Run dnf module list postgresql again; the [e] should now be on the new stream.
• Check service status: systemctl status postgresql should show Active: active (running) and the service description includes the version.
• Query PostgreSQL: SELECT version(); must match the expected major release.

Recovery / Rollback

If the new stream causes problems, revert to the previous one following the same steps: disable the current stream, enable the former stream, reinstall, re‑initialize (or restore a backup), and restart the service. Ensure you have a backup before dropping any cluster.

Limitations & Notes

• Switching streams replaces the PostgreSQL binaries; it does not migrate existing databases automatically. Use pg_dumpall or a file‑system snapshot before switching.
• Data directories may need SELinux context adjustments if moved: semanage fcontext -a -t postgresql_db_t '/new/pgdata(/.*)?' and restorecon -Rv /new/pgdata.
• Only supported streams available in AppStream can be enabled; custom or older versions require manual RPM builds.

Practical Check

After following the steps, a quick sanity test is to connect locally with psql -U postgres -c "SELECT current_setting('server_version');" and confirm the returned string matches the stream you enabled.