A non-CDB is a database created in the classic single-database layout, where one instance owns one data dictionary and one set of user schemas with no container root and no pluggable databases. The multitenant architecture, introduced in 12.1, adds a root container and pluggable databases (PDBs) that can be unplugged, cloned and moved.

Migrations care because a large share of the estates being moved to Azure are 11.2 and 12.x non-CDBs. The architecture changes what assessment tools see (there is no PDB scoping), how users and services are structured (no common users or per-PDB services), and whether the source can be upgraded in place to a release that current migration tooling supports before the move.

Oracle's 12.1 Upgrade Guide lists the non-CDB architecture as deprecated, and the 12.2 guide repeats the deprecation. The 21c Upgrade Guide desupports it (notice dated May 2021): the installer can no longer create a non-CDB, and upgrading to 21c or later requires converting the database into a PDB.

Non-CDB architecture, introduced before 11.2, served as Oracle Database’s standalone architecture before containers became the order of the day. It kept databases outside the CDB-and-PDB arrangement that would succeed it.

Deprecated in 12.1 and desupported in 21c, it yielded to the multitenant architecture of CDBs with pluggable databases. On Exadata Database Service, console and OCI APIs create only CDBs with an initial PDB; dbaascli permits non-CDBs only through 19.x.

It is survived by Oracle Database@Azure only by workaround, and by Oracle Database on Azure Virtual Machines only by workaround. AutoUpgrade can upgrade and convert it into a PDB in one operation, a tidy final arrangement.