I will refer PDB and CDB to refer pluggable database and container database respectively.
Before we delve into deeper details about CDB and PDB, let us get some basic details about multi-tenancy.
It has been an administrative nightmare when multiple databases are running in one machine. Each database has memory footprint and each database has background processes.
So you can have up to 250 pluggable databases or PDBs within one container database or CDB.
It is clear that there is one-to-many relationship between CDB and PDBS.
It is a brand new capability insider a single container database.
So the DBAs and developer community should be familiar with “Container Databases” or CDB and “Pluggable Database” or PDB.
So all database connectivity should use “database service” rather than using the legacy approach of ORACLE_SID based connectivity.
ORACLE_SID ties the application connectivity to a specific database instance and does not give the scalability or high availability. Distinguished Product Manager Bryn Llewellyn, who is shown in the picture, and Architect Kumar Rajamani, who is also the visionary for the pluggable databases, mentioned that the maximum limit to the total number of Pluggable databases within a container database is close to 250.When application connects to the PDB, it will specify the destination PDB via a database service.All home-grown or third party applications typically will have connectivity defined out of the application so it is easier to just change the service name outside of the application code.In Oracle Open World 2012 held in San Francisco, Larry Ellison announced that oracle database also be cloud enabled by introducing pluggable databases for multi-tenancy and easy database movement between systems, platforms or releases.When the database is a cloud ready, it should be hardware agnostic, platform agnostic and release agnostic so that it gives all the characteristics to be in Platform as a Service (Paa S) for middleware.Oracle has leap forwarded the middleware technologies especially the database technology into the cloud.