Container Database (CDB)

Multitenant Container Database (CDB)

🧠 Introduction
What is a Container Database (CDB)?
Think of it like a hotel 🏨:

  • The hotel building is the CDB
  • Each hotel room is a Pluggable Database (PDB)
  • Guests (apps/users) stay in the rooms (PDBs), not in the lobby (root)

Let’s break it down!

🧩 What Is a CDB?
A CDB (Container Database) is an Oracle database that includes multiple separate databases inside it, called PDBs (Pluggable Databases).

Each PDB contains schemas, tables, indexes, and application-specific data.

📌 From an app or user perspective:
➤ The PDB looks and behaves like a standalone database.

📌 From the operating system or Oracle perspective:
➤ The CDB is the actual database instance running on disk.

🧠 What Containers Exist in a CDB?

  1. CDB$ROOT (Root Container)
    ➤ Stores Oracle-supplied metadata
    ➤ Manages common users (users shared across all PDBs)
    ➤ Does not contain user data
  2. PDB$SEED (Seed Pluggable Database)
    ➤ Read-only template used to create new PDBs
    ➤ Cannot be modified
  3. User-Created PDBs (Pluggable Databases)
    ➤ Created by DBAs to run applications
    ➤ Each PDB is logically isolated
    ➤ You can plug multiple PDBs into a CDB
    ➤ But a PDB can’t be plugged into multiple CDBs
  4. Application Containers (Optional)
    ➤ Group of PDBs that share application data and metadata
    ➤ Helps build and manage complex multi-tenant applications
    ➤ Think of it like a sub-hotel within your main hotel

🧱 What’s Shared and What’s Not?

ComponentShared Across PDBsPDB-Specific
Control File✅ Yes❌ No
Redo Log Files✅ Yes❌ No
Data Files❌ No✅ Each container
Tablespaces❌ No✅ Each container
Metadata (Root)✅ Common Users❌ No User Data

📁 Each container has its own data files and tablespaces, but control files and redo logs are shared by the whole CDB.

📌 Important Note
Oracle has desupported Non-CDB architecture in Oracle Database 21c.

➤ You can no longer create non-CDB instances using tools like:

  • Oracle Universal Installer
  • DBCA (Database Configuration Assistant)

The Multitenant Architecture is now mandatory — and honestly, it’s way more flexible and scalable.

🛠️ DBA Tip of the Day
The CDB/PDB model is your go-to for modern Oracle deployments.

✔️ Use it to consolidate multiple apps
✔️ Securely isolate environments
✔️ Clone PDBs quickly for dev/test
✔️ Simplify upgrades and patching

If you’re still thinking in terms of one database = one app, it’s time to think multitenant!

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