Lesson 1: ZDM Overview & Internals Explained

Ashish Agnihotri

Data Analyst at Exaguru 

Introduction

Modern enterprises running mission-critical Oracle Databases cannot afford downtime during migrations. Whether it’s an upgrade (11g → 19c/21c), cloud adoption (OCI/ExaCC), or platform change (Linux, Exadata), the migration strategy must ensure minimal to zero downtime.
Oracle Zero Downtime Migration (ZDM) is an automation tool designed by Oracle for seamless, automated, and low-risk database migrations. It orchestrates end-to-end migration steps—leveraging RMAN, Data Guard, and Oracle GoldenGate (depending on the method)—while providing monitoring, logging, and rollback capabilities.

ZDM Technical Architecture

  The ZDM architecture can be broken into 3 major layers:  

1. ZDM Service Host (Control Node)

  • Installed on a dedicated Linux host (separate from source & target).
  • Acts as the brain of migration.
  • Responsible for:
    • Orchestrating migration steps.
    • Generating configuration templates.
    • Running pre-checks & post-checks.
    • Executing migration workflow.
  • Runs as zdmlogger and zdmadm processes.
👉 Key component: zdmtask framework (handles step-by-step execution & retry logic).

2. Source Database System

  • Can be:
    • On-premises Oracle Database (11gR2 → 19c supported).
    • Exadata on-prem.
    • VM/BM DB systems on OCI.
  • ZDM interacts with the source via:
    • SSH connectivity.
    • Oracle Wallets for secure passwordless operations.
    • RMAN / Data Guard, depending on the migration mode.

3. Target Database System

  • Typically:
    • Oracle Cloud Infrastructure (OCI) DB systems.
    • Exadata Cloud Service / Exadata Cloud@Customer.
    • Autonomous Database (in selective migration modes).
  • ZDM prepares the target with:
    • Matching database home.
    • Required patches.
    • Network configuration.
    • Data Guard broker setup (for physical standby-based migrations).

ZDM Migration Flow (High-Level)

  1. Pre-check Phase
    • Validates connectivity, environment, and prerequisites.
    • Example:
    • $ zdmcli migrate database -sourcedb <SID> -targetdb <CDB> -precheck
  2. Stage Phase
    • RMAN backup / OGG instantiation / Data Guard standby setup.
  3. Migration Phase
    • Continuous sync between source and target.
    • Minimal downtime cutover.
  4. Cutover Phase
    • The source DB stopped.
    • The final sync was applied to the target.
    • The role switched to primary.
  5. Post-Migration Phase
    • Target verification.
    • Source cleanup.

Internal Components Explained

Component Role in ZDM
zdmadm Core ZDM engine, manages task orchestration.
zdmlogger   Logs each step, maintains rollback checkpoints.  
zdmtask   Atomic task execution framework (retry, resume).  
Pre-check Module   Ensures prerequisites (patches, OS settings, DB versions).  
Transport Layer   Handles RMAN / OGG / Data Guard data transfer.  
Cutover Module   Executes role transition with downtime < minutes.  

Example: ZDM Pre-check & Migration Commands

Run a Pre-check:

# Run ZDM migration precheck using Data Guard mode 

   zdmcli migrate database \ 

            -sourcedb myDB \ 

            -targetdb myTargetCDB \ 

            -sourcenode src-host \ 

            -targetnode tgt-host \ 

            -mode dg \ 

            -precheck

Execute Migration (Data Guard mode):

# Run ZDM migration precheck using Data Guard mode 

  zdmcli migrate database \ 

            -sourcedb myDB \ 

            -targetdb myTargetCDB \ 

            -sourcenode src-host \ 

            -targetnode tgt-host \ 

            -mode dg

Monitor Migration:

  zdmcli query job -id <JOB_ID>  

 Why ZDM?

 ✅ Automated end-to-end process (less manual work).
 ✅ Supports cross-version and cross-platform migrations.
 ✅ Uses native Oracle tools (RMAN, OGG, DG).
 ✅ Provides logging, retries, and rollback for safer migrations.
 ✅ Ensures near-zero downtime for business-critical databases.

Best Practices

  • Always run -precheck before execution.
  • Keep ZDM Service Host separate from source/target.
  • Use Data Guard-based migrations for large DBs.
  • For heterogeneous migrations (11g → 19c + endian change), use RMAN/OGG.
  • Monitor jobs via ZDM logs ($ZDM_HOME/logs).
  • Plan cutover during a low-activity window.

Closing Thoughts

  Oracle ZDM is more than a migration utility—it’s an enterprise-grade orchestration engine. Understanding its internals and architecture ensures that architects and DBAs can design robust, low-risk migration strategies for critical workloads, especially when moving to OCI or ExaCC.  

 Next Step: Lesson 2: Cross-Version Migrations with ZDM (11g → 19c)

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