Database Migration Service Complete Guide (2026)

Database Migration Service (DMS) is a GCP service that performs managed migrations of on-prem or other-cloud databases into Cloud SQL, AlloyDB, BigQuery, or Firestore. With CDC-based Continuous Migration it minimizes downtime and also supports heterogeneous conversion (e.g., Oracle → PostgreSQL).

Supported Migration Paths

Source	Target	Type
MySQL 5.5+	Cloud SQL for MySQL	Homogeneous (free)
PostgreSQL 9.4+	Cloud SQL for PostgreSQL / AlloyDB	Homogeneous (free)
SQL Server	Cloud SQL for SQL Server	Homogeneous
Oracle 11g R2+	AlloyDB / Cloud SQL for PostgreSQL	Heterogeneous ($0.10/GB)
MongoDB	Firestore	Heterogeneous
Oracle / SQL Server	BigQuery (CDC)	Datastream

Migration Modes

One-time Migration: one-shot migration (with downtime)
Continuous Migration: initial sync + ongoing CDC delta sync → cutover
Promote: run cutover to promote to production

Oracle → AlloyDB Migration Example

# 1. Database Migration Assessment
gcloud database-migration migration-jobs assessments create \
  --source-database-engine=ORACLE \
  --source-connection-profile=oracle-source \
  --location=asia-northeast1

# Assessment result: migration feasibility + PL/SQL → PL/pgSQL convertibility

# 2. Create connection profile (source)
gcloud database-migration connection-profiles create oracle oracle-source \
  --host=10.0.0.1 --port=1521 --username=migration_user \
  --password=... --location=asia-northeast1

# 3. Create connection profile (target AlloyDB)
gcloud database-migration connection-profiles create alloydb alloydb-target \
  --alloydb-cluster=projects/PROJECT/locations/asia-northeast1/clusters/my-cluster \
  --location=asia-northeast1

# 4. Create migration job (Continuous)
gcloud database-migration migration-jobs create my-job \
  --type=CONTINUOUS \
  --source=oracle-source --destination=alloydb-target \
  --location=asia-northeast1

# 5. Start (initial data transfer + CDC begins)
gcloud database-migration migration-jobs start my-job --location=asia-northeast1

# 6. Check progress
gcloud database-migration migration-jobs describe my-job --location=asia-northeast1

# 7. Cutover (after switching app connections)
gcloud database-migration migration-jobs promote my-job --location=asia-northeast1

Homogeneous (MySQL → Cloud SQL) Flow

Source MySQL: enable binary logging and grant dump privileges
Create a connection profile
Create a migration job (Continuous)
Initial dump → automatic transfer
Continuous delta sync via CDC
Switch the app's connection string to Cloud SQL
Promote the migration job (production cutover)

Elements Converted in Heterogeneous (Oracle → PostgreSQL)

Oracle	PostgreSQL/AlloyDB
VARCHAR2(n)	VARCHAR(n)
NUMBER	NUMERIC
DATE	TIMESTAMP
CLOB / BLOB	TEXT / BYTEA
SEQUENCE	SEQUENCE (syntax differences)
PL/SQL Stored Procedure	PL/pgSQL (auto-converted by Assessment; review required)
Trigger	Trigger (syntax differences)
View / Index	Migrated as-is

Datastream (Real-time CDC)

Streams Oracle / MySQL / SQL Server changes into BigQuery / GCS / Spanner
Near real-time (seconds to minutes)
Use cases: real-time analytics, always-on data-lake updates, replica creation
Can be combined with DMS to continue with CDC after migration

Pricing Examples

Item	Price
MySQL → Cloud SQL	Free (only Cloud SQL charges apply)
PostgreSQL → AlloyDB	Free
Oracle → AlloyDB	$0.10/GB processed data
Datastream	$0.30/GB ingested

Best Practices

Confirm compatibility up front with Migration Assessment
Validate Continuous Migration in a test environment
Cutover plan + rollback plan
Update the app's connection string to go through Cloud SQL Auth Proxy
Schedule the cutover during the lowest-impact business window
Keep the source DB for 1-2 weeks after cutover as an emergency fallback
Optimize post-migration with AlloyDB AI / Index Advisor

What is Database Migration Service (DMS)?

A managed service that migrates on-prem or other-cloud databases to Cloud SQL, AlloyDB, or BigQuery. Minimal downtime plus continuous CDC sync keeps production impact low.

Which migration paths are supported?

MySQL → Cloud SQL / AlloyDB, PostgreSQL → Cloud SQL / AlloyDB, Oracle → AlloyDB / PostgreSQL (Heterogeneous), SQL Server → Cloud SQL for SQL Server, MongoDB → Firestore.

How is it priced?

Homogeneous migrations (e.g., MySQL → MySQL) are free. Heterogeneous (Oracle → PostgreSQL) is $0.10/GB of processed data. Cloud SQL / AlloyDB charges apply separately.

How short can the downtime be?

With Continuous Migration, production keeps running while deltas sync. Only the cutover takes a few to several dozen minutes. A read-replica pattern is also possible.

What gets converted in an Oracle migration?

PL/SQL is auto-converted to PL/pgSQL (via Database Migration Assessment), along with data-type mappings, sequences, triggers, and views. Not 100% automatic, but dramatically less manual work.

How does it relate to Datastream?

Datastream streams CDC (Change Data Capture) into BigQuery / GCS / Spanner. DMS handles full migration. Use Datastream for real-time analytics and DMS for migrations.

How does it compare with AWS DMS?

Both are managed DB migration services. GCP DMS supports AlloyDB and has strong PostgreSQL compatibility; AWS DMS covers more source databases. In multi-cloud setups, you may use both.

How does rollback work on failure?

During Continuous Migration the source DB is still live, so rollback is simply 'cancel the cutover.' Rolling back after cutover requires preserving the source DB state.

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Author

NicheeLab Editorial Team

NicheeLab editorial team focused on data engineering and cloud certification learning. Content is structured around practical study needs and official exam domains.

Database Migration Service (DMS) Complete Guide: Oracle/MySQL/PG → Cloud SQL/AlloyDB