AWS Database Migration Service: A Guide to Migrate Legacy Databases

How much downtime can your business tolerate during a database migration?

For many IT teams, that question shapes the entire migration process. Legacy databases often support critical applications, reporting systems, and operational workloads. Even a short outage can affect users, revenue, or compliance obligations.

AWS Database Migration Service (DMS) is designed to reduce that risk. It helps organizations move data from on-premises databases to AWS while keeping source systems running during the transition. Whether the goal is to modernize database engines or consolidate data warehouses, AWS DMS provides a managed approach to data transfer and continuous data replication without requiring a lengthy outage window.

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What is AWS Database Migration Service?

AWS Database Migration Service (AWS DMS) is a managed migration service that moves data between source and target databases. It supports migrations into AWS, between AWS services, and from one database platform to another.

The service is designed to keep downtime low by continuously replicating changes from the source database while applications remain active. Once the target environment is synchronized, teams can perform a planned cutover instead of scheduling a prolonged outage.

AWS DMS supports both homogeneous and heterogeneous migrations.

• A homogeneous migration moves between similar database engines, such as PostgreSQL to Amazon RDS for PostgreSQL.
• A heterogeneous migration moves between different platforms, such as Oracle to Amazon Aurora or SQL Server to MySQL.

How AWS DMS Works

AWS DMS relies on several core components:

• Replication instance: The compute resource that runs replication tasks.
• Source endpoint: The existing database being migrated.
• Target endpoint: The destination database in AWS.
• Replication tasks: Processes that manage full loads, ongoing replication, or both.

A typical migration starts with a full data transfer from the source system to the target database. After that, AWS DMS continuously replicates changes using Change Data Capture (CDC). This allows the target database to remain synchronized until the final cutover.

Supported Source and Target Databases

AWS DMS supports a wide range of commercial and open source database platforms. Organizations can migrate between on-premises infrastructure, Amazon EC2 environments, and managed AWS services.

AWS DMS is commonly used to migrate:

• Commercial and open source databases
• Legacy on-premises databases
• Hybrid database environments
• Reporting systems and data warehouses
• Applications requiring continuous data replication during migration

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The AWS DMS Migration Process: A Step-by-Step Breakdown

1. Assess the Existing Environment

Before creating migration tasks, review the current database environment and identify technical dependencies.

Key areas to assess:

• Existing database schema and application dependencies
• Unsupported or custom data types
• Database engines currently in use
• Network connectivity between on-premises databases and AWS
• Storage requirements and expected data transfer volumes
• Current backup and disaster recovery procedures
• Reporting systems or downstream applications connected to the database

Recommendations:

• Document application dependencies early
• Run schema assessments before migration begins
• Identify latency-sensitive workloads that may require testing
• Estimate replication load to size infrastructure correctly

2. Prepare the AWS Environment

The target environment should be fully configured before data migration starts.

Typical preparation tasks include:

• Provisioning Amazon RDS, Amazon Aurora, or Amazon Redshift targets
• Configuring VPC networking and routing
• Setting up IAM roles and permissions
• Creating security groups and firewall rules
• Establishing VPN or Direct Connect connectivity for hybrid environments
• Enabling monitoring and logging services

Recommendations:

• Use isolated subnets for database workloads where possible
• Restrict administrative access using least-privilege IAM policies
• Validate connectivity before creating replication tasks

3. Configure AWS DMS

Once infrastructure is ready, teams can configure AWS DMS components.

This usually includes:

• Creating a replication instance
• Defining source and target databases
• Configuring migration tasks
• Selecting full load, CDC replication, or combined migration modes
• Mapping schemas and tables
• Configuring validation settings

Recommendations:

• Start with smaller test datasets before full migration
• Monitor replication performance during initial setup
• Separate large migration tasks where practical to improve troubleshooting

4. Run Initial Data Transfer

The initial migration stage copies existing records from the source database to the target environment.

During this stage:

• AWS DMS performs the initial full load
• Data validation checks compare source and target records
• Teams monitor replication throughput and database performance
• Errors related to schema mismatches or unsupported data types are identified

Recommendations:

• Schedule large transfers during lower usage periods
• Monitor storage growth and replication lag
• Validate row counts and critical application data before proceeding

5. Enable Continuous Replication

After the initial load completes, AWS DMS can continuously replicate database changes using Change Data Capture (CDC).

This allows:

• Ongoing synchronization between source and target databases
• Reduced downtime during cutover
• Parallel testing before production migration

Recommendations:

• Track replication latency throughout testing
• Keep source systems stable during CDC operations
• Test application functionality against the target database before cutover

6. Perform Cutover and Validation

The final stage shifts production traffic to the new database platform.

Typical cutover steps include:

• Pausing application writes temporarily
• Allowing final replication tasks to complete
• Redirecting applications to the target database
• Performing integrity and performance checks
• Monitoring production workloads after migration

Recommendations:

• Prepare rollback procedures before cutover begins
• Communicate maintenance windows clearly to stakeholders
• Validate application performance immediately after migration

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Security and Compliance Considerations

Database migrations often involve sensitive operational, financial, or customer data. Security controls should be planned alongside the migration process rather than added later.

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AWS DMS supports encrypted data transfer between source and target databases.

Security measures typically include:

• SSL/TLS encryption for replication traffic
• Encryption at rest for replication storage
• Encrypted target databases using AWS Key Management Service (KMS)

Recommendations:

• Enforce encrypted connections across all endpoints
• Validate certificate configurations before migration begins
• Review encryption requirements for regulated workloads

Identity and Access Management

Replication tasks require access to both source and target systems. Permissions should be tightly controlled.

Best practices include:

• Using IAM roles instead of long-term credentials
• Applying least-privilege permissions
• Restricting administrative database access
• Separating operational and migration accounts where possible

Recommendations:

• Audit IAM permissions before production migration
• Rotate credentials after migration completion
• Limit direct console access to migration administrators

Monitoring, Logging, and Auditability

Migration visibility is important for troubleshooting and compliance reporting.

AWS services commonly used alongside AWS DMS include:

• Amazon CloudWatch for monitoring replication tasks
• AWS CloudTrail for audit logging
• AWS Config for compliance tracking
• Security Hub for centralized findings

Recommendations:

• Enable detailed logging before migration starts
• Monitor replication lag continuously
• Retain migration logs for audit requirements where applicable

Compliance Considerations

Organizations operating in regulated industries may need additional controls during migration.

Requirements may include:

• Data residency validation
• Retention and logging policies
• Segmented network environments
• Change management documentation
• Validation testing for regulated workloads

AWS DMS is commonly used in environments aligned with:

• HIPAA
• PCI DSS
• NIST 800-53
• FedRAMP-oriented architectures

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Let’s Modernize Your Legacy Database

AWS DMS gives IT teams a structured way to move data between source and target databases while reducing downtime during the transition.

SMS helps organizations modernize legacy infrastructure without turning migration projects into long-term operational problems. Our team works with clients to assess existing environments, design secure AWS architectures, and build migration pathways that are maintainable after the project is complete.

Our goal is to leave clients with infrastructure they can manage, scale, and understand long after the migration is finished.

Talk to a Migration Expert