Database Migration Service

How DMS Moves Data

AWS Database Migration Service keeps a source and target database synchronized while you plan cutover. Three objects make it work: endpoints (connection details for source and target), a replication instance (a managed EC2-like host in your VPC running the replication), and a task (table mappings plus a mode).

Tasks run full load first — bulk copy of existing rows — then change data capture (CDC), which tails the source's transaction log and applies inserts, updates, and deletes to the target until you cut over. Homogeneous moves (MySQL → MySQL) need no schema work. Heterogeneous moves (Oracle → Aurora PostgreSQL) pair DMS with the Schema Conversion Tool to rewrite schema and code first.

Configuring a Task

Placement matters: the replication instance sits in a VPC with network reach to both databases — security groups must allow the traffic, and the same-AZ rule-of-thumb avoids cross-AZ transfer charges. Test each endpoint with the console's connectivity test before starting.

Source access requirements trip people up: MySQL CDC needs binary logging enabled with ROW format; PostgreSQL CDC needs wal_level=logical so DMS can create a replication slot. Most targets also require a primary key on tables for CDC to apply updates and deletes.

At cutover: pause writes, let the task drain remaining changes, flip your application's connection string, then keep the task running briefly as insurance.

Pitfalls and Interview Focus

The SCT misconception: it converts schema, stored procedures, and queries — it does not move data, and conversions are often partial (report exactly what it could not convert so you budget manual work). Large LOB columns in CDC mode crawl; most teams use limited LOB handling or migrate them out-of-band.

Operational traps: forgetting CDC requires log access and primary keys; running an undersized replication instance so large tables miss their window; leaving triggers and constraints active on the target slows applies; and stopping the task before final verification, losing the last minutes of change.

Interview: name the three objects (endpoint, instance, task), the two phases (full load, CDC), and one source prerequisite — that answers most DMS questions.