Pro ยท Lesson 5 of 11
Change data capture reads the source database's change log from a saved position. The log name is different on every engine. If the log is gone, DMS cannot invent the missing changes.
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CDC is reading the shop's diary instead of recounting every item. If someone throws the diary away, you cannot reconstruct Tuesday afternoon.
The task stores a position in the source log. After a restart it tries to continue from that position. Full load plus CDC starts the log reader before or as the load runs, so changes during the load are not dropped on the floor. The exact handshake is in the engine's DMS page. Do not skip that page for the engine you use.
If the position is older than the logs the source still has, CDC cannot resume. Retention is a source-database setting: binary log days on MySQL, a replication slot that stops PostgreSQL from recycling WAL, archive logs on Oracle, and the SQL Server retention for CDC. A slot or a log that fills the disk is the other failure. Watch it.
MySQL and MariaDB: the binary log, usually row format, a file and position or a GTID, and enough retention. PostgreSQL: wal_level = logical, a replication slot, and an LSN. The PostgreSQL course's replication lesson is the database side of slots. Oracle: supplemental logging and an SCN. SQL Server: the CDC or replication feature the DMS source page requires for that edition, and an LSN.
Turning those settings on can require a restart or extra disk. Do it in a change window, not as a surprise during the load.
| Engine | DMS remembers | You must keep |
|---|---|---|
| MySQL | Binlog file and position, or GTID | Binlog retention |
| PostgreSQL | LSN and a slot | WAL for that slot |
| Oracle | SCN | Supplemental logging and archive logs |
| SQL Server | LSN | The CDC or replication feature DMS asked for |
Step 1 of 3The database writes changes to its own log.
Step 2 of 3DMS stores how far it has read.
Step 3 of 3The task applies those changes on the target until you stop it.
Pick one engine. If you do not know the retention setting today, write unknown and the doc page you will open. A PostgreSQL reader can continue in the replication lesson.
Engine:
Log object DMS will read:
Position name (file/pos, GTID, LSN, or SCN):
Who deletes old logs, and after how long:
Disk alarm if a slot or CDC capture holds logs:
Checked against the linked documentation in October 2026. A later release can move a flag, a price, or a supported pair. Use the page for the version you installed.