AWS DMS between Oracle - Sync Latency

Martin,

Thank you for your question about AWS DMS latency between Oracle RDS instances. There are a few things to consider and steps you can take to address this issue:

1. Verify CDC Configuration: Ensure that Change Data Capture (CDC) is properly configured. Even though you're in the "Load completed, replication ongoing" state, there might be issues with CDC setup.

2. Check SCN (System Change Number): Make sure you've specified the correct SCN for the CDC start point. If the SCN is set too high, changes might not be captured.

3. Monitor CloudWatch Metrics: Use AWS CloudWatch to monitor DMS task metrics, particularly the "CDC Latency" metric. This can provide insights into any delays between changes in the source and replication to the target.

4. Review Task Logs: Examine the DMS task logs for any error messages or warnings that might indicate why changes aren't being applied.

5. Verify Source Database Configuration: Ensure that the source Oracle database is properly configured for CDC. This includes setting the correct supplemental logging and having the necessary permissions.

6. Consider Endpoint Settings: While you mentioned no special endpoint settings, you might want to explicitly set "UseLogminerReader" to "Y" in your Oracle source endpoint settings to ensure LogMiner is being used.

7. Check for Triggers or Constraints: Verify if there are any triggers or constraints on the target database that might be preventing updates.

8. Force a Resync: You can try stopping and resuming the task, which sometimes helps to trigger the replication process.

If these steps don't resolve the issue, you may need to recreate the task, ensuring all CDC-related settings are correctly configured. Remember to capture the SCN before starting the full load and use it when setting up the CDC portion of the task.

If the problem persists, consider reaching out to AWS Support for more detailed troubleshooting specific to your setup.

Sources
Migrate Oracle database with AWS DMS ongoing replication - Database Migration Guide
DMS Migration Task(only CDC) for Ongoing Replication Type, Migration progress struck at 0% | AWS re:Post
Using an Oracle database as a source for AWS DMS - AWS Database Migration Service

Hi ,

Did you check those points ?

https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Troubleshooting.html

https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Troubleshooting_Latency_Source_Oracle.html