Delta Migration

This feature is only available with Enterprise subscriptions, within the Data Migration functionality.

It was designed for use in migrations that run in Data Append mode, which is the only option available when the destination database is the Runtime Connection. With this connection, DMM doesn’t have IDENTITY INSERT permissions, so primary key values will change between source and destination. In that case, if you perform multiple migrations with the same entity, records will be duplicated—unlike migrations without Data Append, where primary keys match across environments and records are simply updated if needed.

With this feature, DMM not only ensures that duplicate records aren’t copied, but also updates existing records if needed, or even deletes records that were removed from the source.

The Delta Migration option is available in the Data Migration feature, on the Final Configuration screen.

When you run a migration using this feature, DMM keeps track of the records it migrates for each entity. Later, if you run another migration with this feature enabled, DMM will:

• Insert records that didn’t exist in the previous migration;

• Update records that have changed in the source;

• Delete records that are no longer found in the source.

DMM uses the primary key—or, if you’re using the User Mapped Table feature, the selected attributes—as the key to determine whether a record has already been migrated. It still needs to process all records from the source each time, so even if the source data hasn't changed, the second migration won’t be faster than the first one. This feature ensures the environments stay synchronized.

The migration configurations don’t need to be identical between runs. DMM handles this per entity. For example, you might run a migration where some entities have already been migrated with Delta Migration enabled, while others are being migrated for the first time. In this case, for the already migrated entities, DMM will only insert new records that weren’t migrated previously. For new entities, DMM will insert all records and save their primary key information for future migrations.

To use this feature as intended, all migrations should be performed with the Delta Migration feature enabled.