Module Development

Track Changes to Source Data During a Migration for Drupal 9, 10

If you need to write a migration that is capable of being executed multiple times and picking up changes to previously imported data, you can use the track_changes configuration option of most source plugins. This will tell the migration to keep a hash of each source row, and on subsequent runs of the same migration it will compare the previous hash to the current one. If they don't match this means the source data has changed, and Drupal will reimport that record and update the existing destination record with new data.

Using track_changes differs from calling drush migrate:import --update in that using --update will force every record to be re-imported regardless of whether the source data has changed or not.

In this tutorial we'll:

  • Learn how change tracking works to detect changes in your source data
  • Use the track_changes option in a migration

Note that progress of a migration can also be tracked using highwater marks if the source data has something like a last_updated timestamp column. Using highwater marks in this case is likely more efficient than using track_changes. It's a good idea to understand how both features work, and then choose the appropriate one for each migration.