Lessons Learned from a Cascade Deletion Incident

Setting the Scene

One evening, a project manager reached out to inform me that some data appeared to be missing from our system. Upon investigating, I discovered that an unintended cascade deletion had occurred due to the use of on_delete=models.CASCADE. This resulted in the deletion of not only the target data but also several related records, which were critical to the system’s integrity.

The Restoration Process

  1. Restore from Backup: I quickly connected with our Site Reliability Engineer (SRE) to retrieve the most recent data backups. After confirming the integrity of the backups, I restored them to a staging environment.
  2. Delta Sync: After confirming the backup’s integrity, I performed a delta restoration, syncing the backup with the current database to recover the lost data while preserving recent changes.
  3. Code Fixes: I updated the models, replacing models.CASCADE with safer options like models.PROTECT or models.SET_NULL to prevent accidental deletions in the future.

Lessons Learned

  • Be Cautious with on_delete: Understand the cascading impact when cleaning up or modifying attributes that affect related records.
  • Backups Matter: Regular backups are essential for quick recovery.
  • Test Thoroughly: Ensure changes are tested with edge cases, especially when they involve data relationships.
  • Better Monitoring: Implement more granular monitoring and alerting for destructive changes to the database.

This experience was a wake-up call to double down on best practices, including database integrity checks, better testing, and comprehensive backups. Mistakes happen, but the goal is to learn from them and improve the system.