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Solving data consistency problems from Active Directory tombstones

Active Directory tombstones can cause problems with data consistency. Domain controllers need to be backed up as often as tombstones, if not more.

Can Active Directory tombstones cause problems with AD restoration?

To ensure proper replication between domain controllers, deleted objects are first converted into special objects called "tombstones."

As changes to Active Directory (AD) are replicated between domain controllers, the tombstones are also replicated -- effectively deleting the same objects on other domain controllers. However, tombstones take up space in storage, so they are given a finite lifetime and finally deleted from disk once their lifetime expires (60 days by default).

Active Directory tombstones can cause data consistency problems for AD restoration. If a domain controller is restored to state before an object was deleted, and a tombstone for that deleted object is not replicated to the restored controller (e.g., the tombstone for that object had already expired), then that old object will remain on the restored domain controller and cause severe AD problems. In actual practice, AD may prevent restoration if the backup is older than the tombstone lifetime.

This puts the onus on administrators to ensure domain controllers are backed up at least as frequently as tombstone lifetimes. As a rule, domain controllers should be backed up at least twice within the tombstone lifetime to help ensure all tombstones are available for proper replication to restored domain controllers.

IT administrators must understand how Active Directory iterations are coordinated and maintained, and domain controllers require careful protection using tools designed to accommodate those unique behaviors. Improper backup and restoration can easily result in data consistency and rollback issues that spell catastrophe for the enterprise. It's important to select adequate backup tools for the job, test those tools thoroughly, use the tools frequently, and implement restoration processes that will ensure continued domain controller operation. 

This was first published in August 2014

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