In Cassandra, a delete operation does not delete the requested data immediately. Instead, it puts a marker over the deleted data. This marker is called a tombstone. A tombstone's age is defined by per-column family option
gc_grace_seconds has passed, Cassandra clears data marked with tombstones, and it clears all tombstones during the compaction of the column family.
If a node is down for loger than the time specified by
gc_grace_seconds and is recovered, there is a chance that the deleted data will come back. To avoid such scenarios, nodes dead for such a long time should be replaced instead of being added back to the cluster.