What are Tombstones in Cassandra used for?

Tombstones in Cassandra are used for marking deleted data before it is completely removed from the database. When a record is deleted, instead of immediately erasing it from the storage system, Cassandra marks it with a tombstone, which serves as a flag indicating that the data has been deleted. This is particularly important in distributed systems like Cassandra, where data may be replicated across multiple nodes.

The tombstone serves a dual purpose: it helps maintain consistency across replicas by ensuring that all nodes recognize that the data has been marked for deletion, and it also provides a mechanism for handling eventual consistency. Over time, tombstones will be cleaned up during compaction processes, permanently removing the marked data, but until then, they play a crucial role in the system's ability to manage deletes effectively.

This mechanism allows Cassandra to maintain performance and data integrity while handling large volumes of data across distributed architectures.