Cross-cluster replication (CCR) has been introduced in Elasticsearch 6.7.0, allowing for seamless data replication between Elasticsearch clusters, addressing critical use cases such as disaster recovery, data locality, and centralized reporting. Previously dependent on third-party solutions, CCR is now natively integrated, offering benefits like comprehensive error handling and simplified management via Elasticsearch APIs and Kibana UIs. CCR operates on an active-passive index model, where a "follower index" replicates changes from a "leader index" at the shard level, ensuring efficient, near-real-time data replication without additional load on the leader. This capability enhances the ability to maintain high throughput and scale with production loads, supporting both new and existing indices, and incorporating features like auto-follow patterns and integration with index lifecycle management (ILM) for efficient management of time-based indices. The underlying technology of CCR, including "soft deletes" and shard history retention leases, ensures robust replication by preserving a history of operations and controlling when deleted documents are merged away, which is vital for maintaining synchronization between leader and follower indices.