How Hinted Handoff Works in ScyllaDB

Hinted Handoff is a feature introduced in ScyllaDB 2.1 to address the issue of data inconsistency caused by write failures when replica nodes become unresponsive. This feature allows the coordinator node to store a "hint" whenever it fails to write to a replica, enabling it to later send this hint to the recovered replica to ensure data consistency. While Hinted Handoff reduces inconsistency and speeds up synchronization, it does not eliminate the need for running repair operations, as it operates on a best-effort basis without guaranteeing complete data consistency. The feature is configurable through parameters such as enabling hints for specific data centers, setting a maximum hint window, and specifying a storage directory for hints. Although it mimics the Apache Cassandra implementation, ScyllaDB's version ensures that hints do not consume excessive disk space, and it does not count hints towards the consistency level, except under specific conditions.