This document discusses using Redis for social networking applications. Some key points: - Redis is an in-memory database that can dump data to disk and supports many useful data structures like hashes. It provides fast read and write performance. - Redis is used to store relationship data like follower/following lists in hashes with user IDs as keys and fields. MySQL is also used to complement Redis for persistence. - Challenges include high memory usage, slow disk writes, lack of high availability, and performance issues with large hashes. - Potential solutions explored are using MySQL with queues and processors to keep Redis in sync, splitting data between Redis and MySQL more intelligently, and implementing Redis data sharding and replication