This document discusses using MongoDB as the common store for user-generated content in Adobe Experience Manager Communities. It describes the benefits of MongoDB, such as its flexible JSON document model, efficient searching, built-in replication for high availability, and ability to scale. It then covers different options for implementing MongoDB as the common store, including the MongoMK JCR SRP, Adobe SRP, and Mongo SRP. It also discusses which content is typically stored in the common store versus being replicated separately.