IceCube is a system for optimistic replication, supporting collaborative work and mobile computing. It lets users write to shared data with no mutual synchronisation; however replicas diverge and must be reconciled. IceCube is a general-purpose reconciliation engine, parameterised by ldquoconstraintsrdquo capturing data semantics and user intents. IceCube combines logs of disconnected actions into near-optimal reconciliation schedules that honour the constraints. IceCube features a simple, high-level, systematic API . It seamlessly integrates diverse applications, sharing various data, and run by concurrent users. This paper focus on the IceCube API and algorithms. Application experience indicates that IceCube simplifies application design, supports a wide variety of application semantics, and seamlessly integrates diverse applications. On a realistic benchmark, IceCube runs at reasonable speeds and scales to large input sets.