Merge what you can, fork what you can't: managing data integrity in local-first software

ABSTRACTIn a local-first architecture that prioritizes availability in the presence of network partitions, there is a tension between two goals: merging concurrent changes without user intervention and maintaining data integrity constraints. We propose a synchronization model called forking histories which satisfies both goals in an unconventional way. In the case of conflicting writes, the model exposes multiple event histories that users can see and edit rather than converging to a single state. This allows integrity constraints to be maintained within each history while giving users flexibility in deciding when to manually reconcile conflicts. We describe a class of applications for which these integrity constraints are particularly important and propose a design for a system that implements this model.