We developed our own sync engine for an offline-first IDE for notes/tasks [1] we're building, where the data structure is a tree (or graph actually) to support outlining operations. Conflict resolution is always the challenge, and especially with trees multiple offline players can optimistically commit local changes which would result in an invalid tree state once globally merged.
The way we handle this is by rolling back tree inserts when a client comes online and receives other tree insert deltas. From what I understand from the description of SQLSync, the best way to handle this would be to pull in a latest snapshot and then replay. Pulling in a full snapshot sounds rather expensive though. We have some sort of heuristic where we can do this if the number of incoming deltas would be very large, but for most offline syncing we never need it. Just curious how SQLSync defines these snapshots? Sticking with the note-taking example, in our case we can't really have a snapshot of a single "note" because of graph features like transclusions. Does SQLSync have some clever way to avoid having to send all state in order to "reset and replay"?