[eloff]

Ouch, data sync his close to home for me. That was my first job. Not an easy problem at all.

Tip for those of you with the same problem, if you can, try to sync both the data and the actions taken to produce the data (event sourcing) and try to eliminate sources of indeterminism.

[swyx]

why both? isn't that wasteful? which one wins if they disagree? do you sync both at the same frequency?

[eloff]

It depends on the business logic. If e.g. a user updates a numeric value on an entity, and there's a conflict probably you choose the most recent value. The action taken (user updates value) is pretty useless in this case. In other cases, e.g. inventory reduced by X because X units purchased/consumed - then it's more important to have the action itself, and a conflict would involve applying both actions and possibly alerting someone if that causes the value to be invalid (e.g. inventory below 0.)