Duplicate Updates (Access/SQL cont’d)
Some rejected updates are saved for later analysis
To identify duplicate updates to discard them -
- When applying an update to x, replace x’s array of [replica, version#] pairs by the update’s array.
- To avoid processing the same update via many paths, check version number of arriving update against the array
Consider a rejected update to x at R from R´, where
- [R´, V] describes R´ in x’s array, and
- V´ is the version number sent by R´.
- If V ? V´, then R saw R´’s updates
- If V < V´, then R didn’t see R´’s updates, so store it in the conflict table for later reconciliation