index.rst

Merging

There are two possible cases for the merging of data: fast-forward merges and conflicts.

Fast-forward merges

A "fast-forward" data merge situation means that there is no conflict to resolve. This can be determined by checking if the current version of a record on the receiving device is already contained in the history of the transmitting device, or vice-versa.

In the illustration above:

1. Device A (green) produces a new record, r. It gets assigned record version A1 and history [ A1 ].
2. Next, Device A modifies r. The record version changes to A2 and the history is now [ A2, A1 ].
3. Device B (red) now syncs data with Device A and both the devices have same version and history of record r.
4. Device B modifies its copy of r and sets the record version to B1. The history of r is now [ B1, A2, A1 ] on Device B and still [ A2, A1 ] on Device A.
5. When Device A syncs with Device B again (the arrow), there is no conflict and the update B1 can be incorporated directly.

Merge conflicts

A merge conflict means that two devices have made changes to a record, and it is not clear how to reconcile the two change histories.

In the illustration above:

1. As above, Device A (green) produces a new record r with version A1 and history [ A1 ].
2. Device B (red) now syncs data with Device A and both the devices have same copy of record r.
3. Next, Device B modifies its copy of r. The record version changes to B1 and the history [ B1, A1 ].
4. Device A modifies its own copy of record r and saves it as A2 with history [ A2, A1 ].
5. When Device A syncs data with Device B again (the arrow), there is a conflict because both devices have modified r.

It is up to the implementing application to determine what the merge conflict resolution strategy is.