Resolving Modify Conflicts

Modify conflicts only occur with modification operations.

Operations are globally and logically ordered to determine the outcome of a given set of operations. Change numbers are used to define the order.

The replication conflict resolution functionality ensures that all servers eventually reach the same state, as if all operations were replayed everywhere in the order defined by the change numbers. This remains true even though changes might be replayed in a different order on different servers. In the modify conflict example with the sn values of Smith and Jones, described previously, assume that the value was set to Jones on the second server after it was set to Smith on the first server. The resulting attribute value should be Jones on both servers, even after the replace modification of Smith is replayed on the second server.

Historical information about each entry is retained to check whether a conflicting operation has already been played using a change number newer than that of a current conflicting operation. For each modify operation, historical information is used, first to check if there is a conflict, and, if there is a conflict, to determine the correct result of the operation.

When a modify conflict occurs, the server determines whether the current attribute values must be retained or whether the modification must be applied. The current attribute values alone are not sufficient to make this assessment. The server also determines when (at which change number) prior modifications were made. Historical information therefore includes the following elements:

• The date when the attribute was last deleted

• The date when a given value was last added

• The date when a given value was last deleted

When an attribute is deleted or fully replaced, older information is no longer relevant. At that point the older historical information is removed.

Historical information undergoes the following processing:

• Saved in the ds-sync-hist attribute (This attribute can only be viewed by an administrator.)

• Updated (but not used) for normal operations

• Updated and used for replicated operations

Conflict resolution is carried out when operations are replayed, after the pre-operation during the handleConflictResolution phase.

Conflict resolution is carried out by changing the List<Modification> field of the modifyOperation to match the actual modifications required on the user attributes of the entry, and to change the ds-sync-hist attribute that is used to store historical information.