A conflict occurs when one or more entries are updated simultaneously on multiple servers and the changes are incompatible, or causes some interaction between the updates. Conflict occurs because no update operation is carried out simultaneously on every replica in the replication topology. Instead, updates are first processed on one server, then replicated to other servers.
The following example describes a conflict that occurs when an attribute is modified at the same time on two different directory servers.
Consider a topology with two read-write replicas. A modify operation changes the surname, sn, attribute of an entry to Smith on one server. Before the server that is processing the change can synchronize with the other server, the sn attribute value for that entry is replaced with the value Jones on the other server. Unless the conflict is managed, replication would replay the change (Smith) on the server that now contains the value Jones. At the same time, replication would replay the change (Jones) on the server that contains the value Smith. The servers would therefore end up with inconsistent values for the sn attribute on the modified entry.
The following list describes additional conflicts that can occur.
An entry is deleted on one server while one of its attribute values is modified on another server.
An entry is renamed on one server while one of its attribute values is remodified on another server.
An entry is deleted and another entry with the same Distinguished Name (DN) is added on one server while one of its attribute values is modified on another server.
A parent entry is deleted and a child of that entry is created on another server, either through an add operation or a rename operation.
Two different entries with the same DN are added at the same time on two different servers.
Two different values are used to replace a single-valued attribute on the same entry on different servers at the same time.
Conflicts that involve only modifications of the same entry are called modify conflicts. Conflicts that involve at least one operation other than modify are called naming conflicts.
All modify conflicts and the vast majority of naming conflicts can be solved automatically by replaying the operations in their order of occurrence. However, the following naming conflicts, which have very little chance of occurring, cannot be solved automatically.
Two entries with the same DN are created at the same time on different servers, either by adding new entries or by renaming existing entries.
A parent entry is deleted and a child of the parent entry is created at the same time. The child entry can be created either when a new entry is added or when an existing entry is renamed.