What are Merge Replication Conflicts

What are Merge Replication Conflicts

Merge Replication Conflict in SQL Server

A merge replication conflict in SQL Server occurs when the same data is modified concurrently at different nodes in a merge replication topology, such as the publisher or subscribers, leading to inconsistencies during synchronization. Merge replication allows multiple nodes to independently update data, and the Merge Agent reconciles these changes during synchronization. Conflicts arise when changes cannot be seamlessly merged due to conflicting modifications.

Here is a short video discussing merge replication conflicts and finding them with Database Health Monitor.

Common Conflict Scenarios

Conflicts typically occur in the following situations:

  • Update Conflict: The same row is updated with different values at different nodes. For example, a product price is set to $100 at the publisher and $120 at a subscriber.
  • Insert Conflict: Rows with the same primary key are inserted at different nodes, violating uniqueness constraints.
  • Delete/Update Conflict: One node deletes a row while another updates it, creating ambiguity about the row’s final state.
  • Delete/Insert Conflict: A row is deleted at one node, but a new row with the same key is inserted at another.

Key Aspects of Merge Replication Conflicts

Understanding how SQL Server manages conflicts is essential for effective replication:

  • Detection: SQL Server tracks changes using metadata, such as row GUIDs and lineage columns in replicated tables, to identify conflicts during synchronization. This metadata ensures accurate detection of conflicting changes.
  • Resolution: Conflicts are resolved based on predefined or custom rules, including:
    • Default Resolver: Typically prioritizes the publisher’s changes, though this is configurable.
    • Custom Resolver: Allows user-defined logic or stored procedures to resolve conflicts based on business requirements.
    • Interactive Resolver: Enables manual conflict resolution through tools like SQL Server Management Studio.
  • Conflict Tracking: Conflicts are logged in system tables, such as MSmerge_conflicts_info, for auditing and review. This allows administrators to analyze and address conflicts systematically.

Mitigation and Best Practices

To minimize conflicts, consider the following strategies:

  • Design applications to avoid concurrent updates to the same data, such as by assigning specific data ranges to different nodes.
  • Use logical data partitioning to reduce overlap in updates across nodes.
  • Implement custom conflict resolvers tailored to business logic to ensure consistent outcomes.
  • Regularly monitor conflict tables and use SQL Server’s conflict viewer to identify and resolve recurring issues.

For example, if a row is updated to “Value X” at the publisher and “Value Y” at a subscriber, the Merge Agent detects the conflict and applies the configured resolver to determine the final value. Proper conflict management ensures data integrity and consistency across the replication topology.

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