"Last Writer Wins" Algorithm May Cause Loss of Data for FRS-Replicated Content

The File Replication Service (FRS) is a multi-threaded, multi-master replication engine that replaces the LMREPL service in Microsoft Windows NT 3.x and 4.0. Windows 2000 domain controllers and servers use FRS to replicate system policy and login scripts for Windows 2000 and down-level clients.

Optionally, FRS can replicate files and folders between Windows 2000 servers hosting the same fault-tolerant DFS roots or child node replicas.

This article describes the impact of a multi-master replication model for files and folders replicated between Windows 2000 servers and domain controllers.

FRS uses multi-master replication to keep files and folders residing in SYSVOL and replicated DFS trees consistent between partners. Multi-master replication ensures that changes to a file or folder originating on any partner is replicated to all other "downstream" partners once the changes are received.

Domain controllers and servers hosting System Volume (SYSVOL), fault-tolerant DFS, and DFS replica member shares on which replication has been enabled are as likely to receive updates from an attached network client as from FRS replica partners.

Replication for SYSVOL is always enabled when FRS is running. Replication must be explicitly enabled in the Distributed File System Manager snap-in (Dfsgui.msc) for DFS root and replica members. To enable, disable or view the DFS replication settings:

1. Start the Distributed File System Manager snap-in (Dfsgui.msc).
2. Right-click the level of the DFS tree "backed" by more than one Windows 2000 computer in the left pane and then click Replication Topology.
3. Use the Enable and Disable buttons as required.

FRS uses a "last writer wins" algorithm, meaning that the last update to a file or folder originating on any replica becomes "authoritative" once replication takes place, regardless of the document version number or file size. This makes FRS well suited to replicate files that are updated infrequently such as product specifications, software distribution points, and Web content. Content updated more frequently faces two scenarios:

• Users A and B open the same 100-page document on different replicas: User A adds 100 more pages and saves, while user B deletes 80 pages before saving. The 20-page document saved last becomes authoritative.
• Users open files on FRS replicated SYSVOL and DFS shares in two different Active Directory sites. The replication schedule on the Active Directory or DFS connection object specifies that replication take place at the default setting of once every 3 hours. Updates originating to replica members in site A are not available on replica members in Site B for up to 3 hours based on the last replication period. The minimum replication interval is 15 minutes.

There is no way to deny file sharing or enforce file locking between two users writing the same file on two different replicas. To implement single master replication, the administrator must ACL the root of the replica tree to limit write access appropriately, then establish a policy that a specific computer is the "master" replica.

The current replication method for FRS operates at the file level. Changing a single character in a 5-MB document causes the entire file to replicate.