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Part 2: The Exchange public folder replication methodology explained

Learn how hierarchy and backfill replication work in public folders, and find out what Exchange does when two users try to modify the same public folder content simultaneously.

Hierarchy replication

As the name implies, hierarchy replication refers to replication of the Exchange public folder hierarchy (also called the Exchange public folder tree).

The Exchange Information Store service watches for changes to the public folder hierarchy, including the creation, deletion, or renaming of public folders within the hierarchy.

But these are not the only changes that the service watches for. It also keeps an eye on:

  • Exchange public folder permission changes
  • Changes to a public folder's description
  • Public folder priority settings
  • The public folder replication schedule
  • A public folder's position within the public folder hierarchy

When any of these changes occur, the server sends a replication message to the other Exchange public folder replicas to make them aware of the change.

Backfill replication

Content replication refers to replicating messages, message headers and attachments. An important aspect of content replication is backfill replication.

Backfill replication comes into play any time that an Exchange public folder server becomes disconnected from the network or must be restored from backup. Backfill replication is responsible for replicating any public folder content (or changes to the public folder hierarchy) that might have occurred while the server was unavailable.

To understand why this is important, imagine that an Exchange public folder server fails and must be restored from yesterday's backup. When the server is restored, its public folder content and hierarchy is current through yesterday. Once the server comes back online, any changes that occurred to Exchange public folder replicas on this or other public folder servers will, of course, be replicated in the usual manner. Unfortunately, the server is missing a full day's worth of synchronization changes.

At best, the missing content would be confusing to users. Depending on which Exchange public folder server a user is connected to, they may or may not see content that has been added to the public folder structure between the time that the backup was made and the time it was restored.

At worst, having a day's worth of changes missing from one of the Exchange public folder replicas could result in database corruption. For example, imagine that one of the changes included in the missing data was the creation of a new folder. Once the server is back online, replication begins working normally. However, inbound replication messages contain data that is destined for a public folder that does not exist on the server.

Backfill replication uses sequence numbers to determine what data needs to be replicated to an Exchange public folder server (and in what order), so it can synchronize the replica with the other public folder replicas. Once the backfill process completes, public folder content replication can continue in the normal manner.

Conflict resolution

The whole point of creating Exchange public folder replicas is so users can work with the replica located nearest to them on the network, instead of taxing WAN and public folder resources by using a centralized public folder server.

Because users work off multiple Exchange public folder servers, there is always the chance that two different users could make conflicting modifications to the same folder or message simultaneously.

If this occurs, two different servers have two different versions of the message or folder that has been modified; and both servers believe they have the most recent version. When these servers begin to replicate the modified folder or message, a conflict occurs.

The way that Exchange Server resolves this conflict differs depending on whether the modified object was a public folder or content within a public folder. If the conflict involves a public folder, Exchange resolves it by using the "last change wins" method. The most recent change is saved, overriding all previous changes.

If a conflict relates to folder content, Exchange Server does not automatically resolve the conflict. Instead, it sends a conflict resolution message to the public folder's designated contact. That person must then decide whether to keep one or both versions of the modified message.


TUTORIAL: EXCHANGE PUBLIC FOLDER REPLICATION
 
  Home: Introduction
  Part 1: An overview of the Exchange public folder replication process
  Part 2: The Exchange public folder replication methodology explained
  Part 3: What content should you replicate in Exchange public folders?
  Part 4: How to create Exchange public folder replicas
 Part 5: Related resources on Exchange public folder management
 

 

ABOUT THE AUTHOR:   
 
Brien M. Posey, MCSE
Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Exchange Server, and has previously received Microsoft's MVP award for Windows Server and Internet Information Server (IIS). Brien has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. As a freelance technical writer, Brien has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal Web site at http://www.brienposey.com.
 
This was last published in April 2007

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