Managing Public Folder Replication

Public folders are an afterthought in many Exchange implementations. Administrators tend to concentrate on getting their email infrastructure working properly without giving much thought to deploying public folders. However, public folders offer great functionality. You can use them to host electronic discussions, accept incoming Network News Transport Protocol (NNTP) newsfeeds, and provide outgoing information to newsfeeds. You can associate public folders with electronic forms and use public folders as the basis for applications. You can also use them as most companies do today, as convenient repositories for shared information. You can create public folders to hold documents, spreadsheets, and presentations, and your users can view and manage the data through Exchange clients and the Exchange administration program. (For more information about Exchange, see "Related Articles in Windows NT Magazine," page 190).

Public Folder Basics
Each public folder is a directory object in the Exchange directory. Public folders' properties include access control lists (ACLs), replication schedules, and email addresses. (You can specify email addresses for a public folder to subscribe the folder to Internet mailing lists.) You maintain public folders' properties as you would maintain any other object in the Exchange directory.

Exchange stores the public folder hierarchy and the contents of each public folder in pub.edb, Exchange's public information store. Exchange automatically replicates the public folder hierarchy among all the servers in an organization. When you create a new folder, Exchange enters the folder's details into the directory and sends that information to every other server.

When you add content to a public folder or a server executes a backfill request, Exchange replicates the folder's content to specific servers across an organization. When Exchange needs to replicate an item, it generates a replication message for that item and sends the message to all the other servers in the organization that contain replicas of that item's public folder. You must have directory and messaging connectors in place before public folder replication can work.

Accessing Public Folders
Exchange associates every user mailbox with a home public information store. In many smaller installations, the public information store is on the same server as the user's mailbox. In larger installations, dedicated public folder servers simplify administration and minimize replication traffic.

Messaging API (MAPI) clients such as Outlook establish remote procedure call (RPC) sessions to the home public information store at logon. The clients fetch information about the public folder hierarchy and use this information to build the public folder tree users see. Users might see folders in the hierarchy that they cannot access because either Exchange has not replicated the folder to a server they can access, or an ACL prohibits them from accessing the folder.

When users access public folder content, Exchange attempts to fetch the content from their home server. If a user's home server does not contain a replica of the public folder, Exchange attempts to locate a replica on other servers.

Exchange 5.0 and 5.5 support the concept of subsites. Many companies use sites to represent broad geographical areas, such as North America; subsites let you associate servers with specific locations, such as New York or London. (You specify a server's subsite--its location--as a property of the server, as Screen 1 shows.) If you use subsites, when users request a public folder their home server doesn't have, Exchange attempts to fetch the contents from another server in the same subsite.

If you don't use subsites, or if no server within the subsite contains a replica of the public folder's content, Exchange queries servers at random within the user's site. If Exchange doesn't find a replica within the site, it looks to remote sites using affinity (a mechanism that lets you assign different costs to different remote sites) to decide which site to query next. For example, if a New York site has affinity costs of 10 for San Francisco and 20 for London, an Exchange server in New York attempts to access public folders in San Francisco before attempting to access public folders in London.

Administering Public Folders
Only MAPI clients can create public folders. MAPI clients can also administer folder security, but they can't perform other management operations, such as creating replicas, establishing a replication schedule, or defining which server is a public folder's home server. You must perform most management operations through the Exchange administration program. You can view folder properties from any server, but you can alter properties only on the folder's home server. To view a specific public folder's properties, expand the set of public folders in the Exchange administration program's organizational hierarchy and select the folder, then select Properties from the File menu.

You can use the Exchange administration program to manage all of a particular server's public folders at once. Select the server in the Exchange administration program's hierarchy pane and expand the Public Information Store entry. From there, select the public information store property you want to administer. For example, you can select Folder Replication Status to view the replication status of every public folder and public folder replica on a server, as Screen 2 shows. The Display Name column lists the public folders' names. The Last Received Time column lists the date and time the Exchange administration program last received an update to each folder. The Number column lists each folder's number of replicas within the organization. The Replication Status column shows each folder's current replication status: Local Modified means that a user modified the local folder's content and Exchange has not yet replicated that change to the other servers that host replicas of the folder. In Sync means that all the replicas through- out the organization contain the same data. Remote Modified (which Screen 2 doesn't show) means that a remote server has updates for the local server's replica, but Exchange has not yet replicated that data.

You can also use the Exchange administration program to administer the system folders that the public information store creates on the first server in a site. Exchange hides system folders from users' view. These folders include the Offline Address Book; a folder that publishes Schedule+ free and busy information; a folder that holds permissions for the Event Service; and the electronic forms registry, which is a repository for the electronic forms that all your users access. Screen 3 shows the system folders for a server named Platinum. Exchange can create replicas of system folders on other servers; many organizations replicate Schedule+ free and busy information to save clients from making extended network connections to set up meetings.

Replicating Public Folders
A part of the public information store service called the Public Folder Replication Agent (PFRA) manages public folder replication. Underlying the PFRA are background threads that Exchange activates when the store starts. The PFRA performs several tasks. It maintains folder replica lists, monitors replication schedules, dispatches replication messages at times that the schedules dictate, sends status messages to other servers to ensure that they receive all replicated data, generates backfill requests if a server misses a piece of data, and responds to other servers' backfill requests.

Creating a new replica.
When you create a new replica, Exchange adds an entry for the replica to the public folder's replica list, which is a folder property in the Exchange directory. Exchange automatically replicates the change to other sites through its standard public folder hierarchy replication. Then, clients can see the new replica when they browse the public folder hierarchy. After Exchange replicates a new replica's addition to the hierarchy, it replicates the folder's content.

Pulling and pushing replicas.
Exchange replicates public folder content by either pulling or pushing the content. To replicate folders through the pull method, open the Instances tab in the public information store's Properties box on the server that you want to hold the new replicas. Browse the names of folders on other sites, and select the folders you want to replicate; click Add to add a new instance to the replica list. You can use folder pulling to establish replicas of several folders on a new server.

To replicate a folder using the push method, select the folder on its home server. Open the Replicas tab in the folder's Properties box. Select the sites you want to replicate the folder to, select a server within each site to hold the folder, and click Add. The push method lets you set up replicas of a folder on several servers in one operation.

By default, Exchange 5.5 restricts administrative access for public folders to administrators connected to a folder's home server, as Screen 4 shows. Unless you clear the Limit administrative access to home site check box, Exchange does not accept remote requests to create new replicas, which limits replica creation to the push method. However, the local administrative access property is not an option in Exchange 4.0 or 5.0, and a server with a version of Exchange earlier than 5.5 can pull replicas of folders that you restrict to local administrative access.