Public Folders

Public folder architecture

In Exchange 2013, public folders were re-engineered using mailbox infrastructure to take advantage of the existing high availability and storage technologies of the mailbox database. Public folder architecture uses specially designed mailboxes to store both the public folder hierarchy and the content. This also means that there’s no longer a public folder database. High availability for the public folder mailboxes is provided by a database availability group (DAG). To learn more about DAGs, see Database availability groups.
The main architectural components of public folders are the public folder mailboxes, which can reside in one or more mailbox databases.

Public folder mailboxes

There are two types of public folder mailboxes: the primary hierarchy mailbox and secondary hierarchy mailboxes. Both types of mailboxes can contain content:

• Primary hierarchy mailbox   The primary hierarchy mailbox is the one writable copy of the public folder hierarchy. The public folder hierarchy is copied to all other public folder mailboxes, but these will be read-only copies.
  
• Secondary hierarchy mailboxes   Secondary hierarchy mailboxes contain public folder content as well and a read-only copy of the public folder hierarchy.
  

Note:
Retention policies aren’t supported for public folder mailboxes.

There are two ways you can manage public folder mailboxes:

• In the Exchange admin center (EAC), navigate to Public folders > Public folder mailboxes.
  
• In the Exchange Management Shell, use the *-Mailbox set of cmdlets. The following parameters have been added to the New-Mailbox cmdlet to support public folder mailboxes:
  
  • PublicFolder   This parameter is used with the New-Mailbox cmdlet to create a public folder mailbox. When you create a public folder mailbox, a new mailbox is created with the mailbox type of PublicFolder. For more information, see Create a public folder mailbox.
    
  • HoldForMigration   This parameter is used only if you are migrating public folders from a previous version to Exchange 2013. For more information, see Migrate Public folders from previous versions later in this topic.
    
  • IsHierarchyReady   This parameter indicates whether the public folder mailbox is ready to serve the public folder hierarchy to users. It's set to $True only after the entire hierarchy has been synced to the public folder mailbox. If the parameter is set to $False, users won't use it to access the hierarchy. However, if you set the DefaultPublicFolderMailbox property on a user mailbox to a specific public folder mailbox, the user will still access the specified public folder mailbox even if the IsHierarchyReady parameter is set to $False.
    
  • IsExcludedFromServingHierarchy   This parameter prevents users from accessing the public folder hierarchy on the specified public folder mailbox. For load-balancing purposes, users are equally distributed across public folder mailboxes by default. When this parameter is set on a public folder mailbox, that mailbox isn't included in this automatic load balancing and won't be accessed by users to retrieve the public folder hierarchy. However, if you set the DefaultPublicFolderMailbox property on a user mailbox to a specific public folder mailbox, the user will still access the specified public folder mailbox even if the IsExcludedFromServingHierarchy parameter is set for that public folder mailbox.
    

A secondary hierarchy mailbox will serve only public folder hierarchy information to users if it's specified explicitly on the users' mailboxes using the DefaultPublicFolderMailbox property, or if the following conditions are met:

• The IsHierarchyReady property on the public folder mailbox is set to $True.
  
• The IsExcludedFromServingHierarchy property on the public folder mailbox is set to $False.
  
  https://technet.microsoft.com/en-us/library/jj150538%28v=exchg.150%29.aspx