Exchange Server 2007 Availability Service Explained

When planning an Exchange Server 2007 project with customers the question of Public Folders always comes up. One of the scenarios in which Public Folders are discussed is by customers who do not currently use Public Folders, and who want to know whether they need to use them with Exchange Server 2007.

The question relates to the topic of Free/Busy information, which is the data from mailbox users' calendars that lets others see their availability when trying to schedule meetings in Outlook.

Exchange 2003 stores Free/Busy information in the Public Folder database for all mailbox users, whereas Exchange Server 2007 introduced a new feature called the Availability Service to replace that functionality. The Availability Service runs on the Client Access Server role.

The Availability Service does not store Free/Busy data, rather it retrieves it on request directly from the mailbox in question. This is in contrast to Exchange 2003 which stored the data in a special Public Folder. The data was published to the Public Folder by the Outlook client itself, and so it was not always completely up to date.

Some of the advantages of the Availability Service over the Public Folder publishing method are:

• Makes Free/Busy data sharing available in a more granular fashion for end users (e.g., can choose to just show whether they are free or not, or also show details of the meetings they have planned, etc)
• Simplifies cross-Forest sharing of Free/Busy data by making it directly accessible between organizations, instead of the legacy method of synchronizing Free/Busy data with the Inter-Org Replication Tool
• Exposes Free/Busy data via Exchange Web Services so that it can be accessed by other programs via APIs

The main dependency of the Availability Service is that it can only be accessed by Outlook 2007 and later clients. Outlook 2003 and earlier have no ability to query the Availability Service. This leads to some confusion for customers, especially during a migration project when both Exchange 2003 and 2007 co-exist in the organization.Consider an organization that is in the process of migrating to Exchange Server 2007 and so has mailbox users on both 2003 and 2007 mailbox servers. Access to Free/Busy data will be achieved in the following ways:

• Regardless of the Outlook version, any Exchange 2003 mailbox user will publish Free/Busy data to the Public Folders
• Regardless of the server version, any Outlook 2003 or earlier client will publish Free/Busy data to the Public Folders, and read Free/Busy data from the Public Folders
• Outlook 2007 clients on Exchange 2007 mailbox servers will query the Availability Service for Free/Busy data
• The Availability Service retrieves Free/Busy data directly from Exchange 2007 mailboxes, and from Public Folders for Exchange 2003 mailboxes

What this usually boils down to for customers, when planning for the stage that they are running only Exchange 2007 servers, are these simple rules:

• If you have any Outlook 2003 or earlier clients, you will still need Public Folders for Free/Busy information
• If all your clients are Outlook 2007 or later, you do not need to retain Public Folders for Free/Busy information

It may seem a trivial issue, but being able to remove Public Folders completely makes the environment that little bit easier to deploy and administer. At the very least it is one less database to backup on the server.