Exchange 2007 Availability Services

I do not know about you, but when I think of the Client Access Server role in Exchange 2007, I immediately think of Outlook Web Access, Outlook Anywhere, ActiveSync and other non-MAPI forms of connectivity to the mailbox server. However, the Client Access Server role is also home to other important services such as the Autodiscover and Availability services. These are vital services in the Exchange 2007 infrastructure and in this article we will be taking an introductory and thus high-level look at the Availability service and what its main role is.

What is The Availability Service?

The availability service in Exchange 2007 is largely concerned with how users access the free/busy information of other users. Before we get into the detail of this service, it’s important to review how the free/busy information is stored and accessed in Exchange 2000 and Exchange 2003 so that we can draw comparisons on how the process works in these older versions of Exchange and more importantly what has been improved in Exchange 2007. In these older versions of Exchange, a site folder exists by the name of Schedule+ Free/Busy that is used to store the calendar free/busy information for each user. You can see this folder in Exchange System Manager by viewing the system folders instead of the public folders. Figure 1 shows an example of the Schedule+ Free/Busy system folders as seen in Exchange System Manager.

Figure 1:
The Schedule+ Free/Busy System Folder

You’ll note that there are two of these system folders, one per administrative group. By default, Outlook periodically publishes the calendar free/busy information into the system folder which then allows other users’ Outlook clients to query the data when booking appointments. On smaller systems this is generally fine, although on larger systems the use of public folders as the free/busy storage medium can pose a few problems. For example, there is always the possibility of the lack of up-to-date free/busy information due to public folder replication delays, or, in the worst case, the failure of the public folder replication process for whatever reason.  However, the main problem with this approach going forward is the future of public folders in Exchange. Over the last few years, you may have heard or read about Microsoft’s plans to retire public folders from a future version of Exchange and thus a new mechanism is required to handle free/busy requests. That’s where the availability service in Exchange 2007 comes in.

A Better Free/Busy Method

The availability service in Exchange 2007 is one of the new Web services. Briefly speaking, the Exchange 2007 Web services allow applications to access mailbox contents via HTTP, so clearly application development is aimed at these services in Exchange 2007. As we will discuss in detail, the free/busy information for a user hosted on Exchange 2007 is now stored directly in the mailbox, so accessing free/busy information can be achieved via the Web services and thus, specifically, the Availability service. As we shall see, this new method relies on Outlook 2007 and Exchange 2007, so things are not always achieved via this new method if Outlook 2003 and Exchange 2003 are still in the mix. Outlook 2007 locates the Availability service via the Autodiscover service. 

This article will not focus specifically on the Autodiscover service as this is a huge topic within itself. However, as a brief piece of background information, it is important to understand what the Autodiscover service is if you do not already understand it. Briefly, the Autodiscover service gives Outlook 2007 clients access to specific Exchange 2007 features such as the Availability service as we have already mentioned, plus other common services such as the Offline Address Book (OAB) as well as less common services such as Unified Messaging (UM).  Essentially, Outlook 2007 makes a request to a virtual directory called Autodiscover that is present on a Client Access Server. This Autodiscover service returns to the client many different pieces of information, some of which are URLs for services such as the Availability service.

Version Issues

Different access methods for free/busy information retrieval are used in environments that consist of Outlook 2003 and Outlook 2007 as well as Exchange 2003 and Exchange 2007. For example, when Outlook 2007 is used in conjunction with Exchange 2007, the main improvement is the fact that the free/busy information is now obtained directly from the Exchange 2007 target mailbox rather than from the Schedule+ Free/Busy system folder. This is how free/busy information is much more up-to-date than via the traditional public folder method.  Consider Figure 2 below, where an Outlook 2007 user with an Exchange 2007 mailbox requests free busy information for another Exchange 2007 user. In this case, the connection from Outlook is made to the Availability service running on the Client Access Server which in turn determines which mailbox server hosts the target Exchange 2007 mailbox. A Remote Procedure Call (RPC) connection is then made to that mailbox server and the results returned to the Client Access Server before being passed back to the user.

Figure 2:
Outlook 2007 User Querying Exchange 2007 Free/Busy Information

Figure 2 above assumes that the Client Access Server and mailbox server are in the same Active Directory site. What if the free/busy request is made for a user whose mailbox resides on an Exchange 2007 mailbox server in a different Active Directory site? In this case, the Client Access Server in the Active Directory site of the user who originates the request will proxy the request to a Client Access Server located in the Active Directory site of the target user. The results are returned to the original Client Access Server and then ultimately passed back to the originating user.

There is another important scenario that must be considered. What if a free/busy request was also made to another mailbox at the same time, but that mailbox was still on an Exchange 2003 server? This situation will be very common during any transition from Exchange 2003 to Exchange 2007. In these cases, the free/busy information for the Exchange 2003 user is stored in the Schedule+ Free/Busy system folder as we’ve already seen in this article. Therefore, the Availability service has to obtain the relevant information from this folder and it does this by making HTTP requests to the /Public virtual directory on the target Exchange 2003 mailbox server. This process is depicted in Figure 3. Once the information has been retrieved from the Exchange 2007 and Exchange 2003 servers, the Availability service combines the results and returns them to the Outlook 2007 user.

Figure 3:
Outlook 2007 User Querying Exchange 2003 and Exchange 2007 Free/Busy Information

So far we have covered what happens when a user is running Outlook 2007. How about when the user is running Outlook 2003 but connected to an Exchange 2007 mailbox? In this case, it doesn’t matter whether the target mailbox is on Exchange 2003 or Exchange 2007 as the Outlook 2003 client will attempt to retrieve free/busy information from the Schedule+ Free/Busy system folder. The reason for this is simply that Outlook 2003 always expects to publish the free/busy information in this location and therefore has no knowledge of the Availability service. As you can probably guess, this is also the situation for earlier versions of Outlook, such as Outlook 2002 or Outlook 2000.

Up to this point I’ve only listed Outlook as the client type in use. I should also mention that the principles are the same if you are using Outlook Web Access. In other words, if the target mailbox is on Exchange 2007, the Availability service will make a RPC connection to that mailbox server. If the target mailbox is on Exchange 2003, the Availability service will make HTTP calls and retrieve the information from the Schedule+ Free/Busy system folder. 


The Availability service in Exchange 2007 is important in the fact that it’s responsible for obtaining up-to-date free/busy information for users, providing they are running both Outlook 2007 and Exchange 2007. In this article I’ve covered a high-level introduction to the Exchange 2007 Availability service and how it is used to retrieve this free/busy information.

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