Yes, you should use web-based distribution OAB.

HMC has been in the market for a while and as it grows, we are beginning to see many HMC providers are expanding their infrastructure and having more users and companies subscribing to their environment.

I also observed that the growth is especially obvious recently with the current economy situation where it makes so much sense for many companies to move their email, their SharePoint and corporate IM to the hosted model because of low startup cost and a low and predictable maintenance cost. Not to mention, it is much more feasible to do so nowadays with the current bandwidth and technology.

I noticed that the growth is particularly strong in the MAPI space. Customers are demanding better and richer Outlook email, communication and collaboration experience which HMC offers today rather than those simple POP3 and IMAP4 email technology.

Now, because of the growth in MAPI organizations, I feel that it is probably important for me to briefly talk about the need to move towards web-based distribution OAB instead of continuing scaling out the number of OAB Public Folder server.

We all know that in order to host MAPI clients, having Outlook to access the address list offline, we need to make sure each organization has an Offline Address Book available to be downloaded. Prior to Exchange Server 2007, the storage of the OAB is only in Public Folder Server. In HMC, it is recommended to dedicate Public Folder servers to host OAB and each OAB PF server shouldn’t have anything more than 1000 OABs. If you have more than that, you should either scale out by creating a new OAB PF server to host the new OABs or you can consider increasing the MaxPageSize in Active Directory. The latter, in my opinion is slightly dangerous as it may potentially generate performance impact to your whole Active Directory infrastructure.

So, everything sounds logical here, what’s the concern here? Well, the primary concern is that each Public Folder OAB created consumes a Replication ID (ReplID) in the Public Folder Hierarchy. The Replication ID has a limitation of approximately 32k. What does that mean? It means if you have more than 32k OABs in the environment, you may have problem with your OAB generation and download if they are in Public Folder.

It should also be noted that ReplIDs do not get cleaned up. Once it is created, it stays in the Public Folder Hierarchy until you refresh the Public Folder Hierarchy (which can be a rather tedious exercise) even if you delete the OAB. So, it means, if you create a new OAB and then delete it, it will still take up 1 count.

How do you know how many ReplID count you have now? Easy, go to your Exchange Server, look at the perfmon counter, MSExchangeIS Public\ReplID Count, you should be able to find your ReplID Count there.

I know some of you are probably getting worried knowing that there is a limit there. Good news is, there is a default quota limit being set as documented in this article, you should also take a look at this article, So, chances are, you probably know about this ReplID by now if you have exceeded the limit.

You can definitely increase the default quota limit to allow things to get going as long as it is not over 32k. The long term solution is to move to Web-based distribution which doesn’t have such limitation.

Obviously the caveats of moving towards a pure Web-based distribution OAB is that the environment needs to be upgraded to HMC 4.5 (if you are already running HMC 4.0, moving to 4.5 is a piece of cake but if you are running HMC 3.5, then it needs a bit of planning but not end of the world) and it requires the client to be Outlook 2007 and above. Older version of Outlook will still work for all their functionalities except when it comes to downloading Offline Address Book (OAB) and also any feature that may require OAB (such as selecting contact when you are offline).

Consider the fact that the process to reset the Replication ID should you exceed the limit may require you to reset the whole Public Folder Hierarchy; I will start thinking

1. Switching all my clients to Outlook 2007 only (after all, there is no reason not to do that as Outlook 2007 provides better OOF functionalities and also better free busy and availability data).

2. Move to HMC 4.5 if you haven’t already done so.

3. Then, enable Web-Based Distribution OAB and use that only. You may need to change your control panel to set it to only create Web-based Distribution OAB or something.

It should be noted that this is only applicable to OAB creation; it is not applicable to the normal Public Folder creation.

So, there you go. If you are not using Web-based Distribution OAB today in your HMC environment and you are anticipating rapid growth, you should.