A page appeared on TechNet on February 24, 2014 titled “Limits for Public Folders.” Not much news there, I thought, we all know that public folders have limitations, even the modern variety as used in both Exchange 2013 and Exchange Online. For instance, their lack of any ability to manage conflicts makes public folders a pretty horrible repository for multiple collaborators to work on a document.

But the information in the TechNet article carries some really bad news for enterprise Exchange customers because it includes the news that modern public folders are limited to 100 public folder mailboxes in an organization and (gasp!) 10,000 public folders in the public folder hierarchy.

Update (July 15, 2014): Microsoft announced today that they have increased the limit to 100,000 public folders for Exchange Online. The change is effective immediately for all tenants. Exchange 2013 on-premises customers will have to wait until Exchange 2013 CU6 is delivered.

I had never heard of any such limitation before and this came as an unpleasant shock because there are many deployments of old-style public folders that comfortably pass the 10,000 threshold. Apparently the page was available beforehand but only referenced Exchange Online, so those who read the information might conclude that the limits only referred to that platform. All cloud services impose restrictions to avoid tenants using too many resources to the potential detriment of others and although problems caused by the limitations have surfaced in Office 365 support forums (like this example from August 2013), these were overlooked by the on-premises community. After all, the situation is different when you run an on-premises deployment and control how resources are used. At least, that was the theory.

Some comfort might be gained by the note that “Although you can create more than 10,000 public folders, it isn’t supported”, which I assume to mean that customers can feel free to go ahead and create tons of public folders to allow Microsoft to observe what happens. Microsoft can then decline responsibility if all hell breaks loose. One company who contacted me after this post appeared told me that if you have more than 20,000 folders in a hierarchy modern public folders break comprehensively and totally. This is unverified information, so treat it as such.

The ramifications of this information is that companies who are making plans for the migration of their public folders to Exchange 2013 will now have to prune their existing public folder hierarchy to bring it down under the 10,000 limit before they can begin the move. The process of figuring out what’s good and what’s not so good in a public folder hierarchy is difficult enough as it stands, mostly because of the lack of comprehensive tools in the space, but now it’s impossible to take the attitude of “it’s doesn’t matter – migrate everything.” The work to gather information about public folders, figure out what can be removed, delete unwanted folders (perhaps after saving the data in the folders to a PST first), and then validating that everything is OK before starting the migration simply has to be done.

It’s surprising that Microsoft has taken so long to bring the limitations to the attention of enterprise customers. It’s true that public folder migrations are the very last step in moving older Exchange versions to Exchange 2013, so many companies might not yet have hit the limit. However, you would imagine that performance and scalability testing would have been done and the results documented and shared as part of the development process for the new-style public folders.

Modern public folders use a completely architecture to their ancient cousins that is firmly based on the mailbox database. Lots is known about the performance characteristics of mailbox databases and how well they scale in different circumstances, so I can’t quite work out where the problem might lie. Pure speculation on my part is that the primary public folder mailbox and its role as the host of the only writable copy of the hierarchy might be a bottleneck, but we shall just have to wait until more is known as to where problems lurk.

According to Microsoft sources, the development team is well aware that these limitations will pose problems for enterprise customers who want to move to either Exchange 2013 on-premises or Exchange Online. They’re working on the problem and hope to have some better news “soon.” At least, that’s what the rumor mill says. We live in hope.

The good news (if there is any) is that the same page tells us that modern public folders can store up to 1 million items each. The thought of being able to store quite so many items in a single public folder certainly eases the pain of the other limits.

In the light of this news, I might just have to change my “Ten predictions for the world of Exchange in 2014” because no one might be able to complete a (supported) migration of more than 10,000 public folders. I received some comments after I published “The dirty little secret about migration to modern public folders” to say that companies had completed some migrations, including one spanning 17,000 folders. Is that company now supported?

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