Friday 3 December 2010

Outlook calendar synchronisation conflicts and custom forms

Have you seen this conflict message?

Cannot update the appointment because the corresponding item in the folder you synchronised does not match this item. To resolve the conflict, open the item. Do you want to open the item now?

If you click Yes, you see a box asking you to choose between two seemingly identical items, with this explanation:

Conflicting edits have been made to the same item. To resolve this conflict, select the item in the list below you wish to keep and then choose "Keep This Item", or choose "Keep All" to preserve all the versions.

Sometimes I think Microsoft use some special SEO technique for their best stuff… in this case Search Engine Obfuscation. So many times I’ve tried to find out why and how sync conflicts occur, and it took me a very long time to get any kind of answer.

Why Sync Conflicts Occur

Here’s a quick explanation of what’s going on: when you use Outlook in Cached Exchange mode, sometimes Outlook will see a small difference between the item you’re trying to save (from the cached copy of your mailbox) and the copy on the server. If it’s one of the ‘standard’ types of item (Mail message, Appointment, Task, Contact, etc.) then Outlook knows which fields are likely to be important, which aren’t, and makes a reasonable guess at resolving the item for you if it can without you even knowing it happened.

If you use a custom form instead of one of the standard forms (like if you have some CRM application which puts extra fields on every Appointment or Contact item), then Outlook doesn’t know what the item is supposed to be. It might be pretty obvious to you or me that it’s an Appointment but Outlook doesn’t make any assumptions. So if some little difference occurs between the item you’re editing (from the cached copy of your mailbox) and the copy on the server… it doesn’t try to automatically resolve the conflict and instead prompts you with the message at the top of this post.

Fortunately there’s a registry key where you can add values for each of your custom forms to tell Outlook what ‘standard’ form it’s most like, and it will try to resolve conflicts for you accordingly. This article on msdn is written for Outlook 2007, but the same registry edits work for Outlook 2003 – you just have to put ‘11’ in the key name instead of ‘12’.

About Conflict Resolution for Custom Item Types (MSDN)

Other Sync Conflicts

If you’re just seeing conflict messages once in a blue moon, it’s probably nothing to worry about. If you’re seeing them frequently, but you’re pretty sure you don’t have a custom forms solution and anyway it’s only affecting one or two people, the easiest and most likely solution is to repair your OST file (or even easier, delete or rename it and let Outlook rebuild a new one next time you connect to your mail server). If you’re not sure how to do that, you’d be safer following Microsoft's official instructions. My usual disclaimer applies.

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