问题
I have inherited from a colleague who left a system module that integrates with Exchange Online in Office 365. Essentially what this module does is interact with with the remote Exchange service via EWS Managed API; subscribe for push notifications on changes in a user's calendar.
Change events do get posted to our web service and that is fine. And based on the frequency parameter we defined, the status checks messages also get posted on expected intervals, as per description about the subscription keep-alive behaviour.
The problem is, in observation, the subscription does not perpetuate despite responding with SubscriptionStatusType.OK to keep it ongoing. We never send SubscriptionStatusType.Unsubscribe since there is no error condition found in notification in messages. It seems to only last 9 to 14 hours before the Exchange service stops sending any status checks or change notification messages. When we make subscriptions from two separate web servers (different notification callback URLs), their subscriptions seem to die out around the same time.
Haven't found any clues that would cause the Exchange service to cancel/expire our subscriptions. What other conditions might contribute to this premature unsubscribe?
回答1:
Exchange will "lose" subscriptions on a fairly regular basis, especially in the O365 environment because mailboxes are continually being shuffled onto different servers in order to load balance the entire ecosystem. Even in an on-prem Exchange you can lose subscription if a CAS restarts. Unfortunately to build a reliable app, you have to check periodically that you heard from Exchange via some kind of notification or heartbeat.
来源：https://stackoverflow.com/questions/27976614/exchange-online-push-notification-subscription-unable-to-perpetuate