EWS resource server-side notification handling problems

[Krzysztof Nowicki]

Hi,

When working with the EWS resource I'm experiencing some messy event flow that 
leads to the folder getting out of sync in a fatal way causing the need for a 
full resync. I'm seeking some advice how to fix this.

The problem happens in case of a modify+delete action (e.g. a message is read 
and quickly deleted).

So here is what happens:

 1. The message attributes are modified (e.g. the message is read, causing the
     SEEN attribute to be set).
 2. Akonadi calls itemsFlagsChanged() for the item, which triggers an
     UpdateItem EWS request.
 3. The message is deleted.
 4. Akonadi calls itemsRemoved() for the item, which triggers a DeleteItem EWS
     request.

So far so good, but here is where the server-side notification subscription 
kicks in:

 5. The EWS server sends a notification about the item having been updated (in
     response to the request from step 2. The EWS server notification mechanism
     does not care if the modification originated from the same client.
 6. The event triggers a collection sync for the folder, which retrieves the
     list of changes on the server side and on the Akonadi side.
 7. The synchronization code sees that an item has been modified remotely and
     looks for the matching local item in order to update it (at this point I
     don't know that I have just modified the item). The lookup fails as the
     item is already gone (step 3), at which point the code assumes that we're
     seriously out-of-sync and triggers a full sync.

I have tried to somewhat mitigate this by logging items that were locally 
modified or deleted and in case a notification is received for that operation, 
it is ignored. This reduced the window of opportunity slightly, but with the 
right timing it still happens.

So I was wondering if the logic I use to handle server-side notifications is 
correct. At one time I was thinking whether instead of triggering collection 
syncs, I should let the notification handler apply the changes manually by 
manipulating Akonadi items directly, but that could lead to another 
amplification, as the resource would in turn get calls from Akonadi about item 
operations that it has just made from the notification handler thread.

I was also thinking about a way to check if the local copy of a folder is in 
sync with the remote one (to try to avoid a costly full sync), but the EWS API 
does not offer any reliable way to compare local and remote states.

Any thoughts/ideas?

Regards
Chris