What I want to achieve is basically a reflection of a status of the file system inside the OPC UA Server, on a client side - a "cached" copy of what files and directories the server has. I believe this is a fairly common scenario (for efficiency reasons, offline work etc.). But I do not think it is achievable with 1.04 spec, because the client has no way to tell whether the file contents has changed.

In my scenario it is OK to be able to "sync" files data only when the file is in closed state (it is not necessary, and even not desirable, to be informed about every change).

What does not work:

• You cannot take one of the existing FileType properties such as "Size" and detect changes, because the size does not have to change even if the data had changed.
• You cannot take timestamps of any properties defined on FileType (at least not with the implementations I have tested with) and detect the changes; the timestamps can remain the same even if the file data has changed.
• You cannot subscribe to the OpenCount property to detect that somebody is manipulating the file, for two reasons: a) The server can change file contents "from inside", without opening the file, and b) this does not allow to detect a change while the server was down and/or client was disconnected for any reason.

The proposed solution is to add some kind of version counter on the FileType that the server would have to increment whenever the file data have changed. A "false positive" (incrementing when not needed) would be in principle OK (just leading to an inefficiency in client-side caching). Ideally this would be made mandatory so that server implementors actually bother implementing it; but if not doable, it can stay optional.

An alternative solution would be a something like LastModifiedTime property.

For reliably, it is absolutely necessary that if there is any chance that the file contents have changed during server restart, the server shall increment the version counter or update the lastModifiedTime.