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Subject: [OASIS Issue Tracker] Commented: (ODATA-296) Change tracking may require server-side state, but client cannot advise server to stop tracking in order to allow server to release resources

    [ http://tools.oasis-open.org/issues/browse/ODATA-296?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=33336#action_33336 ] 

Evan Ireland commented on ODATA-296:

In our existing enterprise mobility solution we have found that change tracking (on filtered results) can require per-client (not just per entity-set) logging (on the server).

Suppose a user subscribes (using change tracking, with a filter) to download state "CA" customers. User A downloads customer "Jim", and receives a delta link D.

Now "Jim" moves to state "NY".

Then user A fetches delta link D. The result should include a "deleted-entry" for "Jim".

How can the server know that it should send this "deleted-entry" if it doesn't do per-client logging/tracking (at the server) to recall which entities were previously downloaded by user A?

There are a number of ways to solve this problem (e.g. keeping a change history for each customer with old values and timestamp for each change). Keeping per-client state at the server is a valid (and quite workable) solution, in our experience. While it is accepted that a client might not ever send a DELETE for a variety of reasons, it is also useful for the server to be able to accept it. A not uncommon use case would be to support the situation where the user A moves, and is now interested only in "NJ" customers. So user A could send a DELETE to the delta link D (to allow the server to clean up any per-client state for user A), and then get a new delta link (with a different query) for state "NJ".

> Change tracking may require server-side state, but client cannot advise server to stop tracking in order to allow server to release resources
> ---------------------------------------------------------------------------------------------------------------------------------------------
>                 Key: ODATA-296
>                 URL: http://tools.oasis-open.org/issues/browse/ODATA-296
>             Project: OASIS Open Data Protocol (OData) TC
>          Issue Type: Improvement
>          Components: OData Protocol 
>    Affects Versions: V4.0_WD01
>         Environment: [Proposed]
>            Reporter: Evan Ireland
>             Fix For: V4.0_WD01
> The protocol spec (2013-03-12) states in section 10.3 Requesting Changes:
>   "The delta link MUST conceptually encode the following information..."
> One common implementation of change tracking (where filters are used in the original request) is for the server to persist some state (for each client that it is tracking changes for), e.g. a list of primary keys. Otherwise it can be difficult or expensive to calculate an appropriate delta set when the client wants to receive updates, especially if filtered (non-key) properties have been changed in the interim.
> (In particular, if the server does not persist some state for each client, the size of a delta link may be excessive if the set of tracked entities is large).
> The problem here is that the OData protocol doesn't appear to define a mechanism for a client to notify the server:
> (1) That it no longer wishes the server to track changes for it, or
> (2) That it wishes to modify its filter criteria.
> So for issue (1)  the server doesn't have the opportunity to delete any server resources (e.g. persistent state) that it has retained on behalf of the client.
> And for issue (2) if the client uses a brand new filter with change tracking enabled, the server doesn't have the opportunity to optimize the returned results so as to avoid resending data that happens to exist in both old and new result sets.
> We may decide that the second issue should be deferred to post V4.
> But the first one could easily be addressed if we allowed a DELETE request to be sent to a delta link. If the server doesn't use persistent state for delta links, the DELETE for delta links can be trivially implemented! And if the server does use persistent state, then it would be able to free up resources (assuming co-operative clients).

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