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https://issues.apache.org/jira/browse/JENA-29?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12995083#comment-12995083
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Stephen Allen commented on JENA-29:
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Why is it not reasonable to abandon ORDER BY in the query and do it client
side? This sounds like a specific use case requirement rather than an ARQ
requirement.
The semantics of SPARQL specify that a solution to a query is valid if it meets
all the conditions specified in the BGPs, filters, and solution sequence
modifiers (Order, Project, Distinct, Reduced, Offset and Limit). This means
that only full-and-final results should be returned once those are
fully-and-finally computed. Now in practice, ARQ will build up solutions
incrementally in order to stream results back to a client if it is able to. The
contract of ORDER BY is that first item to be returned actually *is* the first
item in the solution set/bag. I would say that to have it return anything
other is an *incorrect* result not a *partial* result. If that is the case,
then it makes sense to me to simply abandon any work being done and try to
cancel the query as soon as possible. If a client wants to rely on any results
that have been returned to it already, that is its prerogative.
Having said all of that, I'm not saying cancel() support is not important, I
think just the opposite. I see your patch as valuable because it adds the
ability to cancel a query from a different thread by chaining the cancel
request through the iterators.
P.S. Slight bug fix: in QueryIteratorBase and QueryIterRepeatApply, you need to
make the "requestingCancel" field volatile to ensure visibility (see [1] for
more info).
[1] http://www.ibm.com/developerworks/java/library/j-jtp06197.html
> cancellation during query execution
> -----------------------------------
>
> Key: JENA-29
> URL: https://issues.apache.org/jira/browse/JENA-29
> Project: Jena
> Issue Type: Improvement
> Components: ARQ, TDB
> Reporter: Simon Helsen
> Assignee: Andy Seaborne
> Attachments: JENA-29_ARQ_r8489.patch, JENA-29_TDB_r8489.patch,
> JENA-29_tests_ARQ_r8489.patch, jena.patch, jenaAddition.patch,
> queryIterRepeatApply.patch
>
>
> The requested improvement and proposed patch is made by Simon Helsen on
> behalf of IBM
> ARQ query execution currently does not have a satisfactory way to cancel a
> running query in a safe way. Moreover, cancel (unlike a hard abort) is
> especially useful if it is able to provide partial result sets (i.e. all the
> results it managed to compute up to when the cancellation was requested).
> Although the exact cancellation behavior depends on the capabilities of the
> underlying triple store, the proposed patch merely relies on the iterators
> used by ARQ.
> Here is a more detailed explanation of the proposed changes:
> 1) the cancel() method in the QueryIterator initiates a cancellation request
> (first boolean flag). In analogy with closeIterator(), it propagates through
> all chained iterators, so the entire calculation is aware that a cancellation
> is requested
> 2) to ensure a thread-safe semantics, the cancelRequest becomes a real cancel
> once nextBinding() has been called. It sets the second boolean which is used
> in hasNext(). This 2-phase approach is critical since the cancel() method can
> be called at any time during a query execution by the external thread. And
> because the behavior of hasNext() is such that it has to return the *same*
> value until next() is called, this is the only way to guarantee semantic
> safety when cancel() is invoked (let me re-phrase this: it is the only way I
> was able to make it actually work)
> 3) cancel() does not close anything since it allows execution to finish
> normally and the client is responsible to call close() just like with a
> regular execution. Note that the client has to call cancel() explicitly
> (typically in another thread) and has to assume that the returning result set
> may be incomplete if this method is called (it is undetermined whether the
> result is _actually_ incomplete)
> 4) in order to deal with order-by and groups, I had to make two more changes.
> First, I had to make QueryIterSort and QueryIterGroup a slightly bit more
> lazy. Currently, the full result set is calculated during plan calculation.
> With my proposed adjustments, this full result set is called on the first
> call to any of its Iterator methods (e.g. hasNext). This change does not
> AFAIK affect the semantics. Second, because the desired behavior of
> cancelling a sort or group query is to make sure everything is sorted/grouped
> even if the total result set is not completed, I added an exception which
> reverses the cancellation request of the encompassing iterator (as an example
> see cancel() in QueryIterSort). This makes sure that the entire subset of
> found and sorted elements is returned, not just the first element. However,
> it also implies in the case of sort that when a query is cancelled, it will
> first sort the partially complete result set before returning to the client.
> the attached patch is based on ARQ 2.8.5 (and a few classes in TDB 0.8.7 ->
> possibly the other triple store implementations need adjustement as well)
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