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From "Stephen Allen (JIRA)" <>
Subject [jira] Commented: (JENA-29) cancellation during query execution
Date Wed, 16 Feb 2011 00:18:58 GMT


Stephen Allen commented on JENA-29:

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).


> cancellation during query execution
> -----------------------------------
>                 Key: JENA-29
>                 URL:
>             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
> 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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