incubator-jena-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Andy Seaborne (JIRA)" <>
Subject [jira] Commented: (JENA-29) cancellation during query execution
Date Thu, 17 Feb 2011 14:00:32 GMT


Andy Seaborne commented on JENA-29:

This is an outline of the contract of "cancel".  It is not a description of implemenation.

Currently, QueryExecutionBase.cancel() exists (and is deprecated with the comment "do not
use") for testing all this.  It will be moved to .abort() when we're ready.  A phase of renaming
internal methods may happen when the details of implementation make the exact nature of methods
and fields quite clear.

1/ To terminate an execution, something calls .cancel, on the QueryExecution which in turn
calls ".cancelRequest()".

Multiple calls of .cancel result in one call of cancelRequest().

cancelRequest is async to iterator execution.

2/ Internal "cancellation" is also possible i.e. the system chooses to call .cancel itself
(e.g. timeout, if done that way, or limit on total number of resutls [not planned], other

2/ Cancellation is not required to happen immediately, or indeed to happen at all, but would
probably be considered a bad implementation not to do something.  That is, we don't enforce-by-contract
that cancel has a specific effect at any specific point.

3/ When cancelled, .hasNext/.next on the results iterator are undefined.

CONSTRUCT and DESCRIBE will return null.

4/ There are some internal flags to control the behaviour after cancellation is active.

Default behaviour:

A/ Any calls to .hasNext()/.next() throw QueryTerminatedException.  They start doing so at
some point (cancellation is async to execution) but the intention is as soon as reasonably

I read the javadoc for the .hasNext contract as meaning is hasNext is true, then noSuchElementException
will not be thrown.  You can get ConcurrentModificationException from java collections from
.next() anyway regardless of .hasNext().

Continuation behaviour:

B/ The QueryIterator is closed during the next call to .next(). An element is returned.  The
iterator is not explicitly closed by 
QueryIteratorBase if NoSuchElementException is thrown.  Further calls to .hasNext/.next may
return results.

(a little tighter would be to stop if .hasNext has not been called yet for the next solution
- needs another flag for "is hasNext() already decided").

> 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)

This message is automatically generated by JIRA.
For more information on JIRA, see:


View raw message