Hi Stefan,

On Wed, Aug 20, 2008 at 4:05 AM, Stefan Seelmann <seelmann@apache.org> wrote:
Hi Alex,

I would recommend to pass a monitor object to those cursors and
evaluation loops:

public interface Monitor
   public boolean isCanceled();
   public void cancel( String cause );
   public void cancel( Exception cause );
   public Exception getCause();

An implementation SearchLimitMonitor could be constructed with the
search time limit and its isCanceled() method then checks if the time
limit is exceeded. Additional it could have a method cancel() that
could be called by a timer thread, if there is such an requirement in
the future.

After some more thought and discussion with Emmanuel I think this technique is much better.  At first I really did not understand the main benefit gained but essentially you get to avoid having to have an external thread calling close().

Essentially the monitor allows injecting new cancellation handling code specific to the situation when the Cursor does not support every case.  This is just much better than having to deal with the synchronization of yet another thread and it's executor queue.

I will implement this now.  Thanks Stefan.


In the cursors then just check the Monitor.isCanceled(), instead of
compararing the time limit parameters directly. Using this pattern we
are more flexible, we could change the strategy to cancel the search
request by changing the implementation of the monitor.

Kind regards,

> Hi all,
> I have an interesting situation to deal with while trying to find a way to implement search time limits.  I'm a bit perplexed on how best to implement this and so any suggestions are great.  Here's a quick breakdown of the problem.
> Search requests must adhere to search time limits.  The request is used to build a system of nested Cursors and evaluators similar in structure to the search filter.  The final Cursor produced will return candidates accepted by the filter expression.  Cursors can be advanced using first(), last(), next(), and previous() methods.  While advancing a loop performs evaluations to find the first, last, next or previous candidate matching the filter.  Those entries not matching the filter a not returned.  It's a bit more complex than this but this trivial view is sufficient to discuss the problem.
> So we have to stop searching after some time limit.  Previously before the bigbang we used to just check the time limits high up in the code (in the search handler) after getting each candidate.  This did not always work so well because sometimes with very large directories and poor search filters it would take a while to find the next candidate to return so we would overshoot the time limit.   The problem is primarily due to these evaluation loops in the Cursor while trying to advance to a candidate.  While in this loop, going over entries that do not match, there is no way for the Cursor to detect that time is up.
> There are a few ways I thought we can solve this problem.  First these loops will have to check if the time is up between iterations either directly or indirectly through some parameter.  It would be nice if the Cursor can detect this on it's own but tunnelling the size limits deep down through this system of nested Cursors during construction is a real PITA.  I was thinking of using the closed/open state of the Cursor to naturally halt these loops.  This should happen anyway.  The only problem is another thread is needed in the SearchHandler to close the Cursor when time limits are exceeded.
> I talked to Emmanuel about this on IM and he preferred the approach where a Cursor is passed the time limits and handles timing out on it's own.  I'm not too keen on this idea because a Cursor by it's nature is not and may not be time limited.  However it does need to respond to being closed appropriately even while one thread is advancing it.  I think Emmanuel rightfully so dislikes the idea of having a timer thread (really a schedulable executor of tasks for closing search Cursors) in the search handler.  Don't know if others see a better option here or if one way is more preferrable than the other.
> Thoughts?
> Alex
> --
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