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From Alex Karasulu <akaras...@apache.org>
Subject Re: [index] OneLevelIndex removal
Date Thu, 12 Apr 2012 14:14:34 GMT
On Thu, Apr 12, 2012 at 4:35 PM, Emmanuel Lécharny <elecharny@gmail.com>wrote:

> Forgot to reply to this mail, which raises interesting points.
>
> More inside.
>
> Le 4/11/12 10:38 PM, Alex Karasulu a écrit :
>
>> On Wed, Apr 11, 2012 at 4:04 PM, Emmanuel Lécharny<elecharny@gmail.com>**
>> wrote:
>>
>>  I think we should add some mechanism in the server to check that
>>> automatically, to avoid doing it by hand (there are hundreds of tests to
>>> check...). One solution would be to keep a track of every cursor
>>> construction in a HashMap, and to remove them when the cursor is closed.
>>> The remaining cursors are likely not closed.
>>>
>>
>> It would be nice to have a Cursor monitor that every opened Cursor
>> registers with but this needs to happen automatically. Then when out of
>> the
>> creation scope the Cursor is expected to be closed and if not this is
>> handled automatically. However does creation scope work well since
>> sometimes we create Cursors and pass them up?
>>
> We do have a monitor, which is currently used to check that the cursor is
> not closed when we try to use it. We certainly can use this monitor for
> more than just checking such thing.
>
> Now, the pb is that the scope is not as easy to determinate than for a
> variable in Java. For instance, if we consider persistent searches, or
> paged searches, or even an abandonned search request, the scope is pretty
> wide...
>
> Though we can have a set of rules that help us to close the cursor
> automatically :
> - if we get an exception during a SearchRequest, then the cursors must be
> closed immediately. As soon as we store the cursors into the SearchContext,
> this is pretty easy to do
> - an AbandonRequest will close the cursor automatically too (getting the
> cursor from the abandonned request)
> - when we process the SearchResultDone, we can also close the cursor for
> the current search request (this work for PagedSearch too)
> - for pagedSearch, if the user reset the search by sending 0 as the
> expected number of entries to return, then the cursor will be freed
> - for persistent searches, as it will be closed by an unbind or an abandon
> request, we are fine
> - when a client unbinds, then all the pending cursors will be closed.
>
> All in all, we have everything needed to close the cursors automatically,
> assuming we keep all the cursors into the session.
>
>
These are really great suggestions and make the ideas I tried to express
really tangible. Thanks for it Emmanuel.

One technical point, we need to make Cursor close() operations idempotent
if they are not already - meaning if we close a second time this should not
cause an exception or change the outcome.


> On the client side, this is another issue... As cursors are created by the
> client code, we have no easy way to determinate when we should close the
> cursors, except when the connection is closed or an abandon request/unbind
> request is sent. Of course, when the server returns a searchResultDone we
> could also close the cursor. Remains the situations where the client has
> fetched some entries (but not all), and haven't unbind nor abandonned the
> search.
>
>
I think the aspect for automatic closing of cursors is left to be managed
inside the server even though the API overlaps here.


> In any case, this is less critical as we don't have to deal with the txn
> layer. The client will just blow away with some nasty OOM sooner or
> later... but this is not worse than what we get with NamingEnumeration in
> JNDI, nah ?
>
>
Yup +1


> Have I covered all the server options ? Or did I miss something ?
>
>
>> This sounds like something that can be handled nicely using an aspect
>> oriented solution. Now these things are heavy if you use AspectJ or
>> something like that but other simpler solutions exist to bytecode splice
>> compiled code to automatically handle these things. Maybe our past
>> experiences with Aspects might make us reconsider.
>>
> A bit overkilling, IMO?
>
>
I'm feeling the same but thought it should be just put out there. However
we can achieve the same results perhaps with code or using a lighter
mechanism with Proxy's via CGlib or something similar. These are just raw
thought dumps so it's not a we SHOULD recommendation. Something to think
about.

-- 
Best Regards,
-- Alex

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