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From Emmanuel L├ęcharny <elecha...@apache.org>
Subject Re: Storing the txn nto each operationContext
Date Sun, 25 Dec 2011 18:08:47 GMT
On 12/25/11 5:58 PM, Selcuk AYA wrote:
> On Sun, Dec 25, 2011 at 5:37 PM, Emmanuel Lecharny<elecharny@gmail.com>  wrote:
>> Hi,
>>
>> I started to play with this concept. The idea is to able to have
>> encapsulated operations using their own transactions, following these rules
>> :
>>
>> 1) if there is another pending Read transaction, and if the new operation is
>> read only, then reuse the pending transaction
>> 2) if there is another pending Read transaction, and if the new operation is
>> ReadWrite, then create a new transaction
>> 3) if there is another ReadWrite transaction, then generate an error (we
>> can't have such a case)
>>
>> That means we can have a non limited number of encapsulated RO txns, but we
>> can't have more than one RW txn running.
>>
>> RO(RO(RO...(RO)...))) is possible
>> RO(RO(RW))) is possible
>> RO(RO(RW(RO is not possible
>> RW(RO is not possible
>> RW(RW is not possible
>>
>> In order to implement that, we need to add one thing :
>> - a nbRef in readOnly transactions, which will be incremented and
>> decremented as soon as we add new RO txns or abort/commit txns
>>
>> Is that enough ?
> this is also a reply to you previous email.
>
> I suggest we use a txn per operation but we do not have to store the
> txn context pointer in operation context. We can still have the thread
> context stored in thread local variable but we also store a TxnHandle
> ref in EntryFilteringCursor. And we do something like this:
>
> next()
> {
>    get txn handle stored in the cursor and set it as the thread local variable.
> do the next
> unset the thread local variable.
> }

In fact, as each operation except Search are atomic, I don't know if 
it's useful to store the txn in the thread local variable. Regarding the 
search, we just have to store the txn in the cursor, so we don't have to 
store it into the thread local variable either.

Another reason we might not want to use thread local variable is that an 
abandon operation will have to close a txn, and that means grab the txn 
from another thread.It's easier to get the existing cursor, and close 
the cursor. (FYI, we may have more than one thread per session, just in 
order to be able to handle an AbandonRequest)

Unless I'm missing something, of course !
>
>
> Lets say an embedded app developer does something like this:
>
> {
> open cursor1
>
> while ( next on cursor1 ).
>    for some certain entry do a delete
>
> open cursor2
> do a search on cursor2.
>
> close cursor1
> close cursor2
>
> }
>
> if we do something like suggested above, then consistency semantics is
> clear: each cursor sees the world as of the time it is opened. So
> cursor1 does not get affected by the delete and cursor2 sees the
> delete. This implementation does not use txn reuse.
>
> If we deal with reusing the txn contexts and hence ref counts,
> depending on where you close the cursor, what cursor2 see will change.
> As I understand your suggestion, in the above scenario, delete uses
> its own txn and cursor2 reuses cursor1's txn. So cursor2 wont see the
> delete as it see the world as of cursor1's opening time. But if
> cursor1 is closed before opening the cursor2, then cursor2 will see
> the delete. For a developer,  such things make reasoning with
> consistency  I think.

I'm not sure I get what you wrote at the end of your second paragraph 
("For a developer, such things make reasoning with consistency I 
think."). But I think that opening a second cursor inside a first one 
should simply leads to processing the same data, even if some entries 
have been deleted in the mean time.


-- 
Regards,
Cordialement,
Emmanuel L├ęcharny
www.iktek.com


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