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From Mike Matrigali <mikem_...@sbcglobal.net>
Subject Re: conflict detection strategies
Date Wed, 15 Feb 2006 18:33:30 GMT
There are very few cross thread dependencies not managed by locks 
currently.  These things add extra complications to current and
future code.  Also I want to understand clearly the new restrictions
be imposted on the access methods (both current and possible
future).  In the future we would like to do more automatic space
reclamation as part of the zero-admin goal, the ability to do this
in the future internal to access methods is probably affected by
the proposals here.

It is true that the current access methods don't reuse row locations
until a table level lock is granted.  But your project would be the
first dependency on this outside of the access method implementations
themselves.  It is very clear the contract that the access methods
have with their clients while locks are held on the data they are
looking at, what you are proposing is a contract on unlocked data.

Note that the current "in-place" compress will MOVE rows from one
row location to another if one does not have a row lock on the row.
This is done in the 2nd phase and only holds an intent lock, and
exclusive row locks on the rows being moved.
The off-line compress only does work under an X table lock.
So the row that you are updating actually will exist in the table,
but currently you will request the old location and will get back
a delete row indicator.  I think because of this option 1 does not

The state of held cursors across commits is very murky in the standards.
We looked very carefully at forward only held cursors, and the standards
there are carefully worded to basically not promise anything about the 
rows that were viewed that preceded the commit (clearly since the 
standard says the only thing you can do after the commit is a next to 
get a new row or close - never can access rows looked at before the
commit).  What options are legal
implementations of updatable scrollable result sets for held cursors 
across commits?  Do the standards guarantee anything about data in the
cursor looked at before the commit?

Andreas Korneliussen wrote:
> Mike Matrigali wrote:
> ..
>> If possible I would like to see a solution that does not require special
>> messages sent back and forth between modules about state.
> I am not entirely sure what restrictions you want to put on the design, 
> it is a bit unclear to me.
> I have considered some other solutions:
> 1. Change the locking behaviour, so that a table intent lock which is 
> set by an updatable cursor, is kept as long as the cursor is open - this 
> will ensure that the RowLocations are valid.
> 2. After a commit, we could clear all data in the internal table in the 
> SUR. The problem with this approach is that the resultset would not 
> necessarily be repopulated with the same data - it would be sensitive 
> for changes across its own transactions commits, it would be highly 
> ineffecient.
> 3. Let the cursors notify the OnlineCompress module that it should fail 
> any attempt to compress/defragment or purge the table.
> More details on what I suggested yesterday:
> The OnlineCompress class could provide an event mechanism, where 
> subscribers (OnlineCompressListener) register themselves to listen to 
> OnlineCompressEvents. The ScrollInsensitiveResultSet class could then 
> implement the OnlineCompressListener interface, and register itself once 
> it starts populating the table with RowLocations. The OnlineCompress 
> class then simply notifies all listeners once it is doing defragment / 
> compress.
> The listeners should unregister themselves (i.e 
> ScrollInsensitiveResultSet class could do it once it closes). The 
> OnlineCompress class could use a WeakHashMap to put the listeners into, 
> in case they are not well-behaved. I have not checked if derby already 
> has event manager type of modules, if it does, I would attempt to reuse 
> them.
> Please also let me know if any of the other alternatives seems better.
> Andreas
>> Andreas Korneliussen wrote:
>>> Some context: In scrollable updatable resultsets, we populate an
>>> internal table with the following data:
>>> <Position> <RowLocation> <RowUpdated> <RowDeleted> [<DataColumn>]+
>>> Example layeout:
>>>  1         <1,10>         false        false        1,"a",3
>>>  2         <1,11>         false        false        2,"b",2
>>>  3         <1,12>         false        false        3,"c",9
>>> When doing updateRow(), or deleteRow(), we use the RowLocation to
>>> navigate to the row being updated.
>>> Problem:
>>> For holdable cursors, we will release the table intent lock when doing
>>> commit on the transaction for the cursor.
>>> The table intent lock, prevents the system from doing a compress of the
>>> table, causing all RowLocations to be invalid. In addition, it prevents
>>> reuse of RowLocation for deleted + purged rows.
>>> In order to support holdable scrollable updatable cursors, we consider
>>> having a service which allows the system to notify subscribers (i.e
>>> cursors) that it has executed i.e a compress.
>>> If the user then calls updateRow() or deleteRow(), we can then give an
>>> exception like:
>>> "The row could not be updated, because its location has been updated by
>>> the system"
>>> In addition, we consider having a reclaim of locks, so that immediatly
>>> after a commit, the new transaction with the holdable cursor, may
>>> reclaim the table intent lock.  This will reduce the time period which
>>> the system may compress the table, however not completely remove the
>>> possibility of a compress.
>>> Any comments on implementing such strategy ?
>>> An alternative to this strategy, could be to go the other way: cursors
>>> notify the system that it should not do compress.
>>> I would appreciate feedback on this topic, especially if you find any
>>> pitfalls with the proposed strategies, or have better alternatives.
>>> Andreas

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