Mike Matrigali wrote:
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.
I think the complexities are there already. It seems very hard to know
under exactly which conditions, a RowLocation remains valid. We have
assumed that the RowLocation will remain valid as long as we hold a
table intent lock (valid means that it either points to the same row or
to a deleted row).
That is what we concluded from the previous discusssion about validity
of RowLocation. If you in the future need to make code, or there already
is code, which breaks this assumption, we would need to know which other
mechanisms we should use to either verify that the RowLocation is valid,
or to block the system to make it invalid.
Locks can be used to manage cross thread dependencies, however they are
bound to the transaction, and therefore does not help very much for
cursors held across commits. So if the only mechanism we can have to
ensure that the RowLocations are valid, is by the means of locks, I
doubt we can support the feature of scrollable insensitive *holdable*
updatable resultset.
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.
I guess we are the first to use RowLocation without holding a lock on
the row. This is necessary, unless we simply make SUR cursors set locks
for all rows in the cursor independent from isolation level.
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
work.
Are you saing that RowLocations can be invalidated by "in-place"
compress even if we hold a Table intent lock ?
How do you call "in-place" compress today ? Does the system use it
automatically, or do the user have to call it manually ?
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?
I looked at the SQL standard, and for cursors held over commit, it says:
"If the cursor is insensitive, then significant changes are not visible"
Andreas
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
