You seem to be forgetting one important detail… if a Rollback is called, the
underlying data and undo tables will be rolled back automatically. They are,
after all “real” tables in the database, and will be managed as such. Unless
you’re storing data outside the SQLite database, there is nothing for your
function to do… the SQLite engine will rollback all the tables and put them
into their prior state all on its own.
-j
On Nov 20, 2014, at 3:26 AM, Alessandro Marzocchi
<alessandro.marzoc...@gmail.com> wrote:
> Good day,
> I'm implementing a serialization&undo/redo engine based on sqlite. My
> implementation (particulars omitted) will work by adding a layer of virtual
> tables over real sqlite data tables plus a physical table to store the list
> of modifications made to them (to implement undo functionality). Also a
> callback to the real object is provided to notify when values are changed
> (eg for a rollback or undo). My problem comes on implementing rollback
> function. When rollback is called my thought is to restore old data directly
> from the data table or the undo table. For example (still simplified)
>
> 1) Virtual table: dataObjectA(id INTEGER PRIMARY KEY, Foo INTEGER)
> Data table: _dataObjectA(id INTEGER PRIMARY KEY, Foo INTEGER)
> Undo table: _undo(step INTEGER PRIMARY KEY, STRING class, id INTEGER,
> variable STRING, oldValue VARIANT, newValue VARIANT),
> objectA[27]->Foo=3; so _dataObjectA contains a single entry (27,3)
> 2) BEGIN
> 3) User calls objectA[27]->setFoo(4): c++ from this calls make an UPDATE
> dataObjectA SET Foo=4 WHERE id=27;
> 4) My xUpdate gets called. Following queries are made: UPDATE _dataObjectA
> SET Foo=4 WHERE id=27; INSERT INTO _undo VALUES (0 /*incremental step number
> */, 'objectA', 27, 'Foo', 3, 4)
> 5) ROLLBACK
> 6) My xRollback gets called and has to restore previous state (and notify it
> through the callback)
>
> Now what database state will xRollback see? The one before (4) or the one
> after? Looking at code I see xRollback in sqlite3.c (3.8.7.1 amalgamation)
> is called with following call stack:
> 112789: static void callFinaliser(sqlite3 *db, int offset)
> 112838: sqlite3VtabRollback
>
> Which itself may be called from:
> 125934 sqlite3RollbackAll /* After sqlite3BtreeRollback is called */
> 125744 sqlite3Close /* Not meaningfull on my case */
>
> So it seems that when my xRollback will be called it will see old data. Can
> anybody confirm me there are no corner case in which something different
> could happened and may I assume that this behavior will be kept in future
> releases too?
>
> Also, even if I'm afraid the answer is not is there a way to force
> "reloading" of virtual table(s)? In my implementation columns can be added
> (in virtual as in real data table) through another special virtual table
> (following the example before user may do a INSERT INTO classDef VALUES
> ('ObjectA', 'Bar', 'STRING') and this would add a column to _dataObjectA.
> Have I any ways to force (other than forced DROP TABLE/CREATE VIRTUAL TABLE)
> reloading so I can notify sqlite of new schema with sqlite3_declare_vtab?
>
> Thank you for your help
> Alessandro
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--
Jay A. Kreibich < J A Y @ K R E I B I.C H >
"Intelligence is like underwear: it is important that you have it, but showing
it to the wrong people has the tendency to make them feel uncomfortable." --
Angela Johnson
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