On Fri, 18 Mar 2005, Michalis Kamburelis wrote:
Hi
I played with Sqldb and IBConnection units, and now I see the real benefit of them: there's only one dataset and one transaction class for all Firebird, PostgreSQL and MySQL bindings. Each specific database "binding" must only introduce new TSQLConnection descendant. It's great since it gives me even easier switching between various databases. This reminds me the zeos project.
However, there's a problem. In real programs I want to be able to set some FB-specific transaction parameters. By doing this I'm agreeing to sacrifice some portability of my programs across various databases, but I think it's useful, at least with FB, since there are many sensible transaction isolation modes and I want to be able to choose what I want.
When I was playing with Interbase unit yesterday, this was simple, much like with Delphi's IBX: just set appropriate properties of TIBTransaction class, like IsolationLevel.
How to get equivalent functionality with Sqldb and IBConnection ?
I would propose to introduce a enumerated
TSQLTransactionStyle =(tsConcurrent,tsReadCommit, etc.);
Then add a TransactionStyle to TSQLTransaction; This must be mapped by the TSQLConnection when creating the handle.
I implemented some small patches that enable this. The essential thing is new public function TSQLTransaction.SQLHandle that gives an access to TIBTrans class specific to FB database. Note that this forced me to write some additional code to react when user changes value of TSQLConnection.Database property. User of this code must be prepared that changing TSQLConnection.Database resets properties of SQLHandle.
An event 'AfterGetSQLHandle' could be introduced in TTransaction. But I think the abo
( Some notes about the problem with resetting properties of SQLHandle:
This problem is (at least partially) inevitable, since when value of Database property changes, it may change to a different descendant of TSQLConnection so the set of available properties may change anyway. So I think that the current method (that will implicitly reset all properties of SQLHandle, because internally FTrans will be destroyed and then (at the next call to SQLHandle) created again) is OK.
)
Now I can change properties of FB transaction doing e.g. (SQLTransaction.SQLHandle as TIBTrans).IsolationLevel := ilReadCommitted;
Attached patch also fixes a small bug, by changing in TSQLQuery.InternalOpen FieldByName to FindField. See inside the patch for comments what it solves.
One more thing: note that default IsolationLevel of IBConnection.TIBTrans is ilConcurrent, while default IsolationLevel of Interbase.TIBTransaction is ilReadCommitted. It's not my fault :), that's the way they were done. And none of my patches tries to change it, since it would be an incompatible change. However, I would advice changing default IsolationLevel of Interbase.TIBTransaction to ilConcurrent. This way not only IBConnection and Interbase units would use the same IsolationLevel by default, but also this would make Interbase.TIBTransaction a little more compatible to Delphi's IBX and to standard FB behaviour (empty TPB passed to isc_start_transaction is equivalent to isc_tpb_concurrency).
This can be done. None of the DB components are 'fixed', i.e. you are free to submit patches, suggest changes etc.
Michael.
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