ADF: approach needed to do pre-processing on creation of a record

Question

Hi,
i need a proper approach for doing some select and updates from the database when a new record is created. The select and update is not related to the entity object base table.
at the moment i have overriden the create method in the Enitity Implementation class:
1. Select a value from a table
2. Change this value in the database (and commit)
3. use the new value as a default value in the in the EO's new record
The problem is that i want this to be done as two independant transactions, ie. the update in isolation of the entity objects underlying transaction. I have tried using PreparedStatements and postChanges() (which should commit the first transaction without validating the state of the Entity Object?) But i find that the changes are not effected in the database until commit on the entity object is called.
help appreciated,
Brenden

Answer 1

If you are using Oracle database, I would strongly advise you NOT to use this approach.
First, using the code you have given, you can easily get two different records with the same ID. Image two different sessions (users) that are trying to get their IDs. It is possible that the two sessions read the same last number, each increments this number in its own context and then assign the same number to its newly created record. To avoid this, you should try to lock the sequence row before reading the last number, then increment it, save it back in the database and then release the lock. This is very bad approach that limits severely the concurrency.
Secondly, this approach does not allow you to create two new entities in the same transaction. Committing the transaction in the create() method for the second entity will try to post the first one, which might not have its mandatory fields populated, for example. The transaction will fail to commit as there are non-validated entities in its pending list.
So in brief, I suggest that you use native database sequences for this as generating unique numbers in a concurrent and scalable manner is what they have been created for. You can use DBSequence datatype combined with a before-insert trigger for this.