Re: [Springframework-developer] Petclinic Hibernate implementation

[Ken K. <kk...@kk...>]

It might be most useful to implement one of the 2 databases as is using=20
the  MaxValueIncrementer while implementing the other using Hibernate to=20
preserve both types of examples.

Ken

j=FCrgen h=F6ller [werk3AT] wrote:

>Thomas, Trevor,
>
> =20
>
>>Does Hibernate use a different ID strategy for different databases?  I =
think
>>   =20
>>
>what is implemented in the Petclinic is a strategy that would work the s=
ame
>across different databses as long as there is a MaxValueIncrementer impl=
ementation.
>
>I intend to configure Hibernate to use "identity" for all databases. Set=
ting the correct Hibernate dialect will lead to "auto_increment" on MySQL=
 and "identity" on HSQL then. In terms of the data model, this simply mea=
ns dropping the sequence tables and defining the id columns as "auto_incr=
ement" respectively "identity" in the DDL scripts.
>
>Sure, MaxValueIncrementer adopts a very generic approach that will work =
on any database, even if it doesn't support such a thing like identity co=
lumns. The disadvantage is that the mechanism is normally not native to t=
he database and thus somewhat tied to the application.
>
> =20
>
>>If we change the Petclinic JDBC implementation to use identity columns,=
 then we
>>   =20
>>
>need to figure out how to run the insert and subsequent query to retriev=
e the id
>using the same connection whether we are in a transaction or not.
>
>This is very similar with MySQL and HSQL: You simply run the insert with=
out specifying a value for the id field, and query "select last_insert_id=
()" respectively "call identity()" afterwards (I've looked up the latter =
in Hibernate's MySQLDialect and HSQLDialect implementations). Petclinic a=
lready uses MySQLJdbcClinic and HSQLJdbcClinic subclasses; it should be e=
asy to encapsulate the id-fetching query there.
>
>Of course, the range of possible Petclinic JDBC implementations will the=
n be bound to identity-supporting databases. According to the Hibernate d=
ocs, those are "DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL". Ora=
cle is notably absent; I think we can live with Petclinic not running on =
Oracle, if we can allow for a dynamic switch between JDBC and Hibernate o=
n both MySQL and HSQL that way!
>
> =20
>
>>>auto-increment just allows to read in the actual id afterwards
>>>     =20
>>>
>>This is not reliable for concurrent inserts (which is why I generally u=
se sequences.  For the sample this should be sufficient though.
>>   =20
>>
>
>According to what I found out about MySQL, "last_insert_id" returns the =
last used id on the same connection and is thus transactionally safe. Don=
't know about HSQL, but I frankly don't mind for sample purposes.
>
>Juergen
>
>
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