hibernate.hbm2ddl.auto consult and doubts

[dr_pompeii]

Hello guys

<property name="hibernateProperties">
	<props>
       	<!-- SQL dialect -->
	<prop key="hibernate.dialect">org.hibernate.dialect.MySQL5InnoDBDialect</prop>
 	            
	<!-- Enable Hibernate's automatic session context management -->
	<prop key="hibernate.current_session_context_class" >thread</prop>
	<!-- Disable the second-level cache  -->
        <prop key="hibernate.cache.provider_class" >org.hibernate.cache.NoCacheProvider</prop>
        <!-- Echo all executed SQL to stdout -->
	<prop key="hibernate.show_sql" >true</prop>			
					
	<!-- Drop and re-create the database schema on startup -->
 	<prop key="hibernate.hbm2ddl.auto" >create</prop> 
	</props>
</property>

i see that each time that i startup my tomcat,
well the db is created each time and all db data is losted

so to avoid this,
to before to shutdown and startup again,
well i comment the bold line and all the db is keep it with the data

but i am lost with this
if i instead of create value i use
update

well

1:what happens if now in my hbm files i create for the old tables more fields?
2: i create new tables
3: for these new tables maybe can now has relations to the old tables

so for the old tables
which behaviour would has this update in the schema?,
specially with the old rows already inserted in the db?

i am lost with this

some advice of answers are really appreciate

thanks in advanced

[al0]

Any automatic changes in DB schemas outside of personal development environment are big NO-NO-NO

Regards,
Oleksandr

[dr_pompeii]

Any automatic changes in DB schemas outside of personal development environment are big...

so why the <prop key="hibernate.hbm2ddl.auto" > update option?

regards

[al0]

so why the <prop key="hibernate.hbm2ddl.auto" > update option?

regards

To eveolve your schema during development. Small quotation from"Java Persistence with Hibernate" by Christian Bauer and Gavin King, chapter 2 page 64

WARNING We've seen Hibernate users trying SchemaUpdate to update the schema of s production database automatically. This can quickly end in disaster and won't be allowed by your DBA.

To make things clear - SchemaUpdate tool is tool activated by hbm2ddl.auto=true.

Regards,
Oleksandr

[dr_pompeii]

Hello

thanks for the reply,

wondered why this dangerous option is available is has a high risk to kill the db

regards

[al0]

Hello

thanks for the reply,

wondered why this dangerous option is available is has a high risk to kill the db

regards

It is convinient (or at least may be convinient) during development, BTW, the whole HibernateTools package is development, not production oriented. Any decent database (I mean DB size) needs a lot of manual schema tuning to optimize performance which is DB-specific and can not be done with this tool.

There is another reason (IMHO) the whole phylosophy of Hibernate is to provide a user with any and all imaginable options and allow him to select what suits him more. The whole book that I have mentioned is ovecrowded with statements like "Hibernate supports option XXX, but we do not reccomend to use it".

Regards,
Oleksandr