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From Kristian Waagan <kristian.waa...@oracle.com>
Subject Re: Table exists in same JVM after Derby is shutdown
Date Wed, 10 Aug 2011 12:29:19 GMT
On 09.08.11 17:14, Pavel Bortnovskiy wrote:
> Thank you, Kristian.
>
> Can you please clarify some of the points you made (or point me to an
> online primer that describes it):
> - what is the difference between shutting down a specific db and the
> whole engine?

Shutting down a specific database will close all open handles to that 
database and free up resources associated with the database. The next 
connection attempt to this database will cause it to be booted again.
When a database isn't booted, i.e. it has been shut down, it can also 
be safely copied using OS utilities [1].

Shutting down the Derby engine usually makes the Derby classes eligible 
[2] for garbage collection. You won't be able to obtain a new connection 
to a Derby database before you reload the engine/driver [3].
Shutting down the engine also shuts down all booted databases.

> - what does it mean that shutting down the Derby engine which runs an
> in-memory-only DB doesn't drop tables?

It means that the in-memory database will be kept in memory, just as an 
on-disk database will be kept on disk.
The in-memory databases are stored in a static variable. The ways to get 
rid of them are to explicitly drop them, to have the Derby classes 
garbage collected [2] , or to exit the JVM.

> Are you saying that if that engine were to be restarted, the tables
> would be accessible again?

Yes, all in-memory databases would be accessible again.

> Does it also mean that the memory taken by the engine and its tables
> doesn't get reclaimed until it's shutdown (or dropped)?

Shutting down the engine will free up resources like the page cache, and 
most, but not all, instances of Derby classes will be garbage collected.
If in-memory databases have been created, but not dropped, these will 
remain in memory.


Regards,
--
Kristian

[1] There are other ways to safely copy a database, for instance 
freezing it while booted or using the Derby backup routines.

[2] To actually unload the Derby classes, the class loader that loaded 
them has to be eligible for gc too. Simply shutting down the engine is 
not sufficient (see also [3]). Note the difference between garbage 
collecting instances of Derby classes and unloading the Derby classes 
themselves.

[3] This changed somewhat with the addition of the 
"deregister=*true*|false" attribute. If false, the driver will be left 
registered even if the engine is shut down.


> Regards,
> Pavel.
>
>
> From: 	Kristian Waagan <kristian.waagan@oracle.com>
> To: 	derby-user@db.apache.org
> Date: 	08/09/2011 01:42 AM
> Subject: 	Re: Table exists in same JVM after Derby is shutdown
>
>
> ------------------------------------------------------------------------
>
>
>
> On 08.08.11 16:41, Pavel Bortnovskiy wrote:
>  > Thank you, Dag.
>  >
>  > It looks like in order to get this to work, I had to make two calls to
>  > DriverManager.getConnection.
>  > Combining "/shutdown=true;drop=true/" didn't work.
>  > So, is this the correct implementation (if in-line image doesn't work,
>  > please see attachment):
>
> Hello Pavel,
>
> To simply drop an in-memory database, you only need the drop attribute.
> Shutdown of the specified database happens as part of the drop operation.
>
> It is correct that you need two calls to DriverManager.getConnection if
> you want to drop a database and additionally shut down the Derby engine.
> For clarity, note that there is a difference between shutting down a
> specific database and the Derby engine/system.
>
> Further, shutting down an in-memory database does not drop it, neither
> does shutting down the Derby engine. I don't know, maybe it would make
> sense to drop all in-memory databases when the Derby system is shut down
> - but that's not being done currently.
>
>
> Hope this helps,
> --
> Kristian
>
>  >
>  >
>  >
>  >
>  >
>  > From: dag.wanvik@oracle.com (Dag H. Wanvik)
>  > To: "Derby Discussion" <derby-user@db.apache.org>
>  > Date: 08/05/2011 06:36 PM
>  > Subject: Re: Table exists in same JVM after Derby is shutdown
>  >
>  >
>  > ------------------------------------------------------------------------
>  >
>  >
>  >
>  > Pavel Bortnovskiy <PBortnovskiy@Jefferies.com> writes:
>  >
>  > > 1) I create an in-memory database and then a table in it.
>  > > Then the database is shut down.
>  > > I would expect that the shutdown effectively cleans everything up.
>  > > Is it not so?
>  >
>  > Ah, I misunderstood you Pavel. Have a look here:
>  >
>  > http://wiki.apache.org/db-derby/InMemoryBackEndPrimer
>  >
>  > As you can see, you need to provide the "drop=true" connection
>  > attribute to wipe the data.
>  >
>  > Thanks,
>  > Dag
>  >
>  > >
>  > > Thanks,
>  > > Pavel.
>  > >
>  > >
>  > >
>  > > From:
>  > > dag.wanvik@oracle.com (Dag H. Wanvik)
>  > > To:
>  > > "Derby Discussion" <derby-user@db.apache.org>
>  > > Date:
>  > > 08/05/2011 05:57 PM
>  > > Subject:
>  > > Re: Table exists in same JVM after Derby is shutdown
>  > >
>  > >
>  > >
>  > > Pavel Bortnovskiy <PBortnovskiy@Jefferies.com> writes:
>  > >
>  > >> Hello, all:
>  > >>
>  > >> While executing a bunch of JUnit tests within the same JVM (all
>  > executed
>  > >
>  > >> by IntelliJ IDEA) I started seeing strange and unexpected errors
>  > > occurring
>  > >> .
>  > >> Upon closer inspection, I noticed that in many of those tests tables
>  > > with
>  > >> the same names are attempted to be created.
>  > >> Then I realized that although Derby is shutdown and then re-created,
>  > the
>  > >
>  > >> tables remain, thus causing collisions.
>  > >
>  > > The "create=true" connection attribute is ignored (with a warning) if
>  > > the database with the same name alrady exists.
>  > >
>  > >>
>  > >> I've created a digest (attached) which is executed as one JUnit
> test to
>  > >> illustrate what I'm seeing.
>  > >> The behavior I would expect is that once Derby is shutdown, no tables
>  > >> would remain in the JVM, and if a database (with the same name) is
>  > >> re-created, it would be a tabula rasa.
>  > >
>  > > The tables are no longer in memory (or should not be unless you found a
>  > > bug), but they are not erased from the disk image of the database. As
>  > > per the above, one would need to explicitly delete it using OS file
>  > > system tools for the data to be cleared. Some JUnit tests delete tables
>  > > in TestCase#tearDown, others use singleUseDatabaseDecorator to isolate
>  > > itself from the rest of the tests. The tests not necessarily very
>  > > consistent in their patterns for this..
>  > >
>  > > Dag
>  > >
>  > >>
>  > >> Can you please let me know whether my expectations are erroneous and
>  > >> whether I should find workarounds (albeit trivial to implement).
>  > >> However, what would concern me in that case is that the tables and the
>  > >> data remain in the JVM, thus consuming memory
>  > >> (and, if unused, creating memory leaks).
>  > >>
>  > >> Thank you,
>  > >>
>  > >> Pavel.
>  > >>
>
> < snip >
>
>  >
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> contents of this email, including any attachments, are confidential to
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> are not the addressee of this email you may not copy, forward, disclose
> or otherwise use it or any part of it in any form whatsoever. This email
> may be produced at the request of regulators or in connection with civil
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> recipients is prohibited. In the United Kingdom, Jefferies operates as
> Jefferies International Limited; registered in England: no. 1978621;
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