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Re: [eclipselink-users] How eclipselink handles Sequences?
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Doesn't sound correct, the preallocation size must match the increment. But
if it works for you, then fine.
As for runnning out of sequence numbers see,
http://en.wikibooks.org/wiki/Java_Persistence/Identity_and_Sequencing#Running_Out_of_Numbers
The difference of 1 vs 50 should not effect running out, either you would
run out anyway, or more likely it is the difference between 100 millions
years and 20 millions years...
saasira wrote:
>
> Hi James,
>
> I think I found a solution to my problem...
>
> Here is what I did:
>
> Database:Postgres
> Application Server: Glassfish V2.1
>
> Database Sequence: Increment=1,Cache=100
> Sequence Pre Allocation Size Annotation on Entity=100
>
> I tried with parallelly/simultaneously populating the data with three
> different applications: eclipselink based application,pgAdmin, psql
> (command
> line shell for postgres); I'm happy to say that everything worked fine
> with
> out any collisions and no gaps in numbers generated by the sequence. Of
> course, if any of the client has fetched a set of numbers(specified by the
> Cache size) and closed the connection before completely using up the set
> of
> numbers fetched, then there will be unused ids. However, we can eliminate
> this by reducing the cache size to 50 or 25, based on our needs.
>
> So, the solution is not to synchronize allocation size with sequence
> increment value but to sysnchronize the allocation size with Cache size of
> the sequence.
> In this way, we can eliminate the collision of numbers as well as reduce
> the
> gaps that may be produced in case of Increment_Value Vs allocationSize
> sync..
> If you find any issues with this approach, can you please explain those?
>
> Thanks and Regards,
> Samba
>
>
> On Wed, Nov 11, 2009 at 1:28 PM, Samba <saasira@xxxxxxxxx> wrote:
>
>> Hi James,
>> Thanks for your response.
>>
>> I have a need where the same database is used by three different
>> applications: eclipselink , JDBC(legacy), and stored procedures which are
>> used mainly during bulk operations and upgrade scenarios.
>>
>> In such a scenario, I cannot have that luxury of unused id values, as
>> there
>> will be too many over a period of time and the sequence soon
>> becomes exhausted; may be I'm exaggerating when I say that this may
>> happen
>> soon but nevertheless, there will be 50 times lesser available ids,
>> provided
>> allocation size is set at 50.
>>
>> By the way, I suppose that incrementing a sequence value will be DML and
>> not DDL, after all we are not altering the name of the sequence or
>> dropping
>> it, and databases are capable of handling concurrency.
>>
>> In fact, eclipselink is behaving correctly in case of Table Sequences and
>> updating the SEQUIENCE_COUNT column by 50 when ever it prefetches id
>> values
>> by that number.
>>
>> So, why is this differential treatment in case of native Sequences?
>> shouldn't eclipselink increment the sequence value by 50 as it does in
>> case
>> of Table Sequence?
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> On Tue, Nov 10, 2009 at 9:41 PM, James Sutherland
>> <jamesssss@xxxxxxxxx>wrote:
>>
>>>
>>> The purpose of sequence preallocation is to improve performance. If you
>>> want
>>> your increment to be 1, then set your preallocation size to 1, and there
>>> will be no issues. If you want to improve performance, then you need to
>>> set
>>> your increment to the preallocation size, if this is say 50, this will
>>> not
>>> cause any issues with the other application if it assumes it is 1, ids
>>> will
>>> not collide, you will have some unused ids, but that should not be an
>>> issue.
>>>
>>> Changing DDL objects on the fly, is not a very good idea.
>>>
>>>
>>>
>>> saasira wrote:
>>> >
>>> > This following is quoted from the eclipselink User Guide :
>>> >
>>> > "The key difference between Oracle Type Sequencing process and the
>>> process
>>> > involved in table sequencing is that EclipseLink is unaware of the
>>> > INCREMENTconstruct on the
>>> > SEQUENCE object. EclipseLink sequencing and the Oracle SEQUENCE
>>> object
>>> > operate in isolation. To avoid sequencing errors in the application,
>>> set
>>> > the
>>> > EclipseLink preallocation size and the Oracle SEQUENCE object
>>> INCREMENT
>>> to
>>> > the same value. Note that the Oracle sequence object must have a
>>> starting
>>> > value equal to the preallocation size because when EclipseLink gets
>>> the
>>> > next
>>> > sequence value, it assume it has the previous preallocation size of
>>> > values."
>>> >
>>> > We have a scenario where the database is updated not only by an
>>> eclipse
>>> > link
>>> > based application but by stored procedures as well. for that matter, I
>>> > believe that we cannot make an assumption that the eclipse link is
>>> owning
>>> > the database and operates on it in isolation.
>>> >
>>> > So, if we use the increment value of the sequence to the
>>> preallocation
>>> > size
>>> > defined on the eclipse link entity definition, then there will be a
>>> lot
>>> > of
>>> > sequence values unused.
>>> > But if we set the increment value of the sequence to nominal 1, then
>>> there
>>> > is every chance of collision with the values generated by sequence and
>>> > that
>>> > generated by eclipse link.
>>> >
>>> >
>>> > Now my question is why can't the eclipse link update the sequence
>>> object
>>> > by
>>> > the preallocation size when ever it want to prefetch values? why is it
>>> > updating only by the original increment size of the sequence even
>>> though
>>> > it
>>> > is presuming it acquired the amount o values mentioned in the
>>> > preallocation
>>> > size in the entity?
>>> >
>>> > for example, had eclipse link done some thing like this:
>>> >
>>> > on Oracle:
>>> >
>>> > ALTER SEQUENCE SEQUENCE_NAME INCREMENT BY 'preallocation_size'; where
>>> ever
>>> > eclipse link wanted to prefetch the values
>>> > and then resetting it back to
>>> > ALTER SEQUENCE SEQUENCE_NAME INCREMENT BY 'original_increment_value';
>>> >
>>> >
>>> > then there can be no error at any point , who ever may update the
>>> database
>>> > from any where as long as sequence is properly used.
>>> >
>>> > Can this be achieved by any setting by which I can force elipselink to
>>> > always update the sequence value by preallocation size and not the
>>> > INCREMENT
>>> > SIZE?
>>> >
>>> > Thanks and Regards,
>>> > Samba
>>> >
>>> >
>
>
-----
http://wiki.eclipse.org/User:James.sutherland.oracle.com James Sutherland
http://www.eclipse.org/eclipselink/
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