db-derby-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Rick Hillegas (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-5697) Doc changes to account for correctness fixes for sequences
Date Mon, 23 Apr 2012 12:17:36 GMT

    [ https://issues.apache.org/jira/browse/DERBY-5697?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13259562#comment-13259562

Rick Hillegas commented on DERBY-5697:

Hi Kim,

Adjusting derby.language.sequence.preallocator upward may boost your concurrency. At some
point, you will have adjusted it high enough that further upward tweaks won't help your application.
At that point, your throughput will be constrained by other bottlenecks.

I don't see how adjusting derby.language.sequence.preallocator downward would ever reduce
your concurrency. The reason to lower it would be to leak fewer unused preallocated values.
That could be an issue if your application can't guarantee that it will park its databases
gracefully before the VM exits. Thanks.
> Doc changes to account for correctness fixes for sequences
> ----------------------------------------------------------
>                 Key: DERBY-5697
>                 URL: https://issues.apache.org/jira/browse/DERBY-5697
>             Project: Derby
>          Issue Type: Improvement
>          Components: Documentation
>    Affects Versions:
>            Reporter: Rick Hillegas
>            Assignee: Kim Haase
> As a result of the correctness fixes made by DERBY-5493 and DERBY-5494, we should adjust
the user documentation for sequences. I recommend the following changes:
> 1) Reference Guide
> a) Add a section for a new system function. This function lets users observe the instantaneous
current value of a sequence generator without having to query SYSSEQUENCES. Querying SYSSEQUENCES
didn't actually return the current value, it only returned an upper bound on that value, viz.,
the end of the chunk of sequence values which had been pre-allocated but not actually used.
The new function shows you the very next value which will be returned by a NEXT VALUE FOR
clause. Users should never directly query SYSSEQUENCES because that will cause sequence generator
concurrency to slow down to a crawl.
> Syntax
> As with all system functions, schema and object name arguments are case-sensitive.
> Execute privileges
> By default, all users have execute privileges on this function.
> b) SYSSEQUENCES system table
> The Contents description for SYSSEQUENCES.CURRENTVALUE needs to be changed to make a
couple points: This is not the actual next value for the sequence generator. That value can
be obtained by calling SYSCS_PEEK_AT_SEQUENCE(). SYSSEQUENCES.CURRENTVALUE holds the end of
the range of values which have been pre-allocated in order to boost concurrency. Users should
not directly query SYSSEQUENCES because that will slow down the performance of sequence generators.
Instead, users should call SYSCS_PEEK_AT_SEQUENCE().
> c) derby.language.sequence.preallocator
> The default value of this property has changed from 20 to 100.
> In addition, I think that we should point out that if the database is shut down gracefully,
then Derby should not leak unused, preallocated values; they will be thrown away and the sequence
generator will pick up where it left off, once the database re-boots. However, if the database
comes down ungracefully, then unused, preallocated values will be leaked: when the database
comes up again, the sequence generator will skip the unused, preallocated values; this will
leave a gap between the last NEXT VALUE FOR (issued before the database came down ungracefully)
and the next NEXT VALUE FOR (issued after the database re-boots).
> 2) Tuning Guide
> Maybe we should add a small section explaining how to boost the concurrency of sequences:
> a) Don't query SYSSEQUENCES directly. Instead, use SYSCS_PEEK_AT_SEQUENCE().
> b) Adjusting derby.language.sequence.preallocator may boost your concurrency. However,
you should pay special attention to bringing down the database gracefully in order to not
leak unused, preallocated values. See the Reference Manual section on derby.language.sequence.preallocator.

This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators: https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa
For more information on JIRA, see: http://www.atlassian.com/software/jira


View raw message