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IDS Forum

Re: Isolation level on a connection pool data ....

Posted By: Art Kagel
Date: Wednesday, 16 May 2012, at 2:00 p.m.

In Response To: Re: Isolation level on a connection pool data .... (Laird Nelson)

The engine doesn't know that session B doesn't want to see the rows
inserted or updated by session A. Maybe it does but A is taking longer
than usual and shouldn't have started yet. The engine has no way to know.
When Informix was designed, indeed when most RDBMS's were designed back in
the '80s and '90s, the rational went that way. Everyone did it that way
for a long time, including Oracle. Many still do, including Informix.
Versioning began with Interbase in the mid-90's and took off infecting
Oracle and several others.

There are some problems with using versioning for concurrency control which
are not technical but philosophical. Application programmers who only know
versioning databases get lazy. They assume that a row that a session is
working on will never be modified by another user, that rows that they have
read at the beginning of a transaction will never be deleted before they
commit, that no new rows that should interest their transaction will ever
be inserted during their transaction. In many organizations, due to work
flow realities, this is ALMOST ALWAYS true. It's the almost that causes
problems and can result in inconsistent data and balances that don't
balance. That is the likely reason that Informix resisted going to
versioning for all of these years.

The LAST COMMITTED option was added about 3 years ago. It works great to
reduce contention and improve concurrency, but applications still have to
be properly written to avoid inconsistent data. In your case, since
session A is only inserting new rows, and sessions B-Z.... are only
operating on existing rows that I assume are independent of those being
inserted, LAST COMMITTED will work well. If there were dependencies, you
would have to stay with the default isolation model to insure consistency
because of the very long/large transaction.

Art

Art S. Kagel
Advanced DataTools (www.advancedatatools.com)
Blog: http://informix-myview.blogspot.com/

Disclaimer: Please keep in mind that my own opinions are my own opinions
and do not reflect on my employer, Advanced DataTools, the IIUG, nor any
other organization with which I am associated either explicitly,
implicitly, or by inference. Neither do those opinions reflect those of
other individuals affiliated with any entity with which I am affiliated nor
those of the entities themselves.

On Wed, May 16, 2012 at 1:44 PM, Laird Nelson <ljnelson@gmail.com> wrote:

> On Wed, May 16, 2012 at 1:35 PM, Art Kagel <art.kagel@gmail.com> wrote:
>
> > Informix users [sic] locking to enforce data isolation by default
> (whereas
> > Oracle
> > uses row versioning). If you are inserting many rows in a single
> > transaction then those rows will all be locked (or if the table's lock
> mode
> > is page then all of the pages on which those new rows reside will be
> > locked) for the duration of the transaction.
>
> Yes, OK, got that. (And thank you for bearing with me on all this.)
>
> So another transaction (B) comes along and does a SELECT * FROM
>
>
> the_table_where_a_huge_batch_job_is_currently_inserting_records_in_transaction_A.
> I understand that the **rows** that are in the middle of being inserted by
> transaction A should not be seen by transaction B while A is in progress.
> That is just what it means to be uncommitted--if B is any kind of
> COMMITTED READ then there should be no way it can see A's data. I get that.
>
> What I don't get is: why would A's **locks** on those rows get in B's way?
> Under what scenario would this *ever* be what you wanted? B shouldn't
> even see A's locks or be aware of A's existence, right? In my (thick)
> head, B being able to "see" (and hence be screwed up by) A's locks is
> weird. Why wouldn't it just say, oh, hey, these rows are locked by someone
> else doing something transactional that I shouldn't see anyway, so I'll
> just skip them (which is the LAST COMMITTED option behavior)? Why would B
> *ever* want to bomb out because A happens to be inserting rows that B
> shouldn't ever see anyway?
>
> > If they are singleton inserts
> > then the locks will be transitory. Either way, normally in an app
> > accessing Informix, you will issue a "SET LOCK MODE TO WAIT <nseconds>;"
> so
> > that transitory locks do not cause errors.
>
> Sure; in our case this is a huge batch insert process that must all
> complete in one transaction, so we'd be setting a WAIT value of {insert
> ginormous number here} which is impractical.
>
> > This improves concurrency
> > without reducing isolation. On the other hand, if you also set LAST
> > COMMITTED option, then, more like Oracle's versioning, Informix will
> ignore
> > the locks but return the committed version of any locked row.
> >
>
> OK, good, so this is indeed the approach we want. I'm still (thickly)
> trying to come up with some rationale for the default behavior though; I
> just can't see it. I'm sure it's there but a patient explanation of why it
> would be a good idea would be humbly welcomed.
>
> > Best,
> Laird
>
> --
> http://about.me/lairdnelson
>
> --f46d043d67e51765dd04c02ae070
>
>
>
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.
>
>

--047d7b15fe0b3f027404c02b18b8

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