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[Aaron Watters]
> BTW, I would like to see somewhere an explanation of how
> ZODB concurrency control and recovery work. Please inform if
> there is some information. I'm not encouraged by the fact that
> in what documentation I can find the definitions are not the
> standard ones.

Of course you're much more likely to get a useful discussion of this on a
ZODB list, like <mailto:zodb->.

"The standard" ones aren't all that standard. ANSI SQL-92 defines multiple
isolation levels, and they've been (I think) fairly critiqued as incomplete
and partly ambiguous; e.g.,

A Critique of ANSI SQL Isolation Levels
Hal Berenson, et al.
http://citeseer.nj.nec.com/berenson95critique.html

To complicate matters, as Jeremy says here:

http://www.python.org/~jeremy/weblog/030514.html

ZODB uses optimistic concurrency control. It can be hard to
describe what isolation guarantees it provides, because optimistic
implementations don't map naturally onto the ANSI isolation levels.

Jeremy's blog there also contains links to more recent papers.

To complicate matters more (in theory, but nicer in practice for most uses),
ZODB 3.3 introduces so-called Multiversion Concurrency Control as default
behavior:

http://zope.org/Wikis/ZODB/MultiVers...urrencyControl

"Tim Peters" <> wrote in message news:<mailman.284.1073847641.12720.python->...
> [Aaron Watters]
> > BTW, I would like to see somewhere an explanation of how
> > ZODB concurrency control and recovery work. Please inform if
> > there is some information. I'm not encouraged by the fact that
> > in what documentation I can find the definitions are not the
> > standard ones.
>
> Of course you're much more likely to get a useful discussion of this on a
> ZODB list, like <mailto:zodb->.
>
> "The standard" ones aren't all that standard. ANSI SQL-92 defines multiple
> isolation levels, and they've been (I think) fairly critiqued as incomplete
> and partly ambiguous; e.g.,
>
> A Critique of ANSI SQL Isolation Levels
> Hal Berenson, et al.
> http://citeseer.nj.nec.com/berenson95critique.html

Some SQL isolation levels are hacks to allow long running transactions,
etcetera. If you keep to the strictest isolation level you get the
classical behaviour which has been studied and elaborated by many very
smart people over the last several decades and which is very well
understood.

Does ZODB support the strictest isolation levels? If so how? If not
what does it support exactly?
-- Aaron Watters
===
We'll make our way across the galaxy
And head back home on the L.I.E.
-- "Laser show", Fountains of Wayne

On Sun, 2004-01-11 at 20:12, Aaron Watters wrote:
> > "The standard" ones aren't all that standard. ANSI SQL-92 defines multiple
> > isolation levels, and they've been (I think) fairly critiqued as incomplete
> > and partly ambiguous; e.g.,
> >
> > A Critique of ANSI SQL Isolation Levels
> > Hal Berenson, et al.
> > http://citeseer.nj.nec.com/berenson95critique.html
>
> Some SQL isolation levels are hacks to allow long running transactions,
> etcetera. If you keep to the strictest isolation level you get the
> classical behaviour which has been studied and elaborated by many very
> smart people over the last several decades and which is very well
> understood.

I think lower isolation levels are more than hacks for long-running
transactions. They seem like they're pretty important for achieving
good performance. The strictest isolation level is necessary, of
course, for some applications, but not all.

Here's was PostgreSQL has to say about full serializability.
http://www.postgresql.org/docs/curre...ction-iso.html

> The Serializable mode provides a rigorous guarantee that each
> transaction sees a wholly consistent view of the database. However,
> the application has to be prepared to retry transactions when
> concurrent updates make it impossible to sustain the illusion of
> serial execution. Since the cost of redoing complex transactions may
> be significant, this mode is recommended only when updating
> transactions contain logic sufficiently complex that they may give
> wrong answers in Read Committed mode. Most commonly, Serializable mode
> is necessary when a transaction executes several successive commands
> that must see identical views of the database.

Jeremy