From: Ian Jackson <ian.jackson@eu.citrix.com>
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Message-ID: <22609.32147.935746.463570@mariner.uk.xensource.com>
Date: Wed, 14 Dec 2016 17:12:51 +0000
To: Kevin Grittner <kgrittn@gmail.com>
CC: Robert Haas <robertmhaas@gmail.com>, Alvaro Herrera
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Subject: Re: [OSSTEST PATCH 0/1] PostgreSQL db: Retry on constraint
 violation [and 2 more messages] [and 1 more messages]
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Kevin Grittner writes ("Re: [HACKERS] [OSSTEST PATCH 0/1] PostgreSQL db: Retry on constraint violation [and 2 more messages] [and 1 more messages]"):
> On Wed, Dec 14, 2016 at 10:20 AM, Ian Jackson <ian.jackson@eu.citrix.com> wrote:
> 
> > Let me try to summarise my understanding of what the developers think
> > they can and intend to promise, about SERIALIZABLE transactions:
> >
> >  There is a consistent serialisation of all transactions which
> >  successfully commit; or which do not attempt to make any changes.
> >
> >  A "consistent serialisation" means that there is an order in which
> >  the same transactions might have been executed giving exactly the
> >  answers.  This includes, if applicable, the same errors.  (The
> >  database is free to generate synchronisation failure errors 40P01 and
> >  40001 whenever it chooses.)
> 
> I would alter that slightly to:
> 
> There is a consistent serialization of all serializable
> transactions which successfully commit.

Here `serializable' means SERIALIZABLE ?

> >  A transaction which attempts to make any changes, and which does not
> >  commit (whether because the application never asks for COMMIT, or
> >  because of reported synchronisation failure) might see internally
> >  inconsistent data, or an internally-consistent view which is not
> >  compatible with any serialisation of other transactions.  An
> >  application which needs a coherent view should not rely on any of the
> >  information from such a transaction.
> 
> Even a read-only transaction can see a state that is not consistent
> with business rules (as enforced in the software) given that some
> particular later state is reached.
> 
> For examples, please see this Wiki page.  You might be particularly
> interested in the examples in the "Read Only Transactions" section:
> 
> https://wiki.postgresql.org/wiki/SSI

Thanks.  I read that part of the wiki page.  But in that example, we
are told that T1 will be aborted, not T3.

Can it happen that a transaction which does not make any update
attempts, will see "impossible" data, and that this is only detected
at COMMIT ?  Does that apply even to READ ONLY transactions ?

> >  Serialisation failures in subtransactions might cause the parent
> >  transaction to experience a serialisation failure too.
> 
> There is currently at least one bug

Right.  I was trying to capture the intent, modulo bugs.

> Once a serialization failure occurs the transaction is flagged as
> "doomed" and will not, under any circumstances be allowed to
> commit.  If you find any exception to that, please report it as a
> bug.

Right.  I think this prevents any exception-catching arrangements from
suppressing the serialisation failure.  Since AIUI it is not possible
to run the outer COMMIT from within an exception trapping context.

If it /is/ possible to run that outer COMMIT in a way which swallows
the exception then this is not a practical problem but the wording
ought to be changed to refer to the success of the COMMIT statement.

Ian.


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