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Re: MariaDB 10.0.14 got rid of replication safety warning without warning in release notes?


As I have reports of people not seeing this email on maria-developers@
list (probably because Monty sent it from the wrong address) I'm
forwarding it to the list. I'll respond to it in detail later.

On Fri, Oct 17, 2014 at 1:43 PM, Michael Widenius
<michael.widenius@xxxxxxxxx> wrote:
> Hi!
> First sorry for the delayed reply;  I am at a conference in Oxford and
> didn't have an reliable internet connection until now.
>>>>>> "Pavel" == Pavel Ivanov <pivanof@xxxxxxxxxx> writes:
> Pavel> Hi,
> Pavel> I see that in MariaDB 10.0.14 Michael disabled replication safety
> Pavel> warning for the case of INSERT ON DUPLICATE KEY UPDATE with several
> Pavel> unique keys with a comment "We should assume that the store engine
> Pavel> will report the first duplicate key for this case."
> Pavel> http://bazaar.launchpad.net/~maria-captains/maria/10.0/revision/4400.1.3
> Pavel> What does that mean?
> That we don't anymore give a warning for things that can't happen in
> the distributed MariaDB server.
> Pabel> Which key is "the first duplicate key".
> The first key that has a duplicate value in the order of keys in the KEY
> structure that MariaDB provides for the storage engine.
> This is the same order that keys are presented in SHOW CREATE TABLE
> Pavel> and why
> Pavel> MariaDB relies on an engine behavior which AFAIK is not guaranteed?
> This is guaranteed for all storage engines shipped with MariaDB and
> should also be guaranteed for any storage engine that wants to follow
> the MariaDB and even the original MySQL storage engine interface.
> The change is also in line with how I designed the part of the storage
> engine interface that gives the SQL layer the information about which
> key that was conflicted.  As we use this for things like INSERT ... ON
> DUPLICATE KEY, it's critical that all storage engine provides the same
> information in case of conflicts.
> To help engine writes we are constantly adding new test in the storage
> engine test suite, to make it easier for storage engine vendors to
> test that their storage engine follows the interface.
> We have now added a test to check for the correct behaviour for the
> What is important when it comes to the storage engine interface is
> that in all possible cases, a query should work identically against 2
> different storage engines.
> The only 'allowed difference's that I can think of for the moment are:
> - Different storage engines may retrieve rows in different order.
> - In case of errors, changes to non transactional tables will not roll
>   back.
> - Automatically generated key values may be different between storage engines.
>   (but preferably they should not be that). However when things are
>   replicated the slave, it should get the same values as the master.
> In the case of INSERT ... ON DUPLICATE KEY, it was clear that someone
> had broke the storage engine interface, for no good reason.  Instead
> of users having to depend on 'unsafe, different and undocumented
> behavior' for which row as updated, it's better to fix this issue once
> and for all, especially as fixing this is trivial.
> Do you have or use a storage engine that doesn't provide the expected value
> for the conflicting key ?
> If yes, there is a couple of ways to fix this. I am happy to talk with
> you to find a solution that will satisfy your needs and ours.
> I underatand that in many cases it's not critical which unique key the
> storage engine reports as duplicate. However in some cases, it's
> critical to get the first key.
> I could add a new extra call to inform the engine that we now require
> the first duplicate key and not just any conflicting key.  This would
> be used for example for INSERT ... ON DUPLICATE KEY ...
> I would prefer to have the check for which key was duplicated in the
> storage engine.  However, if critically needed, and assuming there is more
> than one storage engine that needs this, we could add a check on the
> handler level that will do a lookup on all unique keys to find the
> first duplicate one.
> What solution would you prefer?
> Pavel> And why this serious change in behavior wasn't mentioned in the
> Pavel> Release Notes?
> I didn't see it as a a serious change as it's only about removing a
> warning that has no value for any normal MariaDB user (who only use
> storage engines supplied with MariadB) and which actually caused
> serious problems for some MariaDB users.
> That said, I agree it was a mistake from my part to not notice that
> this was missing in the release notes.  I will try to do better in the
> future and check that the release notes correctly reflects any changes
> that I have done.
> I appologize for missing this in this case.
> Pavel> On the related note Release Notes mention the error log flood
> Pavel> protection. Why it's not behind flag? How should I disable the feature
> Pavel> and get back to the old behavior?
> We already had in MariaDB flood protection for some error
> messages. What I did was to make it more general and extend it to also
> include other error messages, like the one we are discussing.  As
> there wasn't a flag for the old suppression, I didn't see a need for
> adding a flag for the new suppression.
> I am happy to look into adding a flag that is usable for any kind of
> suppressed messages.
> A couple of ways to do this:
> a) Not suppress anything if you give --log-warnings a value > 2
>    variable, where 0 would mean no suppression.
> Which of the above, or both, would you prefer?
> Or do you have any other suggestion of how to do this?
> For 10.1 we will probably add a mechanism where you can permanently
> suppress any specific error message to be written to the error log.
> (They would still be given to the client).
> Any suggestion or wishes regarding this?
> Regards,
> Monty