Fwd: MariaDB replacing MySQL

[Honza Horak]

On 03/13/2013 09:27 AM, Norvald H. Ryeng wrote:
> On Tue, 12 Mar 2013 18:25:02 +0100, Honza Horak <hhorak at redhat.com> wrote:
>
>> On 03/12/2013 01:14 PM, Norvald H. Ryeng wrote:
>>> On Mon, 11 Mar 2013 19:58:03 +0100, Kevin Kofler
>>> <kevin.kofler at chello.at> wrote:
>>>
>>>> Honza Horak wrote:
>>>>> This doesn't solve all the issues -- if package like akonadi-mysql
>>>>> says
>>>>> "Requires: mysql-server", then Oracle MySQL either wouldn't satisfy
>>>>> that
>>>>> requirement or (in case it includes "Provides: mysql-server") RPM
>>>>> choosing behavior would be ambiguous.
>>>>
>>>> And it should not satisfy it.
>>>>
>>>> We now changed the Requires in akonadi-mysql to mariadb-server to be
>>>> sure of what we get.
>>>
>>> This dependency is a problem. It makes it impossible to install
>>> MySQL-server on a KDE system since mariadb-server and MySQL-server
>>> conflict.
>>
>> I don't think conflict is actually the main problem -- the inability
>> of RPM to un-ambigously choose one of the two packages that provide
>> the same symbol *is* the real problem. If we solved that one,
>> MySQL-server could provide right symbol and KDE system would be happy.
>
> I fully agree that enforcing the default is the main problem. It makes
> the whole ting very difficult.

I've spent some time deep in yum and it seems to be better than I 
thought now. First, the magic about choosing one provider from more 
alternatives is not so dark any more (it was worse few years before) -- 
it's actually documented at [1] now.

However, in scenarios I tested with packages similar to 
mysql/MySQL/mariadb it turned out, that we never reach the point where 
we have to choose one of more alternate providers. The reason is that 
yum chooses right package before going down to [1] (I tracked the code 
and it never came to the part _compare_providers).

I've tested that on sample packages in one repo, that basically looked 
like this:

mysql-5.5.30
   #last version of the package and also package currently installed

mariadb-5.5.29:
   Provides: mysql = 5.5.29
   Obsoletes: mysql < 5.6

MySQL-5.6.10:
   Provides: mysql = 5.6.10
   # doesn't obsolete mysql

The following table shows what actions (cols) yum chose in different 
scenarios -- i.e packages installed (rows):

installed | # yum install mysql | # yum update | # yum shell (*) |
------------------------------------------------------------------
   ---     | mariadb (**)        | ---          | MySQL           |
   mysql   | mariadb             | mariadb      | MySQL           |
   MySQL   | mariadb             | MySQL        | MySQL           |
   mariadb | ---                 | mariadb      | MySQL           |

(*) "yum shell" is needed in order to install MySQL while removing 
current implementation if any (mysql or mariadb) in one transaction.

(**) The reason why mariadb is chosen is most probably this notice 
printed by yum:
"Package mysql is obsoleted by mariadb, trying to install 
mariadb-5.5.29-2.fc18.x86_64 instead"

This means it works as we'd wish even if we let MySQL packages to 
provide mysql symbols. I'll test the real packages after weekend, since 
I'm going to be off until Sunday.

So, to sum up the above, I've started to believe that we will be able to 
add "Provides: mysql" also to MySQL packages, while mariadb would be 
correctly preferred by default.

[1] http://yum.baseurl.org/wiki/CompareProviders

Regards,
Honza