[openstack-dev] [oslo][oslo.db] MySQL Cluster support
mbayer at redhat.com
Mon Feb 6 14:53:38 UTC 2017
On 02/03/2017 11:59 AM, Octave J. Orgeron wrote:
> Hi Mike,
> Comments below..
> On 2/3/2017 7:41 AM, Mike Bayer wrote:
>> On 02/02/2017 05:28 PM, Octave J. Orgeron wrote:
>>> That refers to the total length of the row. InnoDB has a limit of 65k
>>> and NDB is limited to 14k.
>>> A simple example would be the volumes table in Cinder where the row
>>> length goes beyond 14k. So in the IF logic block, I change columns types
>>> that are vastly oversized such as status and attach_status, which by
>>> default are 255 chars.
>> let me give you a tip on IF blocks, that they are a bit of an
>> anti-pattern. If you want a column type to do one thing in one case,
>> and another in another case, create an object that does the thing you
>> some_table = Table(
>> 'some_table', metadata,
>> Column('my_column', VARCHAR(255).with_variant(VARCHAR(50), 'ndb'))
>> I think we might want to look into creating a stub dialect called
>> 'ndb' that subclasses mysql+pymysql. Treating ndb as a whole
>> different database means there's no longer the need for a flag in
>> oslo.db, the 'ndb' name would instead be interpreted as a new backend
>> - the main thing would be ensuring all the mysql-appropriate hooks in
>> oslo.db are also emitted for ndb, but this also gives us a way to pick
>> and choose which hooks apply. It seems like there may be enough
>> different about it to separate it at this level.
>> Not sure if people on the list are seeing that we are simultaneously
>> talking about getting rid of Postgresql in the efforts to support only
>> "one database", while at the same time adding one that is in many ways
>> a new database.
> This is an interesting approach as it would significantly reduce the
> amount of code in my patches today. Do you have any pointers on where
> this should be implemented as a stub? Would we have to take different
> approaches for SQL Alchemy vs. Alembic?
there are simple plugin points for both libraries.
One of the popular 3rd party dialects right now is the
sqlalchemy-redshift dialect, which similarly to a lot of these dialects
is one that acts 95% like a "normal" dialect, in this case postgresql,
however various elements are overridden to provide compatibility with
Amazon's redshift. The overlay of an NDB style dialect on top of
mysql would be a similar idea. The SQLAlchemy plugin point consists
of a setuptools entrypoint (see
and for Alembic, once the dialect is imported you define a special
Alembic class so that Alembic sees the engine name also (see
In this case the NDB dialect seems like it may be a little bit of a
heavy solution but it would solve lots of issues like the "mysql_engine"
flag would no longer be in conflict, special datatypes and naming
schemes can be pulled in, etc. It would at least allow conditionals
against "ndb" in Openstack projects to switch on the same kind of
criteria that they already do for sqlite/postgresql/mysql.
It is possible for the ndb "stub dialect" to be at least temporarily
within oslo.db, however the way to go about this would be to start
getting ndb working as a proof of concept in terms of gerrit reviews.
that is, propose reviews to multiple projects and work at that level,
without actually merging anything. We don't merge anything until it's
actually "done" as a tested and working feature / fix.
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