Hi Mike,
I've had a chance to look through the links you provided. I do think
this is a rather heavy solution that would be more suited if there were
actually significant dialect features to override from MySQL. MySQL and
NDB use the same dialect and the differences really just come down to
operation ordering, no support for savepoints, and no support for nested
transactions. Even if you tried to do those operations today, SQL
Alchemy is able to throw back appropriate errors telling you that you're
doing something wrong or that the feature isn't supported. If we go down
this path, we really only buy two things:
* Ability to use the with_variant for setting column types.
* Do some logic based on the selected dialect, which we would probably
still have to set in oslo.db anyways as the hook.
It doesn't solve the issue of proper ordering of FK, constraints, or
index operations. It doesn't remove the need to do variable
substitutions where things are hard coded. And it doesn't resolve the
issues where we have to intercept savepoints and nested transactions. It
looks like the only major impact it would have is to reduce the number
of if/then logic blocks in the SQL Alchemy and Alembic migration scripts.
But what does it cost to do this? Would the dialect be rolled into SQL
Alchemy for the community, or would it be a separate plugin like
Redshifts? Is it easier to maintain just the patches? Or would it mean
more overhead for me to support the patches and the ndb dialect? I'd
like to keep the overhead simple since it's just me at this point
working on this.
So what I propose is that I'll update my patches for keystone and cinder
next and post those for gerrit review. That will give folks a view into
what the patches will look like and we can figure out if we want to
change the approach. I'm also going to create a spec and blueprint to
cover the changes across the services. I'll post links once all of that
is up for review.
Thanks,
Octave
On 2/6/2017 7:53 AM, Mike Bayer wrote:
On 02/03/2017 11:59 AM, Octave J. Orgeron wrote:
Hi Mike,
Comments below..
Thanks,
Octave
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
want:
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
https://github.com/sqlalchemy-redshift/sqlalchemy-redshift/blob/master/setup.py#L40
,
https://github.com/sqlalchemy-redshift/sqlalchemy-redshift/blob/master/sqlalchemy_redshift/dialect.py#L315)
and for Alembic, once the dialect is imported you define a special
Alembic class so that Alembic sees the engine name also (see
https://github.com/sqlalchemy-redshift/sqlalchemy-redshift/blob/master/sqlalchemy_redshift/dialect.py#L19).
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.
--
Oracle <http://www.oracle.com/>
Octave J. Orgeron | Sr. Principal Architect and Software Engineer
Oracle Linux OpenStack
Mobile: +1-720-616-1550 <tel:+17206161550>
500 Eldorado Blvd. | Broomfield, CO 80021
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<http://www.oracle.com/us/solutions/enterprise-architecture/index.html>
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Octave J. Orgeron | Sr. Principal Architect and Software Engineer
Oracle Linux OpenStack
Mobile: +1-720-616-1550 <tel:+17206161550>
500 Eldorado Blvd. | Broomfield, CO 80021
Certified Oracle Enterprise Architect: Systems Infrastructure
<http://www.oracle.com/us/solutions/enterprise-architecture/index.html>
Green Oracle <http://www.oracle.com/commitment> Oracle is committed to
developing practices and products that help protect the environment
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