[openstack-dev] [oslo.db] [ndb] ndb namespace throughout openstack projects

Michael Bayer mbayer at redhat.com
Sun Jul 23 21:02:31 UTC 2017

I've been working with Octave Oregon in assisting with new rules and
datatypes that would allow projects to support the NDB storage engine
with MySQL.

To that end, we've made changes to oslo.db in [1] to support this, and
there are now a bunch of proposals such as [2] [3] to implement new
ndb-specific structures in projects.

The reviews for all downstream projects except Cinder are still under
review. While we have a chance to avoid a future naming problem, I am
making the following proposal:

Rather than having all the projects make use of
oslo_db.sqlalchemy.ndb.AutoStringTinyText / AutoStringSize, we add new
generic types to oslo.db :


(or similar )

Internally, the ndb module would be mapping its implementation for
AutoStringTinyText and AutoStringSize to these types.   Functionality
would be identical, just the naming convention exported to downstream
consuming projects would no longer refer to "ndb.<name>" for

Reasons for doing so include:

1. openstack projects should be relying upon oslo.db to make the best
decisions for any given database backend, hardcoding as few
database-specific details as possible.   While it's unavoidable that
migration files will have some "if ndb:" kinds of blocks, for the
datatypes themselves, the "ndb." namespace defeats extensibility.  if
IBM wanted Openstack to run on DB2 (again?) and wanted to add a
"db2.String" implementation to oslo.db for example, the naming and
datatypes would need to be opened up as above in any case;  might as
well make the change now before the patch sets are merged.

2. The names "AutoStringTinyText" and "AutoStringSize" themselves are
confusing and inconsistent w/ each other (e.g. what is "auto"?  one is
"auto" if its String or TinyText and the other is "auto" if its
String, and..."size"?)

3. it's not clear (I don't even know right now by looking at these
reviews) when one would use "AutoStringTinyText" or "AutoStringSize".
For example in https://review.openstack.org/#/c/446643/10/nova/db/sqlalchemy/migrate_repo/versions/216_havana.py
I see a list of String(255)'s changed to one type or the other without
any clear notion why one would use one or the other.  Having names
that define simply the declared nature of the type would be most

I can add these names up to oslo.db and then we would just need to
spread these out through all the open ndb reviews and then also patch
up Cinder which seems to be the only ndb implementation that's been
merged so far.

Keep in mind this is really me trying to correct my own mistake, as I
helped design and approved of the original approach here where
projects would be consuming against the "ndb." namespace.  However,
after seeing it in reviews how prevalent the use of this extremely
backend-specific name is, I think the use of the name should be much
less frequent throughout projects and only surrounding logic that is
purely to do with the ndb backend and no others.   At the datatype
level, the chance of future naming conflicts is very high and we
should fix this mistake (my mistake) before it gets committed
throughout many downstream projects.

[1] https://review.openstack.org/#/c/427970/

[2] https://review.openstack.org/#/c/446643/

[3] https://review.openstack.org/#/c/446136/

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