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

Octave J. Orgeron octave.orgeron at oracle.com
Tue Jul 25 19:27:04 UTC 2017

Hi Michael,

I understand that you want to abstract this completely away inside of 
oslo.db. However, the reality is that making column changes based purely 
on the size and type of that column, without understanding what that 
column is being used for is extremely dangerous. You could end up 
clobbering a column that needs a specific length for a value, prevent an 
index from working, etc. It wouldn't make sense to just do global 
changes on a column based on the size. There are far more tables that 
fit in both InnoDB and NDB already than those that don't. As I've stated 
many times before, the columns that I make changes to are evaluated to 

1. What populates it?
2. Who consumes it?
3. What are the possible values and required lengths?
4. What is the impact of changing the size or type?
5. Evaluated against the other columns in the table, which one makes the 
most sense to adjust?

I don't see a way of automating that and making it maintainable without 
a lot more overhead in code and people. If we really want to remove the 
complexity here, why don't we just change the sizes and types on these 
handful of table columns so that they fit within both InnoDB and NDB? 
That way we don't need these functions and the tables are exactly the 
same? That would only leave us with the createtable, savepoint/rollback, 
etc. stuff to address which is already taken care of in the ndb module 
in oslo.db? Then we just fix the foreign key stuff as I've been doing, 
since it has zero impact on InnoDB deployments and if anything ensures 
things are consistent. That would then leave us to really focus on 
fixing migrations to use oslo.db and pass the correct flags, which is a 
more lengthy process than the rest of this.

I don't see the point in trying to make this stuff anymore complicated.


On 7/25/2017 12:20 PM, Michael Bayer wrote:
> On Mon, Jul 24, 2017 at 5:41 PM, Michael Bayer <mbayer at redhat.com> wrote:
>>> oslo_db.sqlalchemy.String(255, ndb_type=TINYTEXT) -> VARCHAR(255) for most
>>> dbs, TINYTEXT for ndb
>>> oslo_db.sqlalchemy.String(4096, ndb_type=TEXT) -> VARCHAR(4096) for most
>>> dbs, TEXT for ndb
>>> oslo_db.sqlalchemy.String(255, ndb_size=64) -> VARCHAR(255) on most dbs,
>>> VARCHAR(64) on ndb
>>> This way, we can override the String with TINYTEXT or TEXT or change the
>>> size for ndb.
>>>> oslo_db.sqlalchemy.String(255)     -> VARCHAR(255) on most dbs,
>>>> TINYTEXT() on ndb
>>>> oslo_db.sqlalchemy.String(255, ndb_size=64)     -> VARCHAR(255) on
>>>> most dbs, VARCHAR(64) on ndb
>>>> oslo_db.sqlalchemy.String(50)     -> VARCHAR(50) on all dbs
>>>> oslo_db.sqlalchemy.String(64)     -> VARCHAR(64) on all dbs
>>>> oslo_db.sqlalchemy.String(80)     -> VARCHAR(64) on most dbs, TINYTEXT()
>>>> on ndb
>>>> oslo_db.sqlalchemy.String(80, ndb_size=55)     -> VARCHAR(64) on most
>>>> dbs, VARCHAR(55) on ndb
>>>> don't worry about implementation, can the above declaration ->
>>>> datatype mapping work ?
>>> In my patch for Neutron, you'll see a lot of the AutoStringText() calls to
>>> replace exceptionally long String columns (4096, 8192, and larger).
>> MySQL supports large VARCHAR now, OK.   yeah this could be
>> String(8192, ndb_type=TEXT) as well.
> OK, no, sorry each time I think of this I keep seeing the verbosity of
> imports etc. in the code, because if we had:
> String(80, ndb_type=TEXT)
> then we have to import both String and TEXT, and then what if there's
> ndb.TEXT, the code is still making an ndb-specific decision, etc.
> I still see that this can be mostly automated from a simple ruleset
> based on the size:
> length <= 64 :    VARCHAR(length) on all backends
> length > 64, length <= 255:   VARCHAR(length) for most backends,
> TINYTEXT for ndb
> length > 4096:  VARCHAR(length) for most backends, TEXT for ndb
> the one case that seems outside of this is:
> String(255)  where they have an index or key on the VARCHAR, and in
> fact they only need < 64 characters to be indexed.  In that case you
> don't want to use TINYTEXT, right?   So one exception:
> oslo_db.sqlalchemy.types.String(255, indexable=True)
> e.g. a declarative hint to the oslo_db backend to not use a LOB type.
> then we just need oslo_db.sqlalchemy.types.String, and virtually
> nothing except the import has to change, and a few keywords.
> What we're trying to do in oslo_db is as much as possible state the
> intent of a structure or datatype declaratively, and leave as much of
> the implementation up to oslo_db itself.
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