[openstack-dev] [oslo.db] [ndb] ndb namespace throughout openstack projects
Octave J. Orgeron
octave.orgeron at oracle.com
Mon Jul 24 19:37:39 UTC 2017
Thanks for putting this together. Comments below..
On 7/23/2017 3:02 PM, Michael Bayer wrote:
> 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  to support this, and
> there are now a bunch of proposals such as   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
I think this would make sense.
> 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.
Agreed that this extra layer of abstraction could be used by DB2,
> 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"?)
For these, here is a brief synopsis:
AutoStringTinyText, will convert a column to the TinyText type. This is
used for cases where a 255 varchar string needs to be converted to a
text blob to make the row fit within the NDB limits. If you are using
ndb, it'll convert it to TinyText, otherwise it leaves it alone. The
reason that TinyText type was chosen is because it'll hold the same 255
varchars and saves on space.
AutoStringText, does the same as the above, but converts the type to
Text and is meant for use cases where you need more than 255 varchar
worth of space. Good examples of these uses are where outputs of
hypervisor and OVS commands are dumped into the database.
AutoStringSize, you pass two parameters, one being the non-NDB size and
the second being the NDB size. The point here is where you need to
reduce the size of the column to fit within the NDB limits, but you want
to preserve the String varchar type because it might be used in a key,
index, etc. I only use these in cases where the impacts are very low..
for example where a column is used for keeping track of status (up,
down, active, inactive, etc.) that don't require 255 varchars.
In many cases, the use of these could be removed by simply changing the
columns to more appropriate types and sizes. There is a tremendous
amount of wasted space in many of the databases. I'm more than willing
to help out with this if teams decide they would rather do that instead
as the long-term solution. Until then, these functions enable the use of
both with minimal impact.
Another thing to keep in mind is that the only services that I've had to
adjust column sizes for are:
The other services that I'm working on like Keystone, Barbican, Murano,
Glance, etc. only need changes to:
1. Ensure that foreign keys are dropped and created in the correct order
when changing things like indexes, constraints, etc. Many services do
these proper steps already, there are just cases where this has been
missed because InnoDB is very forgiving on this. But other databases are
2. Fixing the database migration and sync operations to use oslo.db,
pass the right parameters, etc. Something that should have been done in
the first place, but hasn't. So this more of a house cleaning step to
insure that services are using oslo.db correctly.
The only other oddball use case is deal with disabling nested
transactions, where Neutron is the only one that does this.
On the flip side, here is a short list of services that I haven't had to
make ANY changes for other than having oslo.db 4.24 or above:
> 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
One has to look at what the column is being used for and decide what
appropriate remediation steps are. This takes time and one must research
what kind of data goes in the column, what puts it there, what consumes
it, and what remediation would have the least amount of impact.
> 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.
>  https://review.openstack.org/#/c/427970/
>  https://review.openstack.org/#/c/446643/
>  https://review.openstack.org/#/c/446136/
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