[openstack-dev] [oslo][oslo.db] MySQL Cluster support

Octave J. Orgeron octave.orgeron at oracle.com
Thu Feb 2 19:16:15 UTC 2017

Hi Doug,

Comments below..


On 2/2/2017 11:27 AM, Doug Hellmann wrote:
> Excerpts from Octave J. Orgeron's message of 2017-02-02 09:40:23 -0700:
>> Hi Doug,
>> One could try to detect the default engine. However, in MySQL Cluster,
>> you can support multiple storage engines. Only NDB is fully clustered
>> and replicated, so if you accidentally set a table to be InnoDB it won't
>> be replicated . So it makes more sense for the operator to be explicit
>> on which engine they want to use.
> I think this change is probably a bigger scale item than I understood
> it to be when you originally contacted me off-list for advice about
> how to get started. I hope I haven't steered you too far wrong, but
> at least the conversation is started.
> As someone (Mike?) pointed out on the review, the option by itself
> doesn't do much of anything, now. Before we add it, I think we'll
> want to see some more detail about how it's going used. It may be
> easier to have that broader conversation here on email than on the
> patch currently up for review.

Understood, it's a complicated topic since it involves gritty details in 
SQL Alchemy and Alembic that are masked from end-users and operators 
alike. Figuring out how to make this work did take some time on my part.

> It sounds like part of the plan is to use the configuration setting
> to control how the migration scripts create tables. How will that
> work? Does each migration need custom logic, or can we build helpers
> into oslo.db somehow? Or will the option be passed to the database
> to change its behavior transparently?

These are good questions. For each service, when the db sync or db 
manage operation is done it will call into SQL Alchemy or Alembic 
depending on the methods used by the given service. For example, most 
use SQL Alchemy, but there are services like Ironic and Neutron that use 
Alembic. It is within these scripts under the <service>/db/* hierarchy 
that the logic exist today to configure the database schema for any 
given service. Both approaches will look at the schema version in the 
database to determine where to start the create, upgrade, heal, etc. 
operations. What my patches do is that in the scripts where a table 
needs to be modified, there will be custom IF/THEN logic to check the 
cfg.CONF.database.mysql_storage_engine setting to make the required 
modifications. There are also use cases where the api.py or model(s).py 
under the <service>/db/ hierarchy needs to look at this setting as well 
for API and CLI operations where mysql_engine is auto-inserted into DB 
operations. In those use cases, I replace the hard coded "InnoDB" with 
the mysql_storage_engine variable.

It would be interesting if we could develop some helpers to automate 
this, but it would probably have to be at the SQL Alchemy or Alembic 
levels. Unfortunately, throughout all of the OpenStack services today we 
are hard coding things like mysql_engine, using InnoDB specific features 
(savepoints, nested operations, etc.), and not following the strict SQL 
orders for modifying table elements (foreign keys, constraints, and 
indexes). That actually makes it difficult to support other MySQL 
dialects or other databases out of the box. SQL Alchemy can be used to 
fix some of these things if the SQL statements are all generic and we 
follow strict SQL rules. But to change that would be a monumental 
effort. That is why I took this approach of just adding custom logic. 
There is a president for this already for Postgres and DB2 support in 
some of the OpenStack services using custom logic to deal with similar 

As to why we should place the configuration setting into oslo.db? Here 
are a couple of logical reasons:

  * The configuration block for database settings for each service comes
    from the oslo.db namespace today under cfg.CONF.database.*. Placing
    it here makes the location consistent across all of the services.
  * Within the SQL Alchemy and Alembic scripts, this is one of the few
    common namespaces that are available without bringing in a larger
    number of modules across the services today.
  * Many of the SQL Alchemy and Alembic scripts only import the minimal
    set of python modules. If we imported others, we would also have to
    initialize those name spaces which means a lot more code :(
  * Reduces the amount of overhead required to make these changes.

> Keep in mind that we do not encourage code outside of libraries to
> rely on configuration settings defined within libraries, because
> that limits our ability to change the names and locations of the
> configuration variables.  If migration scripts need to access the
> configuration setting we will need to add some sort of public API
> to oslo.db to query the value. The function can simply return the
> configured value.

Configuration parameters within any given service will make use of a 
large namespace that pulls in things from oslo and the .conf files for a 
given service. So even when an API, CLI, or DB related call is made, 
these namespaces are key for things to work. In the case of the SQL 
Alchemy and Alembic scripts, they also make use of this namespace with 
oslo, oslo.db, etc. to figure out how to connect to the database and 
other database settings. I don't think we need a public API for these 
kinds of calls as the community already makes use of the libraries to 
build the namespace. My oslo.db setting and patches for each service 
just make use of the cfg.CONF.database namespace to determine the 
correct behavior to execute.

> What other behaviors are likely to be changed by the new option?
> Will application runtime behavior need to know about the storage
> engine?

The changes will be transparent to the application runtime behavior. The 
APIs and CLI tools call into the <service>/db/api.py as the entry point 
for database calls. Behind this you usually have a models.py that is 
aware of the database schema to understand the layout of things. So the 
underlining structure is abstracted away from the run-time. These entry 
points sometimes do require minor modifications to handle any hard coded 
issues or intercept functions like savepoints and nested operations. 
Again I use the cfg.CONF.database namespace to check for the appropriate 
behavior and implement IF/THEN logic to do the right thing.

Some of my design objectives for all of these patches are:

  * Zero impact on OpenStack functionality and usability (API, CLI, user
  * No loss in database structure. Consistent foreign keys, constraints,
    indexes, etc.
  * Minimal impact on column size and/or types to fit within NDB table
    row limits. Many columns are over-sized today.
  * Validate functionality of APIs, service processes, and CLI. Tempest
    is our friend :)
  * Zero impact for users not using MySQL Cluster (NDB).

> Doug
>> Thanks,
>> Octave
>> On 2/2/2017 6:46 AM, Doug Hellmann wrote:
>>> Excerpts from Octave J. Orgeron's message of 2017-02-01 20:33:38 -0700:
>>>> Hi Folks,
>>>> I'm working on adding support for MySQL Cluster to the core OpenStack
>>>> services. This will enable the community to benefit from an
>>>> active/active, auto-sharding, and scale-out MySQL database. My approach
>>>> is to have a single configuration setting in each core OpenStack service
>>>> in the oslo.db configuration section called mysql_storage_engine that
>>>> will enable the logic in the SQL Alchemy or Alembic upgrade scripts to
>>>> handle the differences between InnoDB and NDB storage engines
>>>> respectively. When enabled, this logic will make the required table
>>>> schema changes around:
>>>>     * Row character length limits 65k -> 14k
>>>>     * Proper SQL ordering of foreign key, constraints, and index operations
>>>>     * Interception of savepoint and nested operations
>>>> By default this functionality will not be enabled and will have no
>>>> impact on the default InnoDB functionality. These changes have been
>>>> tested on Kilo and Mitaka in previous releases of our OpenStack
>>>> distributions with Tempest. I'm working on updating these patches for
>>>> upstream consumption. We are also working on a 3rd party CI for
>>>> regression testing against MySQL Cluster for the community.
>>>> The first change set is for oslo.db and can be reviewed at:
>>>> https://review.openstack.org/427970
>>>> Thanks,
>>>> Octave
>>> Is it possible to detect the storage engine at runtime, instead of
>>> having the operator configure it?
>>> Doug
>>> __________________________________________________________________________
>>> OpenStack Development Mailing List (not for usage questions)
>>> Unsubscribe: OpenStack-dev-request at lists.openstack.org?subject:unsubscribe
>>> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
> __________________________________________________________________________
> OpenStack Development Mailing List (not for usage questions)
> Unsubscribe: OpenStack-dev-request at lists.openstack.org?subject:unsubscribe
> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev


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
Certified Oracle Enterprise Architect: Systems Infrastructure 
Green Oracle <http://www.oracle.com/commitment> Oracle is committed to 
developing practices and products that help protect the environment

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openstack.org/pipermail/openstack-dev/attachments/20170202/41225219/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: oracle_sig_logo.gif
Type: image/gif
Size: 658 bytes
Desc: not available
URL: <http://lists.openstack.org/pipermail/openstack-dev/attachments/20170202/41225219/attachment.gif>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: oea_logo.jpg
Type: image/jpeg
Size: 2398 bytes
Desc: not available
URL: <http://lists.openstack.org/pipermail/openstack-dev/attachments/20170202/41225219/attachment.jpg>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: green-for-email-sig_0.gif
Type: image/gif
Size: 356 bytes
Desc: not available
URL: <http://lists.openstack.org/pipermail/openstack-dev/attachments/20170202/41225219/attachment-0001.gif>

More information about the OpenStack-dev mailing list