On 02/02/2017 02:16 PM, Octave J. Orgeron wrote:
Hi Doug,
Comments below..
Thanks,
Octave
On 2/2/2017 11:27 AM, Doug Hellmann wrote:
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.
can you please clarify "replace the hard coded "InnoDB" " ? Are you
proposing to send reviews for patches against all occurrences of
"InnoDB" in files like
https://github.com/openstack/nova/blob/master/nova/db/sqlalchemy/migrate_repo/versions/216_havana.py
? The "InnoDB" keyword is hardcoded in hundreds of migration files
across all openstack projects that use MySQL. Are all of these going
to be patched with some kind of conditional?
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.
not really, you can build a hook that intercepts operations like
CreateTable, or that intercepts SQL as it is emitted over a connection,
in order to modify these values on the fly. But that is a specific kind
of approach with it's own set of surprises. Alternatively you can make
an alternate SQLAlchemy dialect that no longer recognizes "mysql_*" as
the prefix for these arguments. There's ways to do this part.
But more critically I noticed you referred to altering the names of
columns to suit NDB. How will this be accomplished? Changing a column
name in an openstack application is no longer trivial, because online
upgrades must be supported for applications like Nova and Neutron. A
column name can't just change to a new name, both columns have to exist
and logic must be added to keep these columns synchronized.
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).
Savepoints aren't InnoDB specific, they are a standard SQL feature and
also their use is not widespread right now. I'm not sure what you mean
by "the strict SQL orders", we use ALTER TABLE as is standard in MySQL
for this and it's behind an abstraction layer that supports other
databases such as Postgresql.
* 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 :(
I'm not sure what this means, can you clarify ?
* Reduces the amount of overhead required to make these changes.
What sort of "overhead", do you mean code complexity, performance ?
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
experience)
* 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
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Oracle Linux OpenStack
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