Comments below..
On 2/2/2017 1:08 PM, Doug Hellmann wrote:
Excerpts from Octave J. Orgeron's message of 2017-02-02 12:16:15 -0700:
Hi Doug,
Comments below..
Thanks,
Octave
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.
So all existing scripts that create or modify tables will need to
be updated? That's going to be a lot of work. It will also be a lot
of work to ensure that new alter scripts are implemented using the
required logic, and that testing happens in the gates for all
projects supporting this feature to ensure there are no regressions
or behavioral changes in the applications as a result of the changes
in table definitions.
I'll let the folks more familiar with databases in general and MySQL
in particular respond to some of the technical details, but I think
I should give you fair warning that you're taking on a very big
project, especially for someone new to the community.
Yes, this is major undertaking and major driver for Oracle to setup a
3rd party CI so that we can automate regression testing against MySQL
Cluster. On the flip side, it helps solve some of the challenges with
larger deployments where an active/passive solution for MySQL DB is not
sufficient. So the pay-off is pretty big from an availability and
scale-out perspective.
But I do realize that I'll have to maintain this long-term and hopefully
get others to help out as more services are added to OpenStack.
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
differences.
As to why we should place the configuration setting into oslo.db? Here
are a couple of logical reasons:
Oh, I'm not questioning putting the option in oslo.db. I think that's
clearly the right place to put it.
* 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.
I may not have been entirely clear. You need to add a function to
oslo.db to allow a user of oslo.db to read the configuration value
without knowing what that option name is. There are two reasons for
this policy:
1. Configuration options are supposed to be completely transparent
to the application developer using the library, otherwise they
would be parameters to the classes or functions in the library
instead of deployer-facing configuration options.
oslo.config allows us to rename configuration options transparently
to deployers (they get a warning about the new name or location
for the option in the config file, but the library knows both
locations).
The rename feature does not work when accessing options
programmatically, because we do not consider configuration options
to be part of the API of a library. That means that cfg.CONF.foo.bar
can move to cfg.CONF.blah.bletch, and your code using it by the
old name will break.
This is correct. Neutron does exactly what you are describing where you
have to look under a neutron namespace instead of the cfg.CONF namespace
to find the actual configured setting from the .conf file.
2. Accessing configuration options depends on having them registered,
and a user of the library that owns a configuration option may not
know which functions in the library to call to register the options.
As a result, they may try to use an option before it is actually
defined. Using an access function to read the value of an option
allows the library to ensure the option is registered before trying
to return the value.
For those reasons, in cases where a configuration option needs to
be exposed outside of the library we require a function defined
inside the library where we can have unit tests that will break if
the configuration option is renamed or otherwise changed, and so
we can handle those changes without breaking applications consuming
the library.
In this case, the migration scripts are outside of oslo.db, so they
will need a public function added to oslo.db to access the configuration
value. The function should first ensure that the new option is
registered, and then return the configured value.
So what do you envision that I'll have to add to the oslo.db library to
make things work as you describe? What would be the best example for me
to build from?
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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