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 /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.mysqlstorageengine setting to make the required modifications. There are also use cases where the api.py or model(s).py under the /db/ hierarchy needs to look at this setting as well for API and CLI operations where mysqlengine is auto-inserted into DB operations. In those use cases, I replace the hard coded "InnoDB" with the mysqlstorage_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 differences.
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 /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).
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 mysqlstorageengine 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: OpenStackfirstname.lastname@example.org?subject:unsubscribe http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
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Octave J. Orgeron | Sr. Principal Architect and Software Engineer
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