Online Schema Migrations

https://blueprints.launchpad.net/neutron/+spec/online-schema-migrations

Author:Mike Bayer <mike.bayer@redhat.com>

This specification discusses the issue of database schema migrations which may proceed while allowing both the previous and the updated version of the Neutron database API to run against that schema at the same time. This is part of a larger approach which is to allow a Neutron application to be upgraded to a new version without incurring downtime while the database schema is migrated.

To achieve this goal fully, several areas must be addressed:

  • The first is that database schema migrations can be applied which don’t impact the old version of the software as it runs. These migrations are referred to as expansive migrations, which only add new elements, never removing any. Once the old software has been entirely replaced with the new version, a second series of migrations known as the contract migrations are run separately; these migrations remove the old elements that are no longer used.
  • The newer version of the software must also be prepared to accommodate for the fact that the old version of the software may still be running as well, meaning it may need to read and persist data from both the old and new schema structures simultaneously.
  • Within the scope of the new software referring to both versions of the schema, a strategy for data migrations must be devised. These migrations can run over time as a function of the data access code itself slowly moving data to the new format, or can run as separate scripts or processes.
  • The sequence of movements between expand/contract, old and new software versions, and migration of data must also be orchestrated fully. Front-end RPC clients and database access services are typically upgraded independently of each other, and additionally at which point “contract” is safe to run must be established.

This blueprint is primarily concerned with only the first bulletpoint, that of organizing schema migrations such that those which are strictly “expansive” may be run separately from those which are “contractual”. The other bullets above will need to be considered separately.

Note that an approach to the problem addressed here has already been accepted for Nova, also called “online schema migrations”. The specification here builds upon the work of Nova’s, proposing essentially the same concept, but implemented slightly differently, in such a way that there is no sharp break from Neutron’s existing system of using Alembic migration scripts, and does not abandon the use of version identifiers which identify an explicit, known state of the schema. There is also a proposal for upstream changes in Alembic so that both Nova’s “live” approach and the “scripted” approach here can share the same codebase against a revised Alembic autogeneration API that allows much greater extensibility.

Problem Description

Database migrations of Neutron and other Openstack applications traditionally involve the replacement of some version of the schema with another one; tables and columns are dropped, new ones added. This change necessarily involves that the software which communicates with the schema must also change at the same time, where the old version is shut down completely before beginning the migration, the migration then proceeds fully offline, and then the new version is started. In terms of a multiple-node Openstack deployment, this means that the entire application on all nodes must be fully shut down and upgraded globally all at once. The offline migration may also be time consuming in terms of what kinds of operations are present and what target database is in use.

The business requirements of many key Openstack consumers is such that the downtime involved with fully upgrading all nodes simultaneously as well as running full schema migrations during that downtime is no longer acceptable; a new approach that allows the application to keep running while the migration goes on must be developed, in particular for key Openstack components such as Nova, Neutron, and Cinder.

Proposed Change

Within this document we will address the goal of organizing schema migrations into “expand” and “contract” phases that are also linked to major release versions. The phases are as follows:

  • Migrations that run under “expand” are “additive” (e.g. tables, columns, indexes and constraints are only created, not dropped) only, and are safe to run while the old version of the application continues to run.
  • Migrations that run under “contract” are “subtractive” (e.g. tables, columns, indexes and constraints are only dropped, not created) and only run once software running on all nodes communicates exclusively with the new version of the schema, and all data has been migrated.

The steps involved in each of the two phases will be rendered as explicit migration directives within Alembic migration scripts, as is already the case for Neutron. The only difference will be that a given migration will be broken out into individual scripts for each phase of operation that the migration includes. These scripts will be assembled into semi-independent lineages that can be run separately. These lineages will also be classified among release versions, so that migration lineages will be targetable at the level of both release and phase, e.g. “expand liberty”, “contract M”, etc.

The new scheme is supported by Alembic’s recently added support for long-lived branches, roots, branch names, and individual file directories. The workflow can be implemented at a proof-of-concept level without writing any new code, by creating the new directory structure and manually assembling new migration files into the appropriate branches using Alembic’s updated command line tools.

However, in order to facilitate the use of Alembic autogenerate, new features will be added upstream to Alembic’s autogenerate API which will allow for the creation of custom autogenerate behaviors and filesystem flows. We will build a new tool that adapts Nova’s current online schema migrations logic to this new API such that the logic used to group migrations into “expand” and “contract” steps may now stream those instructions into individual migration files, targeted into the file structure referred to above. It is hoped that this same tool will also be able to continue to send migration directives directly to a database as well, thus allowing Nova’s current “live” approach to be rolled into the same codebase. Improvements and behavioral contracts for the “expand” / “contract” workflow system will apply both to the “live” and “scripted” approaches, thus making the two approaches that much more interchangeable.

Alembic Migrations

Right now, Neutron makes use of Alembic migrations, which involves a series of migration scripts organized into the neutron/db/migration/alembic_migrations/versions directory. These scripts are organized into a kind of backwards linked list structure, where each script is identified by a six-byte hash scheme, and contains a variable that links it to the previous hash in the series. The rationale for this linked structure is that new versions can be inserted into the middle of the chain without impacting more than one existing migration file; by using a “backwards” linking model, and new versions can be added to the end of the list without impacting any existing versions.

Recent versions of Alembic have been enhanced to reconsider this “backwards linked list” structure as just a specialization of a more flexible structure, the directed acyclic graph, or DAG. In this approach, we remove the requirement that each migration script can only refer to a single anscestor (e.g. dependency), as well as the requirement that only one migration script can refer to a particular ancestor. The structure basically becomes open to the concepts of branching and merging which are very familiar in version control systems. Alembic now has the ability to run upgrades or downgrades along individual branches which are tracked individually within a database schema, meaning a schema’s “head” version may be in fact a series of hashes, each representing the “head” of an individual revision stream. The branches can optionally originate from entirely independent root revisions with no dependencies on each other, and can also be organized into individual subdirectories. Revisions within branches can also refer to specific revisions within other branches as dependencies, and branches may be merged back together into a single revision stream.

Expand and Contract Scripts

The current design of a migration script includes that it indicates a specific “version” of the schema, and includes directives that apply all necessary changes to the database at once. If we look for example at the script 2d2a8a565438_hierarchical_binding.py, we will see:

# .../alembic_migrations/versions/2d2a8a565438_hierarchical_binding.py

def upgrade():

    # .. inspection code ...

    op.create_table(
        'ml2_port_binding_levels',
        sa.Column('port_id', sa.String(length=36), nullable=False),
        sa.Column('host', sa.String(length=255), nullable=False),
        # ... more columns ...
    )

    for table in port_binding_tables:
        op.execute((
            "INSERT INTO ml2_port_binding_levels "
            "SELECT port_id, host, 0 AS level, driver, segment AS segment_id "
            "FROM %s "
            "WHERE host <> '' "
            "AND driver <> '';"
        ) % table)

    op.drop_constraint(fk_name_dvr[0], 'ml2_dvr_port_bindings', 'foreignkey')
    op.drop_column('ml2_dvr_port_bindings', 'cap_port_filter')
    op.drop_column('ml2_dvr_port_bindings', 'segment')
    op.drop_column('ml2_dvr_port_bindings', 'driver')

    # ... more DROP instructions ...

The above script contains directives that are both under the “expand” and “contract” categories, as well as some data migrations. the op.create_table directive is an “expand”; it may be run safely while the old version of the application still runs, as the old code simply doesn’t look for this table. The op.drop_constraint and op.drop_column directives are “contract” directives (the drop column moreso than the drop constraint); running at least the op.drop_column directives means that the old version of the application will fail, as it will attempt to access these columns which no longer exist.

The data migrations in this script are adding new rows to the newly added ml2_port_binding_levels table. Data migrations may or may not be “safe” to run within the “expand” or “contract” phase, depending on the nature of the data. It is expected that most data migrations will run outside of migration scripts going forward, and instead be implemented as part of the model/API layer as the application runs.

Note that this spec suggests, but not requires Neutron to move to live data migrations implemented in the application instead of migration scripts. This part will require a separate consideration and is out of scope for the spec.

Under the proposed plan, the above script, assuming it were added as part of the new architecture, would be stated as two scripts; an “expand” and a “contract” script:

# expansion operations
# .../alembic_migrations/versions/liberty/expand/2bde560fc638_hierarchical_binding.py

def upgrade():

    op.create_table(
        'ml2_port_binding_levels',
        sa.Column('port_id', sa.String(length=36), nullable=False),
        sa.Column('host', sa.String(length=255), nullable=False),
        # ... more columns ...
    )


# contraction operations
# .../alembic_migrations/versions/liberty/contract/4405aedc050e_hierarchical_binding.py

def upgrade():

    op.drop_constraint(fk_name_dvr[0], 'ml2_dvr_port_bindings', 'foreignkey')
    op.drop_column('ml2_dvr_port_bindings', 'cap_port_filter')
    op.drop_column('ml2_dvr_port_bindings', 'segment')
    op.drop_column('ml2_dvr_port_bindings', 'driver')

    # ... more DROP instructions ...

The two scripts would be present in different subdirectories and also part of entirely separate versioning streams, discussed in the section below “New Migration Layout”. The “expand” operations are in the “expand” script, and the “contract” operations are in the “contract” script.

The data migrations are removed, as these are expected to generally not occur within schema migrations any more. However, the approach remains compatible with allowing “safe” data migrations to be manually placed within the expand or contract scripts if deemed appropriate in some cases.

For the time being, until live data migration is accepted in Neutron, data migration rules belong to one of script subtrees.

New Migration Layout

With Alembic’s new capabilities, we can propose a new structure for Neutron’s migration files that is compatible with “expand” / “contract” while at the same time remains compatible with the existing stream of Alembic migration files in Neutron. A new directory/branch structure will be laid out which allows all versions/streams to be apparent:

neutron/db/migration/alembic_migrations/...

...versions/
             <existing version>.py
             <existing version>.py
             <existing version>.py
             ...

versions/liberty/
versions/liberty/expand/
                                <expansion script>.py
                                <expansion script>.py
                                ...

versions/liberty/contract/
                                <contract script>.py
                                <contract script>.py
                                ...

versions/M_release/
versions/M_release/expand/
versions/M_release/contract/
... etc

Above, the existing /versions/ directory with all of its current migration scripts remains intact; these versions are still the scripts that take a Neutron database up through Kilo at least. Following those, a new series of subdirectories are added, organized among major Openstack releases, and within each subdirectory, the “expand” and “contract” series of scripts are themselves separate.

The series of scripts within /expand/ and /contract/ are themselves originating from independent “roots”; that is, the “down” revision for the bottommost script in each directory is None.

The production of these scripts is supported by the alembic revision command, which now includes options to place files in specific directories as well as what the “down revision” of a given revision is to be, including that it may be a “root”, thus allowing the creation of new branches and roots.

Cross-Branch Dependencies

To accommodate for the fact that the scripts in liberty/expand can’t be run until all the old scripts in versions/ have run, as well as that individual scripts in liberty/contract/ can’t run until their correpsonding “expand” has run, Alembic’s cross-branch dependency feature will be used. From a DAG point of view, this is the same as a script declaring another one as a dependency, but from Alembic’s perspective the script is not considered to be any kind of “down revision”; only a script whose version must be invoked within the target schema before the current one can be run. They are indicated in Alembic scripts as a separate directive:

# revision identifiers, used by Alembic.
revision = '2a95102259be'
down_revision = '29f859a13ea'
branch_labels = None
depends_on=('55af2cb1c267', '4fcb78af5a01')

By establishing “depends_on”, a particular script indicates what migration scripts in other branches need to be run first, before this one can. When we instruct Alembic to invoke this migration, it will ensure that all dependency scripts are run first. It is expected that the automated script creation tool will be able to build out these directives automatically.

Alembic branch dependencies are discussed in the Alembic documentation referred to in the References section.

Branch Labels

Alembic also now provides for “branch labels”, meaning that in addition to having our migration files in different directories, versioned across independent branches with independent roots, we can also apply one or more “labels” to a branch as a whole which is then addressable using Alembic’s command line tools. Whereas in Neutron today we can see some migration scripts that are intentionally named juno_release.py and kilo_release.py, we can apply these names to the branches as a whole. Like branch dependencies, these are also indicated as directives within migration scripts; however, the branch label only need be present in any single revision script within the branch. Typically, the first script within the branch is a good choice for placing a label:

# revision identifiers, used by Alembic.
revision = '2a95102259be'
down_revision = None  # because we are a "root"
branch_labels = ('liberty_expand', 'release_expand')
depends_on='55af2cb1c267'

So above, we would apply names such as "liberty_expand" and "liberty_contract" to liberty/expand and liberty/contract branches, appropriately. This allows Alembic commands to be run which refer to the branch as a whole, such as:

alembic upgrade liberty_expand@head

Where above, all migrations up until the liberty_expand branch will be run (including dependent versions from the old series of migration files first, if not already run). This will allow Neutron’s command suite to accommodate specific target points within the new versioning scheme without the need to become aware of specific revisions.

If labels that are agnostic of “release” are desired, such as a branch that indicates “run all the expand steps up to the current release”, we can add additional “latest release” labels that move to new branches as new releases are established.

New Neutron DB Commands

Right now, Neutron allows database upgrades running the neutron-db-manage script, which links into Alembic’s own upgrade command. This script will be enhanced to allow for running individual migration streams by taking advantage of new argument forms that are part of Alembic. The alembic upgrade command will still be used but will now be passed the appropriate branch labels specific to the target operation, such as neutron-db-manage expand or neutron-db-manage contract.

Automation of Scripting

The previous sections essentially make possible the entire “expand” / “contract” workflow completely, in such a way that workflow from Neutron’s existing Alembic versioning scripts is maintained without any backwards incompatibility. However, the addition of new migration scripts would at first be available only by manually targeting each portion of the workflow individually.

We instead can enhance Neutron’s use of Alembic “autogenerate” such that a single revision autogenerate step can produce multiple files as needed; a migration that includes both “expand” and “contract” directives would generate two separate scripts.

Right now, Nova OSM makes use of Alembic autogenerate in order to derive information about how a target database differs from the model established in code. It uses a public method compare_metadata() to achieve this; compare_metadata() returns a simple list of “diffs” which refer to changes in schema objects like tables, columns, and constraints. Nova OSM then keys “operational” objects such as AddTable, DropColumn, AddConstraint, etc. These “operational” objects then link back into Alembic’s API, associated with corresponding “operation” constructs in Alembic such as op.create_table(), op.drop_column(), op.add_constraint().

Nova’s OSM is basically consuming an Alembic “autogenerate diff” stream and streaming it into an Alembic “run operations” stream. It follows that Alembic can provide infrastructure such that an autogenerate diff stream can be supplied directly as a migration operation stream. Both the “live” OSM approach of Nova and the “scripted” approach proposed here can consume this same operation stream, partition it based on operation type into “expand” and “contract” streams, and then direct those streams either to a live database context for “live” migrations or to a series of migration scripts for scripted migrations.

The alembic revision command will also be opened up such that a plugin may establish an open-ended series of revision scripts generated from portions of these operational streams. The end result will be that a single call to alembic revision --autogenerate as performed by Neutron developers today will generate separate “expand” and “contract” scripts directly.

These new APIs are already underway in upstream development branches and are tracked by separate Alembic issues (see References). By closely linking the implementations for “live” migrations and “scripted” migrations, it is hoped that the majority of ongoing effort within OSM can contribute to both approaches simultaneously, thus reducing the risk that work is wasted either if one or the other approach is abandoned or that improvements and workflows begin to diverge if both approaches remain in active use.

Data Model Impact

Expand/contract workflow itself has no direct impact on the data model. The other aspects of online schema migrations, namely support of multiple versions of a schema simultanouesly as well as moving data between those structures at runtime have an enormous impact; however, that’s outside the scope of this document.

REST API Impact

none

Security Impact

none

Other End User Impact

End users will be using a modified workflow when schema upgrades are performed, running “expand” and “contract” steps separately and at the appropriate time.

In terms of backup procedures, there is no difference. Database always represents some specific subset of head revisions (the only difference between the proposed feature and the current state is that the subset has more than one element).

If/once we adopt live data migration in Neutron, it won’t change a lot in terms of database backups either way. The only significant thing is that the same logical object could now be represented by multiple versions of database rows, depending on whether migration for the object is complete. Anyway, backups would still work as usual.

Performance Impact

none

Developer Impact

Developers should continue to use alembic revision --autogenerate in order to create new migration scripts. This operation will create multiple scripts, so to the degree that developers need to manually tune these scripts, they’ll be dealing with more than one script. Since data migrations will generally no longer be within these scripts, and also since we now have the ability to render custom directives via autogenerate, it is hoped that pretty much anything Nova’s “live” OSM approach can automate can also be completely automatic within the “scripted” approach as well.

Alternatives

Live migrations were originally proposed as a replacement for SQLAlchemy-Migrate, which has the additional issues of a very rigid and unworkable numbering scheme as well as verbose migration scripts that rely heavily on full table declarations and reflection. These are not issues for projects that already use Alembic, as Alembic was designed to solve these problems among others.

Live migration also offers the advantages that no script at all needs to be generated or committed into the source repository, and also that because there is literally no way to alter how migrations will proceed on a per-change basis, developers are not given any opportunity to inadvertently produce a migration that is non-expansive or to inappropriately write statements in a migration that result in performing a data migration. This is noted as allowing a “purely declarative” approach to migrations, where the model code is all that’s needed to indicate how to get to the new schema.

However, this advantage only works under limited circumstances. While simple cases are easily automated by both approaches, the issue of accommodating for special cases, unsupported features, and variability in support and/or reliability on various backends is not addressed by the “live” approach. Live migrations only offers in such cases that the upstream migration system must be modified to accommodate for the target case, or the application must be modified to no longer require such a schema migration. Special cases include changes to complex types like ENUMs and precision numerics, operations involving CHECK constraints and some kinds of server defaults, special constructs such as indexes that use vendor-specific extensions, and even simple things like changes of table or column name that can’t be distinguished from an add/drop of two separate objects.

The availability and reliability of the reflection and autogenerate features on backends is not necessarily consistent, nor can the Alembic / SQLAlchemy projects make any such guarantee. In particular, less common backends such as that of IBM DB2, which is published independently of SQLAlchemy or Alembic, may not support some operations correctly at all, and it is not currently known to what extent autogenerate and reflection produce accurate results, especially in thorny cases such as indexes, unique constraints, and column types. Alembic’s autogenerate feature was not intended to be used in the way that live migrations does, and it’s a risky assumption that it will produce correct results perfectly under all cases on thousands of production systems.

While the “scripted” OSM approach maintains reliance upon explicit migration scripts that must be checked in and occasionally edited, the advantage to this approach is that the sequence of migration steps to be run are produced just once, up front, in a controlled environment. Unusual migrations against special types or other constructs are again a non-issue as they can be scripted explicitly as needed. These steps can then be carefully reviewed and tested by developers and then shipped, where there is no risk of them doing something entirely different when run against a backend of a lesser-used vendor or with unusual configurations. They also maintain the advantage that operator-maintained schema structures are unaffected; the “live” approach documents that operators would need to re-create their own schema structures after a “contract” is run.

In order to maintain that scripted migrations stay appropriately expansive / contractual and without inappropriate data migrations in the face of developer intervention, we should require that developers use the autogenerated migration scripts that are generated for them as is, and that they don’t modify these scripts except to support operations that aren’t supported as automatable migrations. Schema migrations should be tested as part of the CI process including that the “online upgrades” are tested against previous API versions, and migration scripts of course go through the usual Gerrit code review process; identifying migrations that are non-expansive or are data migrations is not difficult.

Implementation

Assignee(s)

Primary assignee:
Mike Bayer
Other contributors:
Ann Kamyshnikova Henry Gessau Ihar Hrachyshka

Work Items

  • update neutron-db-manage to support upgrade for multiple heads.
  • update neutron-db-manage revision to generate multiple scripts.
  • get alembic updated to include update stream generated.
  • use the alembic autogenerate feature to filter operations into subtrees.
  • implement new expand/contract neutron-db-manage commands.
  • document the change in the upgrade flow in user and developer docs.
  • introduce testing for expand-only schema upgrade.

Dependencies

Upstream changes to Alembic for the autogenerate integration aspect.

Testing

Functional tests should include that “expand” migrations are run and that the previous version of the API still works fully against an expanded migration.

Documentation Impact

User Documentation

Expand/contract workflow will need to be documented.

Developer Documentation

Usage of autogenerate along with expand/contract workflow can be documented.

References

[1]Alembic’s Branching Model http://alembic.readthedocs.org/en/latest/branches.html
[2]Online Schema Migrations in Nova http://specs.openstack.org/openstack/nova-specs/specs/liberty/approved/online-schema-changes.html
[3]Nova’s Overall Online Upgrade approach http://docs.openstack.org/developer/nova/devref/upgrade.html
[4]Operations as Objects https://bitbucket.org/zzzeek/alembic/issue/302/operations-as-objects
[5]Extensible Revision / Autogenerate strategies https://bitbucket.org/zzzeek/alembic/issue/301/extensible-revision-autogenerate
[6]Neutron patch to rearrange migration directory into subtrees https://review.openstack.org/194198