Alembic is a new database migrations tool, written by the author
of SQLAlchemy. A migrations tool
offers the following functionality:
- Can emit ALTER statements to a database in order to change
the structure of tables and other constructs
- Provides a system whereby “migration scripts” may be constructed;
each script indicates a particular series of steps that can “upgrade” a
target database to a new version, and optionally a series of steps that can
“downgrade” similarly, doing the same steps in reverse.
- Allows the scripts to execute in some sequential manner.
The goals of Alembic are:
- Very open ended and transparent configuration and operation. A new
Alembic environment is generated from a set of templates which is selected
among a set of options when setup first occurs. The templates then deposit a
series of scripts that define fully how database connectivity is established
and how migration scripts are invoked; the migration scripts themselves are
generated from a template within that series of scripts. The scripts can
then be further customized to define exactly how databases will be
interacted with and what structure new migration files should take.
- Full support for transactional DDL. The default scripts ensure that all
migrations occur within a transaction - for those databases which support
this (Postgresql, Microsoft SQL Server), migrations can be tested with no
need to manually undo changes upon failure.
- Minimalist script construction. Basic operations like renaming
tables/columns, adding/removing columns, changing column attributes can be
performed through one line commands like alter_column(), rename_table(),
add_constraint(). There is no need to recreate full SQLAlchemy Table
structures for simple operations like these - the functions themselves
generate minimalist schema structures behind the scenes to achieve the given
- “auto generation” of migrations. While real world migrations are far more
complex than what can be automatically determined, Alembic can still
eliminate the initial grunt work in generating new migration directives
from an altered schema. The --autogenerate feature will inspect the
current status of a database using SQLAlchemy’s schema inspection
capabilities, compare it to the current state of the database model as
specified in Python, and generate a series of “candidate” migrations,
rendering them into a new migration script as Python directives. The
developer then edits the new file, adding additional directives and data
migrations as needed, to produce a finished migration. Table and column
level changes can be detected, with constraints and indexes to follow as
- Full support for migrations generated as SQL scripts. Those of us who
work in corporate environments know that direct access to DDL commands on a
production database is a rare privilege, and DBAs want textual SQL scripts.
Alembic’s usage model and commands are oriented towards being able to run a
series of migrations into a textual output file as easily as it runs them
directly to a database. Care must be taken in this mode to not invoke other
operations that rely upon in-memory SELECTs of rows - Alembic tries to
provide helper constructs like bulk_insert() to help with data-oriented
operations that are compatible with script-based DDL.
- Non-linear versioning. Scripts are given UUID identifiers similarly
to a DVCS, and the linkage of one script to the next is achieved via markers
within the scripts themselves. Through this open-ended mechanism, branches
containing other migration scripts can be merged - the linkages can be
manually edited within the script files to create the new sequence.
- Provide a library of ALTER constructs that can be used by any SQLAlchemy
application. The DDL constructs build upon SQLAlchemy’s own DDLElement base
and can be used standalone by any application or script.
- Don’t break our necks over SQLite’s inability to ALTER things. SQLite
has almost no support for table or column alteration, and this is likely
intentional. Alembic’s design
is kept simple by not contorting its core API around these limitations,
understanding that SQLite is simply not intended to support schema
changes. While Alembic’s architecture can support SQLite’s workarounds, and
we will support these features provided someone takes the initiative
to implement and test, until the SQLite developers decide
to provide a fully working version of ALTER, it’s still vastly preferable
to use Alembic, or any migrations tool, with databases that
are designed to work under the assumption of in-place schema migrations
Documentation and status of Alembic is at http://readthedocs.org/docs/alembic/.
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