Tutorial

Alembic provides for the creation, management, and invocation of change management scripts for a relational database, using SQLAlchemy as the underlying engine. This tutorial will provide a full introduction to the theory and usage of this tool.

To begin, make sure Alembic is installed as described at Installation.

The Migration Environment

Usage of Alembic starts with creation of the Migration Environment. This is a directory of scripts that is specific to a particular application. The migration environment is created just once, and is then maintained along with the application’s source code itself. The environment is created using the init command of Alembic, and is then customizable to suit the specific needs of the application.

The structure of this environment, including some generated migration scripts, looks like:

yourproject/
    alembic/
        env.py
        README
        script.py.mako
        versions/
            3512b954651e_add_account.py
            2b1ae634e5cd_add_order_id.py
            3adcc9a56557_rename_username_field.py

The directory includes these directories/files:

  • yourproject - this is the root of your application’s source code, or some directory within it.

  • alembic - this directory lives within your application’s source tree and is the home of the migration environment. It can be named anything, and a project that uses multiple databases may even have more than one.

  • env.py - This is a Python script that is run whenever the alembic migration tool is invoked. At the very least, it contains instructions to configure and generate a SQLAlchemy engine, procure a connection from that engine along with a transaction, and to then invoke the migration engine, using the connection as a source of database connectivity.

    The env.py script is part of the generated environment so that the way migrations run is entirely customizable. The exact specifics of how to connect are here, as well as the specifics of how the migration enviroment are invoked. The script can be modified so that multiple engines can be operated upon, custom arguments can be passed into the migration environment, application-specific libraries and models can be loaded in and made available.

    Alembic includes a set of initialization templates which feature different varieties of env.py for different use cases.

  • README - included with the various enviromnent templates, should have something informative.

  • script.py.mako - This is a Mako template file which is used to generate new migration scripts. Whatever is here is used to generate new files within versions/. This is scriptable so that the structure of each migration file can be controlled, including standard imports to be within each, as well as changes to the structure of the upgrade() and downgrade() functions. For example, the multidb environment allows for multiple functions to be generated using a naming scheme upgrade_engine1(), upgrade_engine2().

  • versions/ - This directory holds the individual version scripts. Users of other migration tools may notice that the files here don’t use ascending integers, and instead use a partial GUID approach. In Alembic, the ordering of version scripts is relative to directives within the scripts themselves, and it is theoretically possible to “splice” version files in between others, allowing migration sequences from different branches to be merged, albeit carefully by hand.

Creating an Environment

With a basic understanding of what the environment is, we can create one using alembic init. This will create an environment using the “generic” template:

$ cd yourproject
$ alembic init alembic

Where above, the init command was called to generate a migrations directory called alembic:

Creating directory /path/to/yourproject/alembic...done
Creating directory /path/to/yourproject/alembic/versions...done
Generating /path/to/yourproject/alembic.ini...done
Generating /path/to/yourproject/alembic/env.py...done
Generating /path/to/yourproject/alembic/README...done
Generating /path/to/yourproject/alembic/script.py.mako...done
Please edit configuration/connection/logging settings in
'/path/to/yourproject/alembic.ini' before proceeding.

Alembic also includes other environment templates. These can be listed out using the list_templates command:

$ alembic list_templates
Available templates:

generic - Generic single-database configuration.
multidb - Rudimentary multi-database configuration.
pylons - Configuration that reads from a Pylons project environment.

Templates are used via the 'init' command, e.g.:

  alembic init --template pylons ./scripts

Editing the .ini File

Alembic placed a file alembic.ini into the current directory. This is a file that the alembic script looks for when invoked. This file can be anywhere, either in the same directory from which the alembic script will normally be invoked, or if in a different directory, can be specified by using the --config option to the alembic runner.

The file generated with the “generic” configuration looks like:

# A generic, single database configuration.

[alembic]
# path to migration scripts
script_location = alembic

# template used to generate migration files
# file_template = %%(rev)s_%%(slug)s

# max length of characters to apply to the
# "slug" field
#truncate_slug_length = 40

# set to 'true' to run the environment during
# the 'revision' command, regardless of autogenerate
# revision_environment = false

# set to 'true' to allow .pyc and .pyo files without
# a source .py file to be detected as revisions in the
# versions/ directory
# sourceless = false

# version location specification; this defaults
# to alembic/versions.  When using multiple version
# directories, initial revisions must be specified with --version-path
# version_locations = %(here)s/bar %(here)s/bat alembic/versions

# the output encoding used when revision files
# are written from script.py.mako
# output_encoding = utf-8

sqlalchemy.url = driver://user:pass@localhost/dbname

# Logging configuration
[loggers]
keys = root,sqlalchemy,alembic

[handlers]
keys = console

[formatters]
keys = generic

[logger_root]
level = WARN
handlers = console
qualname =

[logger_sqlalchemy]
level = WARN
handlers =
qualname = sqlalchemy.engine

[logger_alembic]
level = INFO
handlers =
qualname = alembic

[handler_console]
class = StreamHandler
args = (sys.stderr,)
level = NOTSET
formatter = generic

[formatter_generic]
format = %(levelname)-5.5s [%(name)s] %(message)s
datefmt = %H:%M:%S

The file is read using Python’s ConfigParser.SafeConfigParser object. The %(here)s variable is provided as a substitution variable, which can be used to produce absolute pathnames to directories and files, as we do above with the path to the Alembic script location.

This file contains the following features:

  • [alembic] - this is the section read by Alembic to determine configuration. Alembic itself does not directly read any other areas of the file.

  • script_location - this is the location of the Alembic environment. It is normally specified as a filesystem location, either relative or absolute. If the location is a relative path, it’s interpreted as relative to the current directory.

    This is the only key required by Alembic in all cases. The generation of the .ini file by the command alembic init alembic automatically placed the directory name alembic here. The special variable %(here)s can also be used, as in %(here)s/alembic.

    For support of applications that package themselves into .egg files, the value can also be specified as a package resource, in which case resource_filename() is used to find the file (new in 0.2.2). Any non-absolute URI which contains colons is interpreted here as a resource name, rather than a straight filename.

  • file_template - this is the naming scheme used to generate new migration files. The value present is the default, so is commented out. Tokens available include:

    • %%(rev)s - revision id
    • %%(slug)s - a truncated string derived from the revision message
    • %%(year)d, %%(month).2d, %%(day).2d, %%(hour).2d, %%(minute).2d, %%(second).2d - components of the create date as returned by datetime.datetime.now()
  • truncate_slug_length - defaults to 40, the max number of characters to include in the “slug” field.

    New in version 0.6.1: - added truncate_slug_length configuration

  • sqlalchemy.url - A URL to connect to the database via SQLAlchemy. This key is in fact only referenced within the env.py file that is specific to the “generic” configuration; a file that can be customized by the developer. A multiple database configuration may respond to multiple keys here, or may reference other sections of the file.

  • revision_environment - this is a flag which when set to the value ‘true’, will indicate that the migration environment script env.py should be run unconditionally when generating new revision files

  • sourceless - when set to ‘true’, revision files that only exist as .pyc or .pyo files in the versions directory will be used as versions, allowing “sourceless” versioning folders. When left at the default of ‘false’, only .py files are consumed as version files.

    New in version 0.6.4.

  • version_locations - an optional list of revision file locations, to allow revisions to exist in multiple directories simultaneously. See Working with Multiple Bases for examples.

    New in version 0.7.0.

  • output_encoding - the encoding to use when Alembic writes the script.py.mako file into a new migration file. Defaults to 'utf-8'.

    New in version 0.7.0.

  • [loggers], [handlers], [formatters], [logger_*], [handler_*], [formatter_*] - these sections are all part of Python’s standard logging configuration, the mechanics of which are documented at Configuration File Format. As is the case with the database connection, these directives are used directly as the result of the logging.config.fileConfig() call present in the env.py script, which you’re free to modify.

For starting up with just a single database and the generic configuration, setting up the SQLAlchemy URL is all that’s needed:

sqlalchemy.url = postgresql://scott:tiger@localhost/test

Create a Migration Script

With the environment in place we can create a new revision, using alembic revision:

$ alembic revision -m "create account table"
Generating /path/to/yourproject/alembic/versions/1975ea83b712_create_accoun
t_table.py...done

A new file 1975ea83b712_create_account_table.py is generated. Looking inside the file:

"""create account table

Revision ID: 1975ea83b712
Revises:
Create Date: 2011-11-08 11:40:27.089406

"""

# revision identifiers, used by Alembic.
revision = '1975ea83b712'
down_revision = None
branch_labels = None

from alembic import op
import sqlalchemy as sa

def upgrade():
    pass

def downgrade():
    pass

The file contains some header information, identifiers for the current revision and a “downgrade” revision, an import of basic Alembic directives, and empty upgrade() and downgrade() functions. Our job here is to populate the upgrade() and downgrade() functions with directives that will apply a set of changes to our database. Typically, upgrade() is required while downgrade() is only needed if down-revision capability is desired, though it’s probably a good idea.

Another thing to notice is the down_revision variable. This is how Alembic knows the correct order in which to apply migrations. When we create the next revision, the new file’s down_revision identifier would point to this one:

# revision identifiers, used by Alembic.
revision = 'ae1027a6acf'
down_revision = '1975ea83b712'

Every time Alembic runs an operation against the versions/ directory, it reads all the files in, and composes a list based on how the down_revision identifiers link together, with the down_revision of None representing the first file. In theory, if a migration environment had thousands of migrations, this could begin to add some latency to startup, but in practice a project should probably prune old migrations anyway (see the section Building an Up to Date Database from Scratch for a description on how to do this, while maintaining the ability to build the current database fully).

We can then add some directives to our script, suppose adding a new table account:

def upgrade():
    op.create_table(
        'account',
        sa.Column('id', sa.Integer, primary_key=True),
        sa.Column('name', sa.String(50), nullable=False),
        sa.Column('description', sa.Unicode(200)),
    )

def downgrade():
    op.drop_table('account')

create_table() and drop_table() are Alembic directives. Alembic provides all the basic database migration operations via these directives, which are designed to be as simple and minimalistic as possible; there’s no reliance upon existing table metadata for most of these directives. They draw upon a global “context” that indicates how to get at a database connection (if any; migrations can dump SQL/DDL directives to files as well) in order to invoke the command. This global context is set up, like everything else, in the env.py script.

An overview of all Alembic directives is at Operation Reference.

Running our First Migration

We now want to run our migration. Assuming our database is totally clean, it’s as yet unversioned. The alembic upgrade command will run upgrade operations, proceeding from the current database revision, in this example None, to the given target revision. We can specify 1975ea83b712 as the revision we’d like to upgrade to, but it’s easier in most cases just to tell it “the most recent”, in this case head:

$ alembic upgrade head
INFO  [alembic.context] Context class PostgresqlContext.
INFO  [alembic.context] Will assume transactional DDL.
INFO  [alembic.context] Running upgrade None -> 1975ea83b712

Wow that rocked! Note that the information we see on the screen is the result of the logging configuration set up in alembic.ini - logging the alembic stream to the console (standard error, specifically).

The process which occurred here included that Alembic first checked if the database had a table called alembic_version, and if not, created it. It looks in this table for the current version, if any, and then calculates the path from this version to the version requested, in this case head, which is known to be 1975ea83b712. It then invokes the upgrade() method in each file to get to the target revision.

Running our Second Migration

Let’s do another one so we have some things to play with. We again create a revision file:

$ alembic revision -m "Add a column"
Generating /path/to/yourapp/alembic/versions/ae1027a6acf_add_a_column.py...
done

Let’s edit this file and add a new column to the account table:

"""Add a column

Revision ID: ae1027a6acf
Revises: 1975ea83b712
Create Date: 2011-11-08 12:37:36.714947

"""

# revision identifiers, used by Alembic.
revision = 'ae1027a6acf'
down_revision = '1975ea83b712'

from alembic import op
import sqlalchemy as sa

def upgrade():
    op.add_column('account', sa.Column('last_transaction_date', sa.DateTime))

def downgrade():
    op.drop_column('account', 'last_transaction_date')

Running again to head:

$ alembic upgrade head
INFO  [alembic.context] Context class PostgresqlContext.
INFO  [alembic.context] Will assume transactional DDL.
INFO  [alembic.context] Running upgrade 1975ea83b712 -> ae1027a6acf

We’ve now added the last_transaction_date column to the database.

Partial Revision Identifiers

Any time we need to refer to a revision number explicitly, we have the option to use a partial number. As long as this number uniquely identifies the version, it may be used in any command in any place that version numbers are accepted:

$ alembic upgrade ae1

Above, we use ae1 to refer to revision ae1027a6acf. Alembic will stop and let you know if more than one version starts with that prefix.

Relative Migration Identifiers

Relative upgrades/downgrades are also supported. To move two versions from the current, a decimal value “+N” can be supplied:

$ alembic upgrade +2

Negative values are accepted for downgrades:

$ alembic downgrade -1

Relative identifiers may also be in terms of a specific revision. For example, to upgrade to revision ae1027a6acf plus two additional steps:

$ alembic upgrade ae10+2

New in version 0.7.0: Support for relative migrations in terms of a specific revision.

Getting Information

With a few revisions present we can get some information about the state of things.

First we can view the current revision:

$ alembic current
INFO  [alembic.context] Context class PostgresqlContext.
INFO  [alembic.context] Will assume transactional DDL.
Current revision for postgresql://scott:XXXXX@localhost/test: 1975ea83b712 -> ae1027a6acf (head), Add a column

head is displayed only if the revision identifier for this database matches the head revision.

We can also view history with alembic history; the --verbose option (accepted by several commands, including history, current, heads and branches) will show us full information about each revision:

$ alembic history --verbose

Rev: ae1027a6acf (head)
Parent: 1975ea83b712
Path: /path/to/yourproject/alembic/versions/ae1027a6acf_add_a_column.py

    add a column

    Revision ID: ae1027a6acf
    Revises: 1975ea83b712
    Create Date: 2014-11-20 13:02:54.849677

Rev: 1975ea83b712
Parent: <base>
Path: /path/to/yourproject/alembic/versions/1975ea83b712_add_account_table.py

    create account table

    Revision ID: 1975ea83b712
    Revises:
    Create Date: 2014-11-20 13:02:46.257104

Viewing History Ranges

Using the -r option to alembic history, we can also view various slices of history. The -r argument accepts an argument [start]:[end], where either may be a revision number, symbols like head, heads or base, current to specify the current revision(s), as well as negative relative ranges for [start] and positive relative ranges for [end]:

$ alembic history -r1975ea:ae1027

A relative range starting from three revs ago up to current migration, which will invoke the migration environment against the database to get the current migration:

$ alembic history -r-3:current

View all revisions from 1975 to the head:

$ alembic history -r1975ea:

New in version 0.6.0: alembic revision now accepts the -r argument to specify specific ranges based on version numbers, symbols, or relative deltas.

Downgrading

We can illustrate a downgrade back to nothing, by calling alembic downgrade back to the beginning, which in Alembic is called base:

$ alembic downgrade base
INFO  [alembic.context] Context class PostgresqlContext.
INFO  [alembic.context] Will assume transactional DDL.
INFO  [alembic.context] Running downgrade ae1027a6acf -> 1975ea83b712
INFO  [alembic.context] Running downgrade 1975ea83b712 -> None

Back to nothing - and up again:

$ alembic upgrade head
INFO  [alembic.context] Context class PostgresqlContext.
INFO  [alembic.context] Will assume transactional DDL.
INFO  [alembic.context] Running upgrade None -> 1975ea83b712
INFO  [alembic.context] Running upgrade 1975ea83b712 -> ae1027a6acf

Next Steps

The vast majority of Alembic environments make heavy use of the “autogenerate” feature. Continue onto the next section, Auto Generating Migrations.