delembic 0.4.2

Data migration framework for versioning ETL and data operations alongside Alembic

Delembic

Data migration framework for Python — Alembic for ETL and data operations.

Alembic versions schema changes. Delembic versions data changes: vocabulary loads, ETL runs, reference data inserts, corrections. Together they describe the complete database state.

Installation

pip install delembic

Quick Start

# 1. Initialize in your project
delembic init

# 2. Edit delembic.ini — set your database URL
# sqlalchemy.url = postgresql+psycopg://user:pass@host/dbname

# 3. Create a migration
delembic revision -m "load vocabulary"

# 4. Fill in the generated file, then run
delembic upgrade head

Configuration

delembic init creates:

your-project/
├── delembic.ini          # config — commit this
└── delembic/
    ├── env.py            # optional connection helpers
    └── versions/         # migration files live here

delembic.ini:

[delembic]
script_location = delembic
sqlalchemy.url = postgresql+psycopg://user:pass@localhost/mydb
alembic_config = alembic.ini

You can name the script folder anything:

delembic init data-migrations

Writing Migrations

from delembic import DataMigration

class LoadVocabulary(DataMigration):

    revision = "D001"
    depends_on = []
    description = "Load OMOP vocabulary tables"

    def upgrade(self, conn):
        conn.execute(...)

    def validate(self, conn):
        count = conn.execute("SELECT COUNT(*) FROM concept").scalar()
        assert count > 0, "vocabulary load produced no rows"

conn is a SQLAlchemy Connection. validate is optional — migration is marked failed if it raises.

Dependency Tracking

Migrations declare explicit dependencies. Delembic builds a DAG and runs them in topological order.

class LoadPerson(DataMigration):
    revision = "D002"
    depends_on = ["D001"]   # waits for D001

Alembic Integration

Point alembic_config at your alembic.ini:

[delembic]
alembic_config = alembic.ini

When you create a migration, Delembic automatically captures the current Alembic head:

delembic revision -m "load person"
# → depends_on = ['3d1e3e6abc12']   (current alembic head)

At upgrade time, Delembic verifies the required Alembic revision has been applied before running:

BLOCKED: Migration D002 requires Alembic revision(s) 3d1e3e6abc12 to be applied first.
Run 'alembic upgrade head' before retrying.

CLI Reference

Command	Description
delembic init [DIR]	Initialize project. DIR defaults to delembic
delembic revision -m "msg"	Generate new migration file
delembic upgrade head	Run all pending migrations
delembic upgrade D003	Run migrations up to D003
delembic current	Show last applied revision
delembic history	List all migrations with status

Metadata Tables

Delembic creates two tables:

delembic_version       -- current status per revision
delembic_run_history   -- full audit log (start/end time, duration, exception, user, host)

Failed migration work is rolled back. The failure record is always committed — audit trail survives transaction failures.

Development

pip install -e ".[dev]"
pytest