Initializing a database

Safir supports simple initialization of a database with a schema provided by the application. By default, this only adds any declared but missing tables, indices, or other objects, and thus does nothing if the database is already initialized. The application may also request a database reset, which will drop and recreate all of the tables in the current schema.

More complex database schema upgrades are not supported by Safir. If those are required, consider using Alembic.

Define the database schema

Database initialization in Safir assumes that the application has defined the database schema via the SQLAlchemy ORM. The recommended way to do this is to add a schema directory to the application containing the table definitions.

In the file schema/base.py, define the SQLAlchemy declarative base:

from sqlalchemy.orm import DeclarativeBase


class Base(DeclarativeBase):
    """Declarative base for SQLAlchemy ORM model of database schema."""

In other files in that directory, define the database tables using the normal SQLAlchemy ORM syntax, one table per file. Each database table definition must inherit from Base, imported from .base.

In schema/__init__.py, import the table definitions from all of the files in the directory, as well as the Base class, and export them using __all__.

Using non-default PostgreSQL schemas

Sometimes it is convenient for multiple applications to share the same database but use separate collections of tables. PostgreSQL supports serving multiple schemas (in the sense of a namespace of tables) from the same database, and SQLAlchemy supports specifying the PostgreSQL schema for a given collection of tables.

The normal way to do this in SQLAlchemy is to modify the DeclarativeBase subclass used by the table definitions to specify a non-default schema. For example:

from sqlalchemy import MetaData
from sqlalchemy.orm import DeclarativeBase

from ..config import config


class Base(DeclarativeBase):
    metadata = MetaData(schema=config.database_schema)

If config.database_schema is None, the default schema will be used; otherwise, SQLAlchemy will use the specified schema instead of the default one.

Safir supports this in database initialization by creating a non-default schema if one is set. If the schema attribute is set (via code like the above) on the SQLAlchemy metadata passed to the schema parameter of initialize_database, it will create that schema in the PostgreSQL database if it does not already exist.

Add a CLI command to initialize the database

The recommended approach to add database initialization to an application is to add an init command to the command-line interface that runs the database initialization code. For applications using Click (the recommended way to implement a command-line interface), this can be done with code like:

import click
import structlog
from safir.asyncio import run_with_asyncio
from safir.database import create_database_engine, initialize_database

from .config import config
from .schema import Base


# Definition of main omitted.


@main.command()
@click.option(
    "--reset", is_flag=True, help="Delete all existing database data."
)
@run_with_asyncio
async def init(reset: bool) -> None:
    logger = structlog.get_logger(config.logger_name)
    engine = create_database_engine(
        config.database_url, config.database_password
    )
    await initialize_database(
        engine, logger, schema=Base.metadata, reset=reset
    )
    await engine.dispose()

This code assumes that main is the Click entry point and .config provides a config object that contains the settings for the application, including the database URL and password as well as the normal Safir configuration settings.

The database URL may be a Pydantic Url type or a str. The database password may be a pydantic.SecretStr, a str, or None if no password is required by the database.

If it receives a connection error from the database, Safir will attempt the initialization five times, two seconds apart, to allow time for networking or a database proxy to start.

To drop and recreate all of the tables, pass the reset=True option to initialize_database.

Run database initialization on pod startup

The recommended pattern for Safir-based applications that use a database but do not use Alembic is to initialize the database every time the pod has been restarted.

Since initialization does nothing if the schema already exists, this is safe to do. It only wastes a bit of time during normal startup. This allows the application to be deployed on a new cluster without any special initialization step.

The easiest way to do this is to add a script (conventionally located in scripts/start-frontend.sh) that runs the init command and then starts the application with Uvicorn:

#!/bin/bash

set -eu

application init
uvicorn application.main:app --host 0.0.0.0 --port 8080

Replace application with the application entry point (the first line) and Python module (the second line). (These may be different if the application name contains dashes.)

Then, use this as the default command for the Docker image:

COPY scripts/start-frontend.sh /start-frontend.sh
CMD ["/start-frontend.sh"]

As a side effect, this will test database connectivity during pod startup and wait for network or a database proxy to be ready if needed, which avoids the need for testing database connectivity during the application startup.