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sqlalchemy / doc / build / core / engines.rst

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Engine Configuration

The Engine is the starting point for any SQLAlchemy application. It's "home base" for the actual database and its :term:`DBAPI`, delivered to the SQLAlchemy application through a connection pool and a Dialect, which describes how to talk to a specific kind of database/DBAPI combination.

The general structure can be illustrated as follows:

sqla_engine_arch.png

Where above, an :class:`~sqlalchemy.engine.Engine` references both a :class:`~sqlalchemy.engine.base.Dialect` and a :class:`~sqlalchemy.pool.Pool`, which together interpret the DBAPI's module functions as well as the behavior of the database.

Creating an engine is just a matter of issuing a single call, :func:`.create_engine()`:

from sqlalchemy import create_engine
engine = create_engine('postgresql://scott:tiger@localhost:5432/mydatabase')

The above engine creates a :class:`.Dialect` object tailored towards PostgreSQL, as well as a :class:`.Pool` object which will establish a DBAPI connection at localhost:5432 when a connection request is first received. Note that the :class:`.Engine` and its underlying :class:`.Pool` do not establish the first actual DBAPI connection until the :meth:`.Engine.connect` method is called, or an operation which is dependent on this method such as :meth:`.Engine.execute` is invoked. In this way, :class:`.Engine` and :class:`.Pool` can be said to have a lazy initialization behavior.

The :class:`.Engine`, once created, can either be used directly to interact with the database, or can be passed to a :class:`.Session` object to work with the ORM. This section covers the details of configuring an :class:`.Engine`. The next section, :ref:`connections_toplevel`, will detail the usage API of the :class:`.Engine` and similar, typically for non-ORM applications.

Supported Databases

SQLAlchemy includes many :class:`~sqlalchemy.engine.base.Dialect` implementations for various backends. Dialects for the most common databases are included with SQLAlchemy; a handful of others require an additional install of a separate dialect.

See the section :ref:`dialect_toplevel` for information on the various backends available.

Engine Creation API

Keyword options can also be specified to :func:`~sqlalchemy.create_engine`, following the string URL as follows:

db = create_engine('postgresql://...', encoding='latin1', echo=True)

Database Urls

SQLAlchemy indicates the source of an Engine strictly via RFC-1738 style URLs, combined with optional keyword arguments to specify options for the Engine. The form of the URL is:

dialect+driver://username:password@host:port/database

Dialect names include the identifying name of the SQLAlchemy dialect which include sqlite, mysql, postgresql, oracle, mssql, and firebird. The drivername is the name of the DBAPI to be used to connect to the database using all lowercase letters. If not specified, a "default" DBAPI will be imported if available - this default is typically the most widely known driver available for that backend (i.e. cx_oracle, pysqlite/sqlite3, psycopg2, mysqldb). For Jython connections, specify the zxjdbc driver, which is the JDBC-DBAPI bridge included with Jython.

Postgresql

The Postgresql dialect uses psycopg2 as the default DBAPI:

# default
engine = create_engine('postgresql://scott:tiger@localhost/mydatabase')

# psycopg2
engine = create_engine('postgresql+psycopg2://scott:tiger@localhost/mydatabase')

# pg8000
engine = create_engine('postgresql+pg8000://scott:tiger@localhost/mydatabase')

# Jython
engine = create_engine('postgresql+zxjdbc://scott:tiger@localhost/mydatabase')

More notes on connecting to Postgresql at :ref:`postgresql_toplevel`.

MySQL

The MySQL dialect uses mysql-python as the default DBAPI:

# default
engine = create_engine('mysql://scott:tiger@localhost/foo')

# mysql-python
engine = create_engine('mysql+mysqldb://scott:tiger@localhost/foo')

# OurSQL
engine = create_engine('mysql+oursql://scott:tiger@localhost/foo')

More notes on connecting to MySQL at :ref:`mysql_toplevel`.

Oracle

cx_oracle is usualjy used here:

engine = create_engine('oracle://scott:tiger@127.0.0.1:1521/sidname')

engine = create_engine('oracle+cx_oracle://scott:tiger@tnsname')

More notes on connecting to Oracle at :ref:`oracle_toplevel`.

Microsoft SQL Server

There are a few drivers for SQL Server, currently PyODBC is the most solid:

engine = create_engine('mssql+pyodbc://mydsn')

More notes on connecting to SQL Server at :ref:`mssql_toplevel`.

SQLite

SQLite connects to file based databases. The same URL format is used, omitting the hostname, and using the "file" portion as the filename of the database. This has the effect of four slashes being present for an absolute file path:

# sqlite://<nohostname>/<path>
# where <path> is relative:
engine = create_engine('sqlite:///foo.db')

# or absolute, starting with a slash:
engine = create_engine('sqlite:////absolute/path/to/foo.db')

To use a SQLite :memory: database, specify an empty URL:

engine = create_engine('sqlite://')

More notes on connecting to SQLite at :ref:`sqlite_toplevel`.

Others

See :ref:`dialect_toplevel`, the top-level page for all dialect documentation.

URL API

Pooling

The :class:`.Engine` will ask the connection pool for a connection when the connect() or execute() methods are called. The default connection pool, :class:`~.QueuePool`, will open connections to the database on an as-needed basis. As concurrent statements are executed, :class:`.QueuePool` will grow its pool of connections to a default size of five, and will allow a default "overflow" of ten. Since the :class:`.Engine` is essentially "home base" for the connection pool, it follows that you should keep a single :class:`.Engine` per database established within an application, rather than creating a new one for each connection.

Note

:class:`.QueuePool` is not used by default for SQLite engines. See :ref:`sqlite_toplevel` for details on SQLite connection pool usage.

For more information on connection pooling, see :ref:`pooling_toplevel`.

Custom DBAPI connect() arguments

Custom arguments used when issuing the connect() call to the underlying DBAPI may be issued in three distinct ways. String-based arguments can be passed directly from the URL string as query arguments:

db = create_engine('postgresql://scott:tiger@localhost/test?argument1=foo&argument2=bar')

If SQLAlchemy's database connector is aware of a particular query argument, it may convert its type from string to its proper type.

:func:`~sqlalchemy.create_engine` also takes an argument connect_args which is an additional dictionary that will be passed to connect(). This can be used when arguments of a type other than string are required, and SQLAlchemy's database connector has no type conversion logic present for that parameter:

db = create_engine('postgresql://scott:tiger@localhost/test', connect_args = {'argument1':17, 'argument2':'bar'})

The most customizable connection method of all is to pass a creator argument, which specifies a callable that returns a DBAPI connection:

def connect():
    return psycopg.connect(user='scott', host='localhost')

db = create_engine('postgresql://', creator=connect)

Configuring Logging

Python's standard logging module is used to implement informational and debug log output with SQLAlchemy. This allows SQLAlchemy's logging to integrate in a standard way with other applications and libraries. The echo and echo_pool flags that are present on :func:`~sqlalchemy.create_engine`, as well as the echo_uow flag used on :class:`~sqlalchemy.orm.session.Session`, all interact with regular loggers.

This section assumes familiarity with the above linked logging module. All logging performed by SQLAlchemy exists underneath the sqlalchemy namespace, as used by logging.getLogger('sqlalchemy'). When logging has been configured (i.e. such as via logging.basicConfig()), the general namespace of SA loggers that can be turned on is as follows:

  • sqlalchemy.engine - controls SQL echoing. set to logging.INFO for SQL query output, logging.DEBUG for query + result set output.
  • sqlalchemy.dialects - controls custom logging for SQL dialects. See the documentation of individual dialects for details.
  • sqlalchemy.pool - controls connection pool logging. set to logging.INFO or lower to log connection pool checkouts/checkins.
  • sqlalchemy.orm - controls logging of various ORM functions. set to logging.INFO for information on mapper configurations.

For example, to log SQL queries using Python logging instead of the echo=True flag:

import logging

logging.basicConfig()
logging.getLogger('sqlalchemy.engine').setLevel(logging.INFO)

By default, the log level is set to logging.WARN within the entire sqlalchemy namespace so that no log operations occur, even within an application that has logging enabled otherwise.

The echo flags present as keyword arguments to :func:`~sqlalchemy.create_engine` and others as well as the echo property on :class:`~sqlalchemy.engine.Engine`, when set to True, will first attempt to ensure that logging is enabled. Unfortunately, the logging module provides no way of determining if output has already been configured (note we are referring to if a logging configuration has been set up, not just that the logging level is set). For this reason, any echo=True flags will result in a call to logging.basicConfig() using sys.stdout as the destination. It also sets up a default format using the level name, timestamp, and logger name. Note that this configuration has the affect of being configured in addition to any existing logger configurations. Therefore, when using Python logging, ensure all echo flags are set to False at all times, to avoid getting duplicate log lines.

The logger name of instance such as an :class:`~sqlalchemy.engine.Engine` or :class:`~sqlalchemy.pool.Pool` defaults to using a truncated hex identifier string. To set this to a specific name, use the "logging_name" and "pool_logging_name" keyword arguments with :func:`sqlalchemy.create_engine`.

Note

The SQLAlchemy :class:`.Engine` conserves Python function call overhead by only emitting log statements when the current logging level is detected as logging.INFO or logging.DEBUG. It only checks this level when a new connection is procured from the connection pool. Therefore when changing the logging configuration for an already-running application, any :class:`.Connection` that's currently active, or more commonly a :class:`~.orm.session.Session` object that's active in a transaction, won't log any SQL according to the new configuration until a new :class:`.Connection` is procured (in the case of :class:`~.orm.session.Session`, this is after the current transaction ends and a new one begins).