# Configuration

## Configuration Keys

Configuration is loaded from the Flask `app.config` when `SQLAlchemy.init_app()` is called. The configuration is not read again after that. Therefore, all configuration must happen before initializing the application.

flask_sqlalchemy.config.SQLALCHEMY_DATABASE_URI

  The database connection URI used for the default engine. It can be either a string or a SQLAlchemy `URL` instance. See below and Engine Configuration for examples.

  At least one of this and `SQLALCHEMY_BINDS` must be set.

  Changed in version 3.0: No longer defaults to an in-memory SQLite database if not set.

flask_sqlalchemy.config.SQLALCHEMY_ENGINE_OPTIONS

  A dict of arguments to pass to `sqlalchemy.create_engine()` for the default engine.

  This takes precedence over the `engine_options` argument to `SQLAlchemy`, which can be used to set default options for all engines.

  Changed in version 3.0: Only applies to the default bind.

  Added in version 2.4.

flask_sqlalchemy.config.SQLALCHEMY_BINDS

  A dict mapping bind keys to engine options. The value can be a string or a SQLAlchemy `URL` instance. Or it can be a dict of arguments, including the `url` key, that will be passed to `sqlalchemy.create_engine()`. The `None` key can be used to configure the default bind, but `SQLALCHEMY_ENGINE_OPTIONS` and `SQLALCHEMY_DATABASE_URI` take precedence.

  At least one of this and `SQLALCHEMY_DATABASE_URI` must be set.

  Added in version 0.12.

flask_sqlalchemy.config.SQLALCHEMY_ECHO

  The default value for `echo` and `echo_pool` for every engine. This is useful to quickly debug the connections and queries issued from SQLAlchemy.

  Changed in version 3.0: Sets `echo_pool` in addition to `echo`.

flask_sqlalchemy.config.SQLALCHEMY_RECORD_QUERIES

  If enabled, information about each query during a request will be recorded. Use `get_recorded_queries()` to get a list of queries that were issued during the request.

  Changed in version 3.0: Not enabled automatically in debug or testing mode.

flask_sqlalchemy.config.SQLALCHEMY_TRACK_MODIFICATIONS

  If enabled, all `insert`, `update`, and `delete` operations on models are recorded, then sent in `models_committed` and `before_models_committed` signals when `session.commit()` is called.

  This adds a significant amount of overhead to every session. Prefer using SQLAlchemy’s ORM Events directly for the exact information you need.

  Changed in version 3.0: Disabled by default.

  Added in version 2.0.

Changed in version 3.1: Removed `SQLALCHEMY_COMMIT_ON_TEARDOWN`.

Changed in version 3.0: Removed `SQLALCHEMY_NATIVE_UNICODE`, `SQLALCHEMY_POOL_SIZE`, `SQLALCHEMY_POOL_TIMEOUT`, `SQLALCHEMY_POOL_RECYCLE`, and `SQLALCHEMY_MAX_OVERFLOW`.

## Connection URL Format

See SQLAlchemy’s documentation on Engine Configuration for a complete description of syntax, dialects, and options.

A basic database connection URL uses the following format. Username, password, host, and port are optional depending on the database type and configuration.

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

Here are some example connection strings:

    # SQLite, relative to Flask instance path
    sqlite:///project.db

    # PostgreSQL
    postgresql://scott:tiger@localhost/project

    # MySQL / MariaDB
    mysql://scott:tiger@localhost/project

SQLite does not use a user or host, so its URLs always start with _three_ slashes instead of two. The `dbname` value is a file path. Absolute paths start with a _fourth_ slash (on Linux or Mac). Relative paths are relative to the Flask application’s `instance_path`.

## Default Driver Options

Some default options are set for SQLite and MySQL engines to make them more usable by default in web applications.

SQLite relative file paths are relative to the Flask instance path instead of the current working directory. In-memory databases use a static pool and `check_same_thread` to work across requests.

MySQL (and MariaDB) servers are configured to drop connections that have been idle for 8 hours, which can result in an error like `2013: Lost connection to MySQL server during query`. A default `pool_recycle` value of 2 hours (7200 seconds) is used to recreate connections before that timeout.

## Engine Configuration Precedence

Because Flask-SQLAlchemy has support for multiple engines, there are rules for which config overrides other config. Most applications will only have a single database and only need to use `SQLALCHEMY_DATABASE_URI` and `SQLALCHEMY_ENGINE_OPTIONS`.

- If the `engine_options` argument is given to `SQLAlchemy`, it sets default options for all engines. `SQLALCHEMY_ECHO` sets the default value for both `echo` and `echo_pool` for all engines.

- The options for each engine in `SQLALCHEMY_BINDS` override those defaults.

- `SQLALCHEMY_ENGINE_OPTIONS` overrides the `None` key in `SQLALCHEMY_BINDS`, and `SQLALCHEMY_DATABASE_URI` overrides the `url` key in that engine’s options.

## Timeouts

Certain databases may be configured to close inactive connections after a period of time. MySQL and MariaDB are configured for this by default, but database services may also configure this type of limit. This can result in an error like `2013: Lost connection to MySQL server during query`.

If you encounter this error, try setting `pool_recycle` in the engine options to a value less than the database’s timeout.

Alternatively, you can try setting `pool_pre_ping` if you expect the database to close connections often, such as if it’s running in a container that may restart.

See SQAlchemy’s docs on dealing with disconnects for more information.