Configuration
*************

The following configuration values exist for Flask-SQLAlchemy. Flask-
SQLAlchemy loads these values from your main Flask config which can be
populated in various ways.  Note that some of those cannot be modified
after the engine was created so make sure to configure as early as
possible and to not modify them at runtime.


Configuration Keys
==================

A list of configuration keys currently understood by the extension:

+------------------------------------+---------------------------------------------+
| "SQLALCHEMY_DATABASE_URI"          | The database URI that should be used for    |
|                                    | the connection.  Examples:  *               |
|                                    | "sqlite:////tmp/test.db"  *                 |
|                                    | "mysql://username:password@server/db"       |
+------------------------------------+---------------------------------------------+
| "SQLALCHEMY_BINDS"                 | A dictionary that maps bind keys to         |
|                                    | SQLAlchemy connection URIs.  For more       |
|                                    | information about binds see Multiple        |
|                                    | Databases with Binds.                       |
+------------------------------------+---------------------------------------------+
| "SQLALCHEMY_ECHO"                  | If set to *True* SQLAlchemy will log all    |
|                                    | the statements issued to stderr which can   |
|                                    | be useful for debugging.                    |
+------------------------------------+---------------------------------------------+
| "SQLALCHEMY_RECORD_QUERIES"        | Can be used to explicitly disable or enable |
|                                    | query recording.  Query recording           |
|                                    | automatically happens in debug or testing   |
|                                    | mode.  See "get_debug_queries()" for more   |
|                                    | information.                                |
+------------------------------------+---------------------------------------------+
| "SQLALCHEMY_NATIVE_UNICODE"        | Can be used to explicitly disable native    |
|                                    | unicode support.  This is required for some |
|                                    | database adapters (like PostgreSQL on some  |
|                                    | Ubuntu versions) when used with improper    |
|                                    | database defaults that specify encoding-    |
|                                    | less databases.                             |
+------------------------------------+---------------------------------------------+
| "SQLALCHEMY_POOL_SIZE"             | The size of the database pool.  Defaults to |
|                                    | the engine's default (usually 5)            |
+------------------------------------+---------------------------------------------+
| "SQLALCHEMY_POOL_TIMEOUT"          | Specifies the connection timeout in seconds |
|                                    | for the pool.                               |
+------------------------------------+---------------------------------------------+
| "SQLALCHEMY_POOL_RECYCLE"          | Number of seconds after which a connection  |
|                                    | is automatically recycled. This is required |
|                                    | for MySQL, which removes connections after  |
|                                    | 8 hours idle by default.  Note that Flask-  |
|                                    | SQLAlchemy automatically sets this to 2     |
|                                    | hours if MySQL is used. Some backends may   |
|                                    | use a different default timeout value. For  |
|                                    | more information about timeouts see         |
|                                    | Timeouts.                                   |
+------------------------------------+---------------------------------------------+
| "SQLALCHEMY_MAX_OVERFLOW"          | Controls the number of connections that can |
|                                    | be created after the pool reached its       |
|                                    | maximum size.  When those additional        |
|                                    | connections are returned to the pool, they  |
|                                    | are disconnected and discarded.             |
+------------------------------------+---------------------------------------------+
| "SQLALCHEMY_TRACK_MODIFICATIONS"   | If set to "True", Flask-SQLAlchemy will     |
|                                    | track modifications of objects and emit     |
|                                    | signals.  The default is "None", which      |
|                                    | enables tracking but issues a warning that  |
|                                    | it will be disabled by default in the       |
|                                    | future. This requires extra memory and      |
|                                    | should be disabled if not needed.           |
+------------------------------------+---------------------------------------------+

New in version 0.8: The "SQLALCHEMY_NATIVE_UNICODE",
"SQLALCHEMY_POOL_SIZE", "SQLALCHEMY_POOL_TIMEOUT" and
"SQLALCHEMY_POOL_RECYCLE" configuration keys were added.

New in version 0.12: The "SQLALCHEMY_BINDS" configuration key was
added.

New in version 0.17: The "SQLALCHEMY_MAX_OVERFLOW" configuration key
was added.

New in version 2.0: The "SQLALCHEMY_TRACK_MODIFICATIONS" configuration
key was added.

Changed in version 2.1: "SQLALCHEMY_TRACK_MODIFICATIONS" will warn if
unset.


Connection URI Format
=====================

For a complete list of connection URIs head over to the SQLAlchemy
documentation under (Supported Databases).  This here shows some
common connection strings.

SQLAlchemy indicates the source of an Engine as a URI combined with
optional keyword arguments to specify options for the Engine. The form
of the URI is:

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

Many of the parts in the string are optional.  If no driver is
specified the default one is selected (make sure to *not* include the
"+" in that case).

Postgres:

   postgresql://scott:tiger@localhost/mydatabase

MySQL:

   mysql://scott:tiger@localhost/mydatabase

Oracle:

   oracle://scott:tiger@127.0.0.1:1521/sidname

SQLite (note that platform path conventions apply):

   #Unix/Mac (note the four leading slashes)
   sqlite:////absolute/path/to/foo.db
   #Windows (note 3 leading forward slashes and backslash escapes)
   sqlite:///C:\\absolute\\path\\to\\foo.db
   #Windows (alternative using raw string)
   r'sqlite:///C:\absolute\path\to\foo.db'


Using custom MetaData and naming conventions
============================================

You can optionally construct the "SQLAlchemy" object with a custom
"MetaData" object. This allows you to, among other things, specify a
custom constraint naming convention in conjunction with SQLAlchemy
0.9.2 or higher. Doing so is important for dealing with database
migrations (for instance using alembic as stated here. Here's an
example, as suggested by the SQLAlchemy docs:

   from sqlalchemy import MetaData
   from flask import Flask
   from flask_sqlalchemy import SQLAlchemy

   convention = {
       "ix": 'ix_%(column_0_label)s',
       "uq": "uq_%(table_name)s_%(column_0_name)s",
       "ck": "ck_%(table_name)s_%(constraint_name)s",
       "fk": "fk_%(table_name)s_%(column_0_name)s_%(referred_table_name)s",
       "pk": "pk_%(table_name)s"
   }

   metadata = MetaData(naming_convention=convention)
   db = SQLAlchemy(app, metadata=metadata)

For more info about "MetaData", check out the official docs on it.


Timeouts
========

Certain database backends may impose different inactive connection
timeouts, which interferes with Flask-SQLAlchemy's connection pooling.

By default, MariaDB is configured to have a 600 second timeout. This
often surfaces hard to debug, production environment only exceptions
like "2013: Lost connection to MySQL server during query".

If you are using a backend (or a pre-configured database-as-a-service)
with a lower connection timeout, it is recommended that you set
*SQLALCHEMY_POOL_RECYCLE* to a value less than your backend's timeout.
