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<h1>Databases</h1>

<p>A database is an abstraction on top of an operating system's file system to

ease creating, reading, updating, and deleting persistent data. The

database storage abstraction most commonly used in Python web development is

sets of relational tables. Alternative storage abstractions are explained in

the <a class="reference external" href="../no-sql-datastore.html">NoSQL</a> section of this guide.</p>

<p>Relational databases store all data in a series of tables. Interconnections

between the tables are specified as <em>foreign keys</em>.</p>

<p>Databases storage implementations vary in complexity. SQLite, a database

included with Python, creates a single file for all data per database.

Other databases such as Oracle, PostgreSQL, and MySQL have more complicated

persistence schemes while offering additional advanced features that are

useful for web application data storage.</p>

<p><a class="reference external" href="http://www.postgresql.org/">PostgreSQL</a> and

<a class="reference external" href="http://www.mysql.com/">MySQL</a> are two of the most common open source

databases for storing Python web application data.</p>

<p><a class="reference external" href="http://www.sqlite.org/">SQLite</a> is a database that is stored in a single

file on disk. SQLite is built into Python but is only built for access

by a single connection at a time. Therefore is highly recommended to not

<a class="reference external" href="https://docs.djangoproject.com/en/dev/ref/databases/#database-is-locked-errors">run a production web application with SQLite</a>.</p>

<div class="section" id="id1">

<h2>PostgreSQL</h2>

<p>PostgreSQL is the recommended relational database for working with Python

web applications. PostgreSQL's feature set, active development and stability

contribute to its usage as the backend for millions of applications live

on the Web today.</p>

<div class="section" id="postgresql-resources">

<h3>PostgreSQL resources</h3>

<p>This post on &quot;<a class="reference external" href="http://killtheyak.com/use-postgresql-with-django-flask/">Use PostgreSQL with Django or Flask</a>&quot;

is a great quickstart guide for either framework.</p>

<p><a class="reference external" href="http://postgresweekly.com/">PostgreSQL Weekly</a> is a weekly newsletter of

PostgreSQL content from around the web.</p>

<p><a class="reference external" href="https://www.braintreepayments.com/braintrust/scaling-postgresql-at-braintree-four-years-of-evolution">Scaling PostgreSQL at Braintree</a>. Fascinating

inside look at the evolution of the database's usage at Braintree.</p>

<p><a class="reference external" href="http://www.ibm.com/developerworks/library/os-postgresecurity/">Total security in a PostgreSQL database</a>.

There is no such thing as total security but this is a good article anyway.</p>

<p><a class="reference external" href="http://www.craigkerstiens.com/2012/10/01/understanding-postgres-performance/">Understanding PostgreSQL performance</a></p>

<p><a class="reference external" href="http://instagram-engineering.tumblr.com/post/40781627982/handling-growth-with-postgres-5-tips-from-instagram">Handling growth with Postgres</a>

provides 5 specific tips from Instagram's engineering team on how to scale

the design of your PostgreSQL database.</p>

<div class="section" id="id2">

<h2>MySQL</h2>

<p>MySQL is another viable open source database backend option for Python web

applications. MySQL has a slightly easier initial learning curve than

PostgreSQL. The database is deployed in production at some of the highest

trafficked sites such as

<a class="reference external" href="https://blog.twitter.com/2012/mysql-twitter">Twitter</a>,

<a class="reference external" href="https://www.facebook.com/notes/facebook-engineering/mysql-and-database-engineering-mark-callaghan/10150599729938920">Facebook</a>

and <a class="reference external" href="http://www.mysql.com/customers/">many others major organizations</a>.

However, since the company focused on MySQL development,

<a class="reference external" href="http://en.wikipedia.org/wiki/MySQL_AB">MySQL AB</a>, was

purchased by Sun Microsystems (which was in turn purchased by Oracle), there

have been major defections away from the database by

<a class="reference external" href="http://www.zdnet.com/wikipedia-moving-from-mysql-to-mariadb-7000008912/">Wikipedia</a>

and <a class="reference external" href="http://readwrite.com/2013/09/14/google-waves-goodbye-to-mysql-in-favor-of-mariadb">Google</a>.

MySQL remains a viable database option but I always recommend new Python

developers learn PostgreSQL if they do not already know MySQL.</p>

<div class="section" id="mysql-resources">

<h3>MySQL resources</h3>

<p><a class="reference external" href="http://designm.ag/tutorials/28-beginners-tutorials-for-learning-about-mysql-databases/">28 Beginner's Tutorials for Learning about MySQL Databases</a>

is a curated collection on various introductory MySQL topics.</p>

<p>This tutorial shows how to install <a class="reference external" href="http://www.cs.wcupa.edu/rkline/index/mysql-lin.html">MySQL on Ubuntu</a>.</p>

<div class="section" id="connecting-to-a-database-with-python">

<h2>Connecting to a database with Python</h2>

<p>To work with a relational database using Python, you need to use a code

library. The most common libraries for relational databases are:</p>

<p><a class="reference external" href="http://initd.org/psycopg/">psycopg2</a> for PostgreSQL</p>

<p><a class="reference external" href="https://pypi.python.org/pypi/MySQL-python/1.2.4">MySQLdb</a> for MySQL</p>

<p><a class="reference external" href="http://cx-oracle.sourceforge.net/">cx_Oracle</a> for Oracle</p>

<p>SQLite support is built into Python 2.7+ and therefore a separate library

is not necessary. Simply &quot;import sqlite3&quot; to begin interfacing with the

single file-based database.</p>

<div class="section" id="object-relational-mapping">

<h2>Object-Relational Mapping</h2>

<p>Object-relational mappers (ORMs) allow developers to access data from a

backend by writing Python code instead of SQL queries. Each web

application framework handles integrating ORMs differently.</p>

<p>Django provides an ORM with its core functionality. Flask leaves using an

ORM up to an extension, such as

<a class="reference external" href="http://pythonhosted.org/Flask-SQLAlchemy/">Flask-SQLALchemy</a>.</p>

<p>Developers can also use ORMs without a web framework, such as when

creating a data analysis tool or a batch script without a user interface.

Currently, the most widely used stand-alone ORM written for Python is

<a class="reference external" href="http://www.sqlalchemy.org/">SQLAlchemy</a>.</p>

<div class="section" id="database-third-party-services">

<h2>Database third-party services</h2>

<p>Numerous companies run scalable database servers as a hosted service.

Depending on the provider, there can be several advantages to using a

hosted database third-party service:</p>

<ol class="arabic simple">

<li>automated backups and recovery</li>

<li>tightened security configurations</li>

<li>easy vertical scaling</li>

<p><a class="reference external" href="http://aws.amazon.com/rds/">Amazon Relational Database Service (RDS)</a>

provides pre-configured MySQL and PostgreSQL instances. The instances can

be scaled to larger or smaller configurations based on storage and performance

needs.</p>

<p><a class="reference external" href="https://developers.google.com/cloud-sql/">Google Cloud SQL</a> is a service

with managed, backed up, replicated, and auto-patched MySQL instances. Cloud

SQL integrates with Google App Engine but can be used independently as well.</p>

<div class="section" id="database-resources">

<h3>Database resources</h3>

<p><a class="reference external" href="http://db-engines.com/en/ranking">DB-Engines</a> ranks the most popular

database management systems.</p>

<p><a class="reference external" href="http://dbweekly.com/">DB Weekly</a> is a new (as of Feb 2014) weekly roundup

of general database articles and resources.</p>

<br/>

Next read the

<a href="/wsgi-servers.html">wsgi servers</a> section.

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