= Multiple Database Support =

The most recent (September 2008) discussion about multi-db support can be seen on this thread: http://groups.google.com/group/django-developers/browse_thread/thread/9f0353fe0682b73

== Requirements ==

An ideal solution would address all of the following:

 * '''Different models/applications living on different databases''', for example a 'blog' application on db1 and a forum application on db2. This should include the ability to assign a different database to an existing application without modifying it, e.g. telling Django where to keep the django.contrib.auth.User table.
 * '''Master-slave replication''', where writes go to a single master while reads are spread across one or more slaves. Due to replication lag it may sometimes be necessary for reads that directly follow a related write to be directed to the master.
 * '''Talking to existing or legacy databases''' while still allowing newer functionality to be developed on a different database dedicated solely to Django.
 * '''Sharding''', where rows within a single model are spread across multiple databases for improved write performance.
 * '''Moving models between databases''' - e.g. for converting from MySQL to Postgres, or for shuffling items between shards.
 * '''Transparently handling database failure''' - many large applications like to handle unavailable databases in their application code, switching to another slave if the first one is unavailable for example. See [http://blog.digg.com/?p=213 Digg Database Architecture] for a high profile example.

== Problems to solve ==

 * '''Connection definitions''' - how are multiple database connections defined? Does this new method replace Django's existing DATABASE_* family of settings?
 * '''Connection selection''' - what is the API for telling Django which database a query should be executed against?
 * '''Associating connections with models''' - for the common case where a model is assigned to a different database, how is that assignment made? How can existing apps such as contrib.auth be assigned to a different database in a clean way? This is one facet of the connection selection problem.
 * '''Related managers''' - how do we deal with the case when the User model lives on one database but its related !BlogEntrys live on a different database?
 * '''Joins across different databases''' - do we try to get these working? If not, how do we detect them and what kind of error or warning do we present?
 * '''Should models be aware of which database they were loaded from?''' - if so, model.save() will be able to automatically remember the correct database connection. The ability to over-ride this (e.g. with `model.save(using='archive')`) would be useful for moving models between databases.

== Implementations ==

 * [http://groups.google.com/group/django-developers/msg/078099f199bdfb79 Jan Oberst demonstrates part of his sharding API]
 * [http://softwaremaniacs.org/soft/mysql_replicated/en/ mysql_replicated] implements master/slave replication triggered by HTTP GET v.s. POST as a custom database backend, works against Django 1.0 with no patching required.
 * [http://trac.woe-beti.de/ Django Multiple Database support] - most recent (last modified May 2008)
 * PreviousMultipleDatabaseBranch - obsolete branch, last modified 2006