Version 22 (modified by simon, 15 years ago) (diff)

Added note about moving models between databases and making models aware of where they were loaded from

Multiple Database Support

The most recent (September 2008) discussion about multi-db support can be seen on this thread:


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.

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, will be able to automatically remember the correct database connection. The ability to over-ride this (e.g. with'archive')) would be useful for moving models between databases.


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