Test framework needs a way to restrict the number of databases created in a multi-db setup

[Russell Keith-Magee]

Brett Hoerner ​raised an interesting point on django-dev about testing under multi-db:

If you have a master/slave setup, your DATABASES setup will describe multiple databases, but unless you're actually testing the master/slave setup, you don't need multiple databases to test general application logic - you just need a single data source.

Django should provide a way to say "only sync this subset of databases during testing".

The initial suggestion is to allow TEST_NAME=None to be interepreted as "don't create this database under testing".

[Jeff Balogh]

My solution was to write a master/slave engine that doesn't depend on having slaves: ​http://gist.github.com/271740. If no slaves are available, everything goes to default. I'm pretty sure it works, but I obviously haven't figured out how to test it automatically. ;)

One thing I discovered playing around with this: if TEST_NAME=None databases are skipped during test setup, but still exist in DATABASES, they'll all have to point at the same connection. If the "slave" databases are pointing at the master, but with separate connections, the slaves won't see anything happening in the master's transaction.

[Russell Keith-Magee]

RC1 of a patch allowing for test mirrors for testing master/slave setups

[Russell Keith-Magee]

(In [12289]) Fixed #12542 -- Added the TEST_MIRROR setting, allowing testing of read slave databases.