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How to run Django's test database only in memory?

My Django unit tests take a long time to run, so I'm looking for ways to speed that up. I'm considering installing an SSD, but I know that has its downsides too. Of course, there are things I could do with my code, but I'm looking for a structural fix. Even running a single test is slow since the database needs to be rebuilt / south migrated every time. So here's my idea...

Since I know the test database will always be quite small, why can't I just configure the system to always keep the entire test database in RAM? Never touch the disk at all. How do I configure this in Django? I'd prefer to keep using MySQL since that's what I use in production, but if SQLite 3 or something else makes this easy, I'd go that way.

Does SQLite or MySQL have an option to run entirely in memory? It should be possible to configure a RAM disk and then configure the test database to store its data there, but I'm not sure how to tell Django / MySQL to use a different data directory for a certain database, especially since it keeps getting erased and recreated each run. (I'm on a Mac FWIW.)

If you set your database engine to sqlite3 when you run your tests, Django will use a in-memory database.

I'm using code like this in my settings.py to set the engine to sqlite when running my tests:

if 'test' in sys.argv:
    DATABASE_ENGINE = 'sqlite3'

Or in Django 1.2:

if 'test' in sys.argv:
    DATABASES['default'] = {'ENGINE': 'sqlite3'}

And finally in Django 1.3 and 1.4:

if 'test' in sys.argv:
    DATABASES['default'] = {'ENGINE': 'django.db.backends.sqlite3'}

(The full path to the backend isn't strictly necessary with Django 1.3, but makes the setting forward compatible.)

You can also add the following line, in case you are having problems with South migrations:

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    • Yes, exactly. I should have put that in my answer! Combine that with SOUTH_TESTS_MIGRATE = False and your tests should be a lot faster.
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    • this is awesome. on newer django setups use this line: 'ENGINE': 'sqlite3' if 'test' in sys.argv else 'django.db.backends.mysql',
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    • @Tomasz Zielinski - Hmm, it depends what you are testing. But I totally agree that, at the end and from time to time, you need to run the tests with your real database (Postgres, MySQL, Oracle...). But running your tests in-memory with sqlite can save you a lot of time.
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    • I reverse -1 to +1: as I see it now, it's much faster to use sqlite for quick runs and switch to MySQL for e.g. final daily tests. (Note that I had to do a dummy edit to unlock voting)
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    • Caution with this "test" in sys.argv; it may trigger when you don't want it to, e.g. manage.py collectstatic -i test. sys.argv[1] == "test" is a more precise condition that should not have that problem.

I usually create a separate settings file for tests and use it in test command e.g.

python manage.py test --settings=mysite.test_settings myapp

It has two benefits:

  1. You don't have to check for test or any such magic word in sys.argv, test_settings.py can simply be

    from settings import *
    # make tests faster
    DATABASES['default'] = {'ENGINE': 'django.db.backends.sqlite3'}

    Or you can further tweak it for your needs, cleanly separating test settings from production settings.

  2. Another benefit is that you can run test with production database engine instead of sqlite3 avoiding subtle bugs, so while developing use

    python manage.py test --settings=mysite.test_settings myapp

    and before committing code run once

    python manage.py test myapp

    just to be sure that all test are really passing.

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    • Hi Anurag, I tried this but my other databases mentioned in the settings are also executed. I am not able to figure out the exact reason.
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    • It's a good approach, but not DRY. Django already knows that you're running tests. If you could 'hook into' this knowledge somehow, you'd be set. Unfortunately, I believe that requires extending the management command. It would probably make sense to make this generic in the core of the framework, by, for example having a setting MANAGEMENT_COMMAND set to the current command whenever manage.py is called, or something to that effect.
    • @DylanYoung you can make it dry by including main settings into test_settings and just overriding things you want for test.
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    • I like this approach. I have a bunch of different settings files and use them for different server environments but I had not thought about using this method to choose a different test database. Thanks for the idea.

MySQL supports a storage engine called "MEMORY", which you can configure in your database config (settings.py) as such:

    'USER': 'root',                      # Not used with sqlite3.
    'PASSWORD': '',                  # Not used with sqlite3.
    'OPTIONS': {
        "init_command": "SET storage_engine=MEMORY",

Note that the MEMORY storage engine doesn't support blob / text columns, so if you're using django.db.models.TextField this won't work for you.

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I can't answer your main question, but there are a couple of things that you can do to speed things up.

Firstly, make sure that your MySQL database is set up to use InnoDB. Then it can use transactions to rollback the state of the db before each test, which in my experience has led to a massive speed-up. You can pass a database init command in your settings.py (Django 1.2 syntax):

    'default': {
            'OPTIONS':{"init_command": "SET storage_engine=INNODB" } 

Secondly, you don't need to run the South migrations each time. Set SOUTH_TESTS_MIGRATE = False in your settings.py and the database will be created with plain syncdb, which will be much quicker than running through all the historic migrations.

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    • Great tip! It reduced my tests from 369 tests in 498.704s to 369 tests in 41.334s . This is more than 10 times faster!
    • @EdwardNewell Not exactly. But you can use --keep to persist the database and not require your complete set of migrations to be reapplied on every test run. New migrations will still run. If you're switching between branches frequently, it's easy to get into an inconsistent state though (you can revert new migrations before you switch by changing the database to the test database and running migrate, but it's a bit of a pain).

You can do double tweaking:

  • use transactional tables: initial fixtures state will be set using database rollback after every TestCase.
  • put your database data dir on ramdisk: you will gain much as far as database creation is concerned and also running test will be faster.

I'm using both tricks and I'm quite happy.

How to set up it for MySQL on Ubuntu:

$ sudo service mysql stop
$ sudo cp -pRL /var/lib/mysql /dev/shm/mysql

$ vim /etc/mysql/my.cnf
# datadir = /dev/shm/mysql
$ sudo service mysql start

Beware, it's just for testing, after reboot your database from memory is lost!

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    • Hmm. I've tried customising the local profile to give mysqld access to the /dev/shm/mysql path and its contents, but the service can only start in 'complain' mode (aa-complain command) and not 'enforce', for some reason... A question for another forum! What I can't understand is how there are no 'complaints' at all when it does work, implying that mysqld isn't violating the profile...
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    • thanks! works for me. I can't use sqlite, because I'm using features specific to mysql (full-text indexes). For ubuntu users, you'll have to edit your apparmor config to allow mysqld access to /dev/shm/mysql

Extending on Anurag's answer I simplified the process by creating the same test_settings and adding the following to manage.py

if len(sys.argv) > 1 and sys.argv[1] == "test":
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mysite.test_settings")
    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "mysite.settings")

seems cleaner since sys is already imported and manage.py is only used via command line, so no need to clutter up settings

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    • Caution with this "test" in sys.argv; it may trigger when you don't want it to, e.g. manage.py collectstatic -i test. sys.argv[1] == "test" is a more precise condition that should not have that problem.
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    • @keturn this way it generates an exception when running ./manage.py without arguments (eg to see which plugins are available, same as --help)
    • @DylanYoung Yes, that's exactly what I wanted Alvin to add to his solution but he isn't particularly interested in improving it. Anyway it looks more like a quick hack than the legit solution.

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