mirror of
https://github.com/carrotquest/django-clickhouse.git
synced 2024-11-25 10:33:47 +03:00
78 lines
3.8 KiB
Markdown
78 lines
3.8 KiB
Markdown
# Migrations
|
|
Migration system allows to make migrate ClickHouse table schema based on `ClickHouseModel`.
|
|
Library migrations are based on [infi.clickhouse_orm migration system](https://github.com/Infinidat/infi.clickhouse_orm/blob/develop/docs/schema_migrations.md),
|
|
but makes it a little bit more django-like.
|
|
|
|
## File structure
|
|
Each django app can have optional `clickhouse_migrations` package.
|
|
This is a default package name, it can be changed with [CLICKHOUSE_MIGRATIONS_PACKAGE](configuration.md#clickhouse_migrations_package) setting.
|
|
|
|
Package contains py files, starting with 4-digit number.
|
|
A number gives an order in which migrations will be applied.
|
|
|
|
Example:
|
|
```
|
|
my_app
|
|
>> clickhouse_migrations
|
|
>>>> __init__.py
|
|
>>>> 0001_initial.py
|
|
>>>> 0002_add_new_field_to_my_model.py
|
|
>> clickhouse_models.py
|
|
>> urls.py
|
|
>> views.py
|
|
```
|
|
|
|
## Migration files
|
|
Each file must contain a `Migration` class, inherited from `django_clickhouse.migrations.Migration`.
|
|
The class should define an `operations` attribute - a list of operations to apply one by one.
|
|
Operation is one of [operations, supported by infi.clickhouse-orm](https://github.com/Infinidat/infi.clickhouse_orm/blob/develop/docs/schema_migrations.md).
|
|
|
|
```python
|
|
from django_clickhouse import migrations
|
|
from my_app.clickhouse_models import ClickHouseUser
|
|
|
|
class Migration(migrations.Migration):
|
|
operations = [
|
|
migrations.CreateTable(ClickHouseUser)
|
|
]
|
|
```
|
|
|
|
## MigrationHistory ClickHouseModel
|
|
This model stores information about applied migrations.
|
|
By default, library uses `django_clickhouse.migrations.MigrationHistory` model,
|
|
but this can be changed using `CLICKHOUSE_MIGRATION_HISTORY_MODEL` setting.
|
|
For instance, if you want to make it replicated, you have to redeclare tables engine.
|
|
|
|
MigrationHistory model is stored in default database.
|
|
|
|
|
|
## Automatic migrations
|
|
When library is installed, it tries applying migrations every time,
|
|
you call [django migrate](https://docs.djangoproject.com/en/3.0/ref/django-admin/#django-admin-migrate). If you want to disable this, use [CLICKHOUSE_MIGRATE_WITH_DEFAULT_DB](configuration.md#clickhouse_migrate_with_default_db) setting.
|
|
|
|
By default migrations are applied to all [CLICKHOUSE_DATABASES](configuration.md#clickhouse_databases), which have no flags:
|
|
* `'migrate': False`
|
|
* `'readonly': True`
|
|
|
|
Note: migrations are only applied, with django `default` database.
|
|
So if you call `python manage.py migrate --database=secondary` they wouldn't be applied.
|
|
|
|
## Migration algorithm
|
|
- Get a list of databases from `CLICKHOUSE_DATABASES` setting. Migrate them one by one.
|
|
- Find all django apps from `INSTALLED_APPS` setting, which have no `readonly=True` attribute and have `migrate=True` attribute. Migrate them one by one.
|
|
* Iterate over `INSTAALLED_APPS`, searching for [clickhouse_migrations package](#file-structure)
|
|
* If package was not found, skip app.
|
|
* Get a list of migrations applied from [MigrationHistory model](#migrationhistory-clickhousemodel)
|
|
* Get a list of unapplied migrations
|
|
* Get [Migration class](#migration-files) from each migration and call it `apply()` method
|
|
* `apply()` iterates operations, checking if it should be applied with [router](routing.md)
|
|
* If migration should be applied, it is applied
|
|
* Mark migration as applied in [MigrationHistory model](#migrationhistory-clickhousemodel)
|
|
|
|
## Security notes
|
|
1) ClickHouse has no transaction system, as django relational databases.
|
|
As a result, if migration fails, it would be partially applied and there's no correct way to rollback.
|
|
I recommend to make migrations as small as possible, so it should be easier to determine and correct the result if something goes wrong.
|
|
2) Unlike django, this library is enable to unapply migrations.
|
|
This functionality may be implemented in the future.
|