Added migrations documentation

docs/migrations.md

@@ -1,2 +1,71 @@
 # Migrations
-Migration system is based on 
+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#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
+```
+
+## 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
+
+class Migration(migrations.Migration):
+    operations = [
+        migrations.CreateTable(ClickHouseTestModel),
+        migrations.CreateTable(ClickHouseCollapseTestModel)
+    ]
+```
+
+## MigrationHistory ClickHouse model
+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 `python manage.py migrate`. If you want to disable this, use [CLICKHOUSE_MIGRATE_WITH_DEFAULT_DB](configuration.md#migrate_with_default_db) settings.
+
+Note: migrations are only applied, when `default` database is migrated.
+So if you call `python manage.py migrate --database=secondary` they wouldn't be applied.
+
+## Migration algorithm
+- Gets a list of databases from `CLICKHOUSE_DATABASES` settings. Migrate them one by one.  
+  - Find all django apps from `INSTALLED_APPS` settings, which have no `readonly=True` setting and have `migrate=True` settings.  
+    Migrate them one by one.  
+    * Iterate over `INSTAALLED_APPS`, searching for `clickhouse_migrations` package  
+    * If package was not found, skip app.  
+    * Get a list of migrations applied from `MigrationHistory` model   
+    * Get a list of unapplied migrations
+    * Get `Migration` class from each migration and call it `apply()` method
+    * `apply()` iterates operations, checking if it should be applied with [router](router.md)
+    * If migration should be applied, it is applied
+    * Mark migration as applied in `MigrationHistory` model
+
+## 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.