Database routing

One of this libraries goals was to create easy and extendable automatic database routing.

Motivation

In original infi.clickhouse-orm you had to explicitly create Database objects and set database to each query with objects_in(db) method. But common projects use a quite little number of database connections. As a result, it's easier to setup routing once and use it as django does.
Unlike traditional relational databases, ClickHouse has per table replication. This means that:

Each model can have it's own replication scheme
Some migration queries are replicated automatically, others - not.
To make system more extendable we need default routing, per model routing and router class for complex cases.

Introduction

All database connections are defined in CLICKHOUSE_DATABASES setting. Each connection has it's alias name to refer with. If no routing is configured, CLICKHOUSE_DEFAULT_DB_ALIAS is used.

Router

Router is a class, defining 3 methods:

def db_for_read(self, model: ClickHouseModel, **hints) -> str
Returns database alias to use for given model for SELECT queries.
def db_for_write(self, model: ClickHouseModel, **hints) -> str
Returns database alias to use for given model for INSERT queries.
def allow_migrate(self, db_alias: str, app_label: str, operation: Operation, model: Optional[ClickHouseModel] = None, **hints: dict) -> bool Checks if migration operation should be applied in django application app_label on database db_alias. Optional model field can be used to determine migrations on concrete model.

By default CLICKHOUSE_DATABASE_ROUTER is used. It gets routing information from model fields, described below.

ClickHouseModel routing attributes

Default database router reads routing settings from model attributes.

from django_clickhouse.configuration import config
from django_clickhouse.clickhouse_models import ClickHouseModel

class MyModel(ClickHouseModel):
    # Servers, model is replicated to. 
    # Router takes random database to read or write from.
    read_db_aliases = (config.DEFAULT_DB_ALIAS,)
    write_db_aliases = (config.DEFAULT_DB_ALIAS,)
    
    # Databases to perform replicated migration queries, such as ALTER TABLE.
    # Migration is applied to random database from the list.
    migrate_replicated_db_aliases = (config.DEFAULT_DB_ALIAS,)
    
    # Databases to perform non-replicated migrations (CREATE TABLE, DROP TABLE).
    # Migration is applied to all databases from the list.
    migrate_non_replicated_db_aliases = (config.DEFAULT_DB_ALIAS,)

Settings database in QuerySet

Database can be set in each [QuerySet](# TODO) explicitly by using one of methods:

With infi approach: MyModel.objects_in(db_object).filter(id__in=[1,2,3]).count()
With using() method: MyModel.objects.filter(id__in=[1,2,3]).using(db_alias).count()

If no explicit database is provided, database connection to use is determined lazily with router's db_for_read or db_for_write method, depending on query type.

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