Django Horizon

Purpose

Simple database sharding (horizontal partitioning) library for Django applications.

• Free software: MIT license
• Documentation: https://django-horizon.readthedocs.io.
• Inspired by django-sharding. Thank you so much for your cool solution :)

Features

• Shard (horizontal partitioning) by some ForeignKey field like user account.

Installation

To install Django Horizon, run this command in your terminal:

$ pip install django-horizon

This is the preferred method to install Django Horizon, as it will always install the most recent stable release.

If you don't have pip installed, this Python installation guide can guide you through the process.

Usage

Setup

Add database router configuration in your settings.py:

Horizontal database groups and a metadata store

HORIZONTAL_CONFIG = {
    'GROUPS': {
        'group1': {  # The name of database horizontal partitioning group
            'DATABASES': {
                1: {
                    'write': 'member1-primary',
                    'read': ['member1-replica-1', 'member1-replica-2'],  # Pick randomly by router
                },
                2: {
                    'write': 'member2-primary',
                    'read': ['member2-replica'],
                },
                3: {
                    'write': 'a3',  # Used by 'read' too
                },
            },
            'PICKABLES': [2, 3],  # Group member keys to pick new database
        },
    },
    'METADATA_MODEL': 'app.HorizontalMetadata',  # Metadata store for horizontal partition key and there database
}

Database router

DATABASE_ROUTERS = (
    'horizon.routers.HorizontalRouter',
    ...
)

Example models

Horizontal partitioning by user

Metadata store

from horizon.models import AbstractHorizontalMetadata

class HorizontalMetadata(AbstractHorizontalMetadata):
    pass

In the example, metadata store save followings.

• group: Group name for horizontal partitioning.
• key: Determines the distribution of the table's records among the horizontal partitioning group.
• index: Choosed database index in horizontal partitioning groups.

Sharded model

from django.conf import settings

from horizon.manager import HorizontalManager  # For Django<1.10
from horizon.models import AbstractHorizontalModel


class SomeLargeModel(AbstractHorizontalModel):
    user = models.ForeignKey(
        settings.AUTH_USER_MODEL,
        on_delete=models.DO_NOTHING,
        db_constraint=False,  # May be using anothor database
    )
    ...

    objects = HorizontalManager()  # For Django<1.10

    class Meta(object):
        horizontal_group = 'group1'  # Group name
        horizontal_key = 'user'  # Field name to use group key

In many cases use UUIDField field for id. The AbstractHorizontalModel uses UUIDField as a them id field in default.

Using a model

from django.contrib.auth import get_user_model


user_model = get_user_model()
user = user_model.objects.get(pk=1)

# Get by foreign instance
SomeLargeModel.objects.filter(uses=user)

# Get by foreign id
SomeLargeModel.objects.filter(uses_id=user.id)

Model limitations

• django.db.utils.IntegrityError occured when not specify horizontal key field to filter

  SomeLargeModel.objects.all()

• Cannot lookup by foreign key field, cause there are other (like default) database

  list(user.somelargemodel_set.all())