python 48 lines · 3 tabs

Django database read-write split with routers

Priya Sharma Jan 2026

3 tabs

import random


class ReadWriteRouter:
    """Route reads to replicas, writes to primary."""

    def db_for_read(self, model, **hints):
        """Send reads to random replica."""
        return random.choice(['replica1', 'replica2'])

    def db_for_write(self, model, **hints):
        """Send all writes to primary."""
        return 'default'

    def allow_relation(self, obj1, obj2, **hints):
        """Allow relations between objects in same database."""
        db_set = {'default', 'replica1', 'replica2'}
        if obj1._state.db in db_set and obj2._state.db in db_set:
            return True
        return None

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        """Only run migrations on primary."""
        return db == 'default'

DATABASES = {
    'default': {  # Primary for writes
        'ENGINE': 'django.db.backends.postgresql',
        'NAME': 'mydb',
        'HOST': 'primary.db.example.com',
    },
    'replica1': {  # Read replica
        'ENGINE': 'django.db.backends.postgresql',
        'NAME': 'mydb',
        'HOST': 'replica1.db.example.com',
    },
    'replica2': {  # Read replica
        'ENGINE': 'django.db.backends.postgresql',
        'NAME': 'mydb',
        'HOST': 'replica2.db.example.com',
    },
}

DATABASE_ROUTERS = ['core.routers.ReadWriteRouter']

# Read from primary (for consistency)
user = User.objects.using('default').get(id=1)

# Explicitly use replica
posts = Post.objects.using('replica1').filter(status='published')

3 files · python Explain with highlit

Database routers enable read-write splitting for scaling. I direct reads to replicas and writes to primary. The router examines hints and model to make routing decisions. For read-heavy apps, this distributes load across multiple databases. I use using() to force specific databases when needed. Connection pooling is important with multiple databases. I monitor replica lag to ensure data consistency. For critical reads, I can force primary database. This architecture scales Django beyond single-database limits.

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