I’m trying to direct my read queries to my read replica in a production setup. Django allows defining multiple databases, so I added two databases like:

DATABASES = {
    "default": {
        "NAME": "ironbank",
        "ENGINE": "django_postgrespool2",
        "USER": "postgres",
        "PASSWORD": os.getenv("DATABASE_PASSWORD", "postgres"),
        "HOST": os.getenv("DATABASE_HOST", "localhost"),
        "PORT": os.getenv("DATABASE_PORT", "5432"),
        "CONN_MAX_AGE": 0,
    },
    "replica": {
        "NAME": "ironbank_replica",
        "ENGINE": "django_postgrespool2",
        "USER": "postgres",
        "PASSWORD": os.getenv("DATABASE_PASSWORD", "postgres"),
        "HOST": os.getenv("REPLICA_DATABASE_HOST", "localhost"),
        "PORT": os.getenv("REPLICA_DATABASE_PORT", "5432"),
        "TEST": {
            "MIRROR": "default",
        },
        "CONN_MAX_AGE": 0,
    },
}

Here I’m using two different databases to test it setup locally.

I also have a database router configured as

file: settings.py
DATABASE_ROUTERS = ["ironbank.router.PrimaryReplicaRouter"]
---
file: ironbank/router/PrimaryReplicaRouter.py
class PrimaryReplicaRouter:
    @staticmethod
    def db_for_read(model, **hints):
        """
        Reads go to a randomly-chosen replica.
        """
        # some model specific logic here, but removed for simplicity
        return "replica"

    @staticmethod
    def db_for_write(model, **hints):
        """
        Writes always go to primary.
        """
        return "default"

    @staticmethod
    def allow_relation(obj1, obj2, **hints):
        """
        Relations between objects are allowed if both objects are
        in the primary/replica pool.
        """
        return True

    @staticmethod
    def allow_migrate(db, app_label, model_name=None, **hints):
        """
        Allow migrations on default database only.
        """
        return db == "default"

Findings

• I’ve been using shell_plus and manual data inspection to check the result being returned by the query is from master or replica. shell_plus directly shows the default database being used for all queries. Doing manual query has also verified data to be coming from master.
• I suspect django is not considering the DATABASE_ROUTERS configuration, as I filled in a random string, and got no error when doing a database query.
• Even doing a syntax error in the class definition tells me django never actually read the class.
• Django version is 3.2, which supports the configuration.
• Using .using("replica") also did not work. As per docs, it overrides database router definition, and that doesn’t work either