# Configure backends

A `default` instance of every interface is built from the flat settings: `DATABASE_URL`, `CACHE_URL`, `BROKER_URL`, `STORAGE_*`, `EVENT_BUS_BACKEND` and `NOTIFIER_BACKEND`. For most applications that is all the configuration there is.

The composition root (`create_app`, the worker, `sandloom migrate`, the test harness) builds the registry. There is no implicit default registry: `cache()`, `notifier()` or any accessor with none bound raises, in every environment, with a hint naming those roots.

The `INFRA` setting (JSON) adds named instances or overrides drivers and options:

```
{"database": {"analytics": {"driver": "sqlalchemy",
                            "url": "postgresql+asyncpg://.../analytics"}},
 "cache":    {"sessions":  {"driver": "redis", "url": "redis://.../3"}}}
```

`NOTIFIER_BACKEND` accepts only the drivers Sandloom ships; a driver package is selected through `INFRA`. Keeping the driver merges new options over the old ones, and naming another driver starts with only the options given here. Driver options are schema-validated; an unknown driver, instance or option fails startup. The `sqlalchemy` option `tenant_rls: true` turns on PostgreSQL row-level security for that instance, with `system_url` / `require_system_channel` for its system channel (see [Multi-tenancy](<https://sandloom.dev/docs/modules/database/multi-tenancy/index.md>)). PostgreSQL URLs always name their driver: `postgresql+asyncpg://` for the app; migrations derive `postgresql+psycopg2://` from it. Never rely on a bare `postgresql://`.

`/diagnostics/infra` reports the configured capabilities, instances, stability, registered drivers and per-instance health.

## Use a named instance

Modules ask for an instance by name, never by engine or connection string:

```
from typing import Annotated
from sqlalchemy.ext.asyncio import AsyncSession

from sandloom.sdk.infra import cache
from sandloom.sdk.web import Database

@router.get("/reports/latest")
async def latest_report(
    analytics: Annotated[AsyncSession, Database("analytics")],
) -> ReportOut:
    await cache("sessions").set("reports:last", "...", ttl_seconds=60)
    return await services.load(analytics)
```

Every setting is in the [configuration reference](<https://sandloom.dev/docs/reference/configuration/index.md>). See also [what is included](<https://sandloom.dev/docs/infrastructure/#what-is-included>).