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# 04.07.2026 PostgreSQL Read Replica Routing

> Route read-only queries to a PostgreSQL read replica to reduce load on the primary under high ingestion.

**Available in arize-phoenix 14.0.0+**

Phoenix now routes read-only queries to an optional PostgreSQL read replica when `PHOENIX_SQL_DATABASE_READ_REPLICA_URL` is set. This reduces CPU, I/O, and connection pool pressure on the primary database under high span ingestion load.

## Configuration

Set both environment variables before starting the server:

```bash theme={null}
export PHOENIX_SQL_DATABASE_URL="postgresql://user:pass@primary-host:5432/phoenix"
export PHOENIX_SQL_DATABASE_READ_REPLICA_URL="postgresql://user:pass@replica-host:5432/phoenix"

phoenix serve
```

When `PHOENIX_SQL_DATABASE_READ_REPLICA_URL` is not set, Phoenix falls back to the primary for all queries — no configuration change is required for existing deployments.

## What Routes to the Replica

The following are routed to the read replica when configured:

* **Dataloaders** — span and trace attribute lookups
* **GraphQL query resolvers** — all read-only queries
* **REST read endpoints** — spans, traces, and sessions
* **Generative model store daemon** — periodic model list refresh

Writes (span ingestion, mutations, migrations) always go to the primary.

## Notes

* `PHOENIX_SQL_DATABASE_READ_REPLICA_URL` is only supported for PostgreSQL. Setting it with a SQLite database logs a warning and is ignored.
* The replica connection uses the same `asyncpg` driver as the primary.
* Replication lag is not managed by Phoenix — reads may reflect slightly stale data depending on your PostgreSQL replication setup.