Fly.io Core Workflow B: Postgres, Volumes & Networking Skill

Fly.io Core Workflow B: Postgres, Volumes & Networking

Overview

Set up Fly Postgres, persistent Fly Volumes, and private networking between apps. Fly Postgres runs as a regular Fly app with automated replication. Volumes provide persistent NVMe storage attached to specific machines.

Instructions

Step 1: Create Fly Postgres

# Create a Postgres cluster
fly postgres create --name my-db --region iad --vm-size shared-cpu-1x --volume-size 10

# Attach to your app (sets DATABASE_URL secret automatically)
fly postgres attach my-db -a my-app

# Connect directly
fly postgres connect -a my-db
# psql> SELECT version();

# Proxy to local machine for dev tools
fly proxy 5432 -a my-db
# Now connect: psql postgres://postgres:password@localhost:5432

Step 2: Create Persistent Volumes

# Create a volume (same region as your machine)
fly volumes create data --size 10 --region iad -a my-app

# List volumes
fly volumes list -a my-app

# Mount in fly.toml

# fly.toml
[mounts]
  source = "data"
  destination = "/data"

# Deploy to pick up mount
fly deploy

# Verify mount inside machine
fly ssh console -C "df -h /data"

Step 3: Private Networking (6PN)

# Apps in the same org can reach each other via .internal DNS
# my-app can reach my-db at: my-db.internal:5432

# Internal DNS format: <app-name>.internal
# Machine-specific: <machine-id>.vm.<app-name>.internal

# Example: connect from app code
DATABASE_URL=postgres://postgres:password@my-db.internal:5432/my_db

// Access internal services (no public internet)
const dbUrl = `postgres://postgres:${process.env.DB_PASSWORD}@my-db.internal:5432/mydb`;
const apiUrl = `http://my-api.internal:3000/health`;  // Internal HTTP

Step 4: Postgres Backups and Failover

# List backups
fly postgres barman list-backups -a my-db

# Create manual backup
fly postgres barman backup -a my-db

# Check replication status
fly postgres barman check -a my-db

# Failover to standby (if primary fails)
fly postgres failover -a my-db

Error Handling

| Error | Cause | Solution | |-------|-------|----------| | volume not found | Volume in different region | Create volume in same region as machine | | connection refused on .internal | App not running | Check fly status -a target-app | | database does not exist | Not yet created | Run CREATE DATABASE mydb; via fly postgres connect | | disk full | Volume full | Extend: fly volumes extend vol_xxx --size 20 |

Resources

Next Steps

For common errors, see flyio-common-errors.

Agent Skills: Fly.io Core Workflow B: Postgres, Volumes & Networking

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