Postgres on Fly

Postgres, or PostgreSQL, is a powerful open-source object relational database system.

About Postgres on Fly

Postgres on Fly is a regular app, with an automated creation process and some extensions to simplify management. It relies on building blocks available to all Fly apps, like flyctl, volumes, private networking, health checks, logs, metrics, and more. The source code is available on GitHub to view and fork.

About Free Postgres on Fly

You can use Fly's free resource allowance in one place, or split it up. The following Postgres configurations fit within the free volume usage limit:

  • single node, 3gb volume (single database)
  • 2 x 1gb volumes (database in two regions, or a primary and replica in the same region)
  • 3 x 1gb volumes (database in three regions)

If you want to keep your whole project free, save some compute allowance for your other apps.

See also How to convert your not-free Postgres to free Postgres.

Creating a Postgres app

To create a Postgres cluster, use the flyctl postgres create command. The command will walk you through the creation with prompts for name, region, and VM resources.

flyctl postgres create
? App Name: c-pg-test
Automatically selected personal organization: Chris Nicoll
? Select region:  [Use arrows to move, type to filter]
> ams (Amsterdam, Netherlands)
  cdg (Paris, France)
  dfw (Dallas, Texas (US))
  ewr (Secaucus, NJ (US))
  fra (Frankfurt, Germany)
  gru (São Paulo)
? Select region: mia (Miami, Florida (US))
For pricing information visit:

During this process, you get to choose from several preset resource configurations for the app:

? Select configuration:  [Use arrows to move, type to filter]
> Development - Single node, 1x shared CPU, 256MB RAM, 1GB disk
  Development - Single node, 1x shared CPU, 512MB RAM, 10GB disk
  Production - Highly available, 1x shared CPU, 256MB RAM, 10GB disk
  Production - Highly available, 1x Dedicated CPU, 2GB RAM, 50GB disk
  Production - Highly available, 2x Dedicated CPU's, 4GB RAM, 100GB disk
  Specify custom configuration

The "Production" options give you a two-node cluster in a leader-replica configuration. A single-node "Development" instance can readily be scaled and expanded to more regions.

Creating postgres cluster c-pg-test in organization personal
Postgres cluster c-pg-test created
  Username:    postgres
  Password:    8a93cbc09798f3805056333072bd2b35be7eb634b13a05c3
  Hostname:    c-pg-test.internal
  Proxy Port:  5432
  PG Port: 5433
Save your credentials in a secure place, you won't be able to see them again!

Monitoring Deployment

1 desired, 1 placed, 1 healthy, 0 unhealthy [health checks: 3 total, 3 passing]
--> v0 deployed successfully

Connect to postgres
Any app within the personal organization can connect to postgres using the above credentials and the hostname "c-pg-test.internal."
For example: postgres://postgres:8a93cbc09798f3805056333072bd2b35be7eb634b13a05c3@c-pg-test.internal:5432

See the postgres docs for more information on next steps, managing postgres, connecting from outside fly:

After answering all the prompts, you'll see a message saying that the cluster is being created, followed by a deployment monitor watching as the app is launched. Take heed of the reminder to save your password in a safe place!

Your new Postgres cluster is ready to use once the deployment is complete.

Before we get any further, note that the automated Postgres creation process doesn't generate a fly.toml file in the working directory. This means that when you use flyctl commands with Fly Postgres apps, you'll have to specify the app, like so:

flyctl <command> -a <postgres-app-name>

Connecting to Postgres

How you connect to Postgres depends on the tools you're using. Connection string URIs are a common way to describe a connection to a postgres server.

Connection strings have the following format:


The output from flyctl postgres create contains all the values you need to make a connection string to your database.

Connecting to Postgres From Within Fly

As a application, your Postgres app is accessible through Fly's private networking. This means applications within the same organization can look up the app at appname.internal. This name, when looked up, can return one or more IPv6 addresses.

Connecting to Postgres From Outside Fly

On a Machine With flyctl Installed

To connect to your Postgres database from outside your Fly organization, you need a WireGuard connection. However, flyctl on your local machine can connect using user-mode WireGuard magic, without you having to set up your own WireGuard tunnel.

For a psql shell, you can just use the flyctl postgres connect command:

flyctl postgres connect -a <postgres-app-name>

You can also forward the server port to your local system with flyctl proxy:

flyctl proxy 5432 -a <postgres-app-name>

Then connect to your Postgres server at localhost:5432. Using psql again, as a trivial example, it would look like this:

psql postgres://postgres:<password>@localhost:5432

If you already have something else listening on port 5432, you can run this instead:

flyctl proxy 15432:5432 -a <postgres-app-name>

Then connect to localhost:15432.

As with all your apps, you can get a root console on your app's VM using flyctl ssh.

With Your Own WireGuard Tunnel

If you have an active WireGuard tunnel to your organization on our private network, you can connect to your Postgres cluster the same way you would from a Fly app within the same organization. For example, the following command would start an interactive terminal session on the cluster leader with psql:

psql postgres://postgres:secret123@appname.internal:5432

Attaching an App to a Postgres App

Using the superuser credentials, you can create databases, users, and whatever else you need for your apps. But we also have the flyctl postgres attach shortcut:

flyctl postgres attach --app <app-name> --postgres-app <postgres-app-name>

When you attach an app to Postgres, a number of things happen:

  • A database and user are created in the Postgres App. If the attached app in named "myapp", both the database and the user are named "myapp" too.
  • The user is allocated a generated password.

When the Attached app starts it will find an environment variable DATABASE_URL set to a Postgres connection URI with the username, password, host, port and dbname filled in.

Detaching an App From Postgres

Use flyctl postgres detach to remove postgres from the app.

flyctl postgres detach --app <app-name> --postgres-app <postgres-app-name>

This will revoke access to the attachment's role, remove the role, and remove the DATABASE_URL secret. The database will not be removed.

High Availability

Fly Postgres uses stolon for leader election and streaming replication between 2+ postgres servers. It provides a number of things, including a “keeper” that controls the postgres process, a "sentinel" that builds the cluster view, and a “proxy” that always routes connections to the current leader.

5433 is the port the keeper tells postgres to listen on. Connecting there goes straight to Postgres, though it might be the leader or the replica. Since clients need writes, the proxy is listening on the default 5432 port so clients are connected to the current leader.

If the leader becomes unhealthy (eg network or hardware issues), the proxy drops all connections until a new leader is elected. Once it’s ready, new connections go to the new leader automatically. The previoius leader's VM will be replaced by another VM which will rejoin the cluster as a replica.

In general, your clients should connect to port 5432.

Users / Roles

A Postgres cluster is configured with three users when created:

  • postgres - a role with superuser and login privileges that was created for you along with the cluster. Since the postgres role has superuser rights, it's recommended that you only use it for admin tasks and create new users with access restricted to the minimum necessary for applications
  • flypgadmin - this role is used internally by fly to configure and query the postgres cluster
  • repluser - this is the user replica servers us for replication from the leader

You can view a list of users using flyctl

flyctl postgres users list c-pg-test
Running flyadmin user-list
flypgadmin true      postgres
postgres   true      postgres
repluser   false     postgres


One Postgres cluster can host multiple databases

Listing Databases

You can view a list of databases with flyctl:

flyctl postgres db list c-pg-test
Running flyadmin database-list
postgres flypgadmin,postgres,repluser

Connection Examples

Connecting With Ruby (docs)

Ruby apps use the pg gem to connect to postgres.

require 'pg'

# Output a table of current connections to the DB
conn = PG.connect("postgres://postgres:secret123@postgresapp.internal:5432/yourdb")
conn.exec( "SELECT * FROM pg_stat_activity" ) do |result|
  puts "     PID | User             | Query"
  result.each do |row|
    puts " %7d | %-16s | %s " %
      row.values_at('pid', 'usename', 'query')

Connecting With Rails (docs)

Rails apps automatically connect to the database specified in the DATABASE_URL environment variable.

You can set this variable manually with flyctl secrets set

flyctl secrets set DATABASE_URL=postgres://postgres:secret123@postgresapp.internal:5432/yourdb

or by attaching the postgres database to your fly app.

Connecting With Go (docs)

pgx is the recommended driver for connecting to postgres. It supports the standard database/sql interface as well as directly exposing low level / high performance APIs.

First, add as a module depepdency.

go get

The following program will connect to the database in DATABASE_URL and run a query.

package main

import (

    _ ""

func main() {
    db, err := sql.Open("pgx", os.Getenv("DATABASE_URL"))
    if err != nil {
        fmt.Fprintf(os.Stderr, "Unable to connect to database: %v\n", err)
    defer db.Close()

    var greeting string
    err = db.QueryRow("select 'Hello, world!'").Scan(&greeting)
    if err != nil {
        fmt.Fprintf(os.Stderr, "QueryRow failed: %v\n", err)


Connecting With Node.js (docs)

You'll use the pg npm module to connect to postgres from a node.js app.

const { Client } = require('pg')
const client = new Client({connectionString: process.env.DATABASE_URL})

await client.connect()
const res = await client.query('SELECT $1::text as message', ['Hello world!'])
console.log(res.rows[0].message) // Hello world!
await client.end()

Connecting With Prisma – Node.js (docs)

Prisma is an open-source object-relational mapper (ORM) for Node.js and works with both JavaScript and TypeScript. It consists of 3 components:

  • Prisma Client - a type-safe query builder
  • Prisma Migrate - a data modeling and migration tool
  • Prisma Studio - a modern intuitive GUI for interacting with your database
Set up Prisma in your project

Install the Prisma CLI and Prisma Client dependencies in your project

npm i --save-dev prisma
npm i @prisma/client

Initialize Prisma in your project:

npx prisma init

This command does the following:

  • Creates a folder called prisma at the root of your project
  • Creates a .env file at the root of your project if it doesn't exist
  • Creates a schema.prisma file inside the prisma folder. This is the file that you will use to model your data

Update the DATABASE_URL in the .env to your PostgreSQL database


If you are working in a brownfield project, you can introspect your database to generate the models in your schema.prisma file:

npx prisma db pull

Assuming you have the following model in your schema.prisma file:

Add a model to your schema.prisma file:

datasource db {
  provider = "postgresql"
  url      = env("DATABASE_URL")

generator client {
  provider  = "prisma-client-js"

model Post {
  id       Int     @id @default(autoincrement())
  title    String
  content  String?

You can query your database using Prisma as follows:

import { PrismaClient } from '@prisma/client'

const prisma = new PrismaClient()

async function main() {
  const posts = await

  const newPost = await{
    data: {
      title: 'PostgreSQL on Fly',
      content: ''

  .catch((e) => {
    throw e
  .finally(async () => {
    await prisma.$disconnect()



You can use flyctl status to see a list of VMs and their status. The output for each VM includes it's role within the cluster.

$ flyctl status
  Name     = test-postgres
  Owner    = superfly
  Version  = 6
  Status   = running
  Hostname =

Deployment Status
  ID          = 044e9269-fabb-27a6-9d53-b25cd2f2e4c2
  Version     = v6
  Status      = successful
  Description = Deployment completed successfully
  Instances   = 2 desired, 2 placed, 2 healthy, 0 unhealthy

6b97fa06 6       iad    run     running (replica) 3 total, 3 passing 0        2021-02-10T23:31:49Z
da8141e7 6       iad    run     running (leader)  3 total, 3 passing 0        2021-02-10T23:21:21Z

To view the status of an individual VM:

$ flyctl vm status da8141e7
  ID            = da8141e7
  Version       = 6
  Region        = iad
  Desired       = run
  Status        = running (leader)
  Health Checks = 3 total, 2 passing, 1 critical
  Restarts      = 0
  Created       = 2021-02-10T23:21:21Z

Recent Events
2021-02-10T23:21:18Z Received   Task received by client
2021-02-10T23:21:48Z Task Setup Building Task Directory
2021-02-10T23:21:49Z Started    Task started by client

vm   app     critical [✗] system spent 31.9 of the last 10 seconds waiting for cpu
                      [✓] 20.53 GB (83.9%) free space on /data/
                      [✓] load averages: 0.29 0.35 0.50
                      [✓] memory: 0.9s waiting over the last 60s
                      [✓] io: 0.0s waiting over the last 60s

pg   app     passing  [✓] replication: currently leader
                      [✓] connections: 37 used, 3 reserved, 100 max

role app     passing  leader

Recent Logs
  2021-02-19T22:53:35Z [info] [HEALTHCHECK] [vm: failing] [✗] system spent 6.3 of the last 10 seconds waiting for memory↩︎[✗] system spent 27.5 of the last 10 seconds waiting for cpu↩︎[✓] 20.53 GB (83.9%) free space on /data/↩︎[✓] load averages: 0.29 0.35 0.58↩︎[✓] io: 0.1s waiting over the last 60s↩︎
  2021-02-19T22:54:35Z [info] [HEALTHCHECK] [vm: passing] [✓] 20.66 GB (84.4%) free space on /data/↩︎[✓] load averages: 0.28 0.31 0.28↩︎[✓] memory: 0.7s waiting over the last 60s↩︎[✓] cpu: 8.6s waiting over the last 60s↩︎[✓] io: 0.1s waiting over the last 60s↩︎


To view a list of health checks for a Fly Postgres app, run:

flyctl checks list -a pg-app
Health Checks for pg-app
vm   passing 6b97fa06   iad    SCRIPT 1m12s ago            [✓] 20.68 GB (84.5%) free
                                                           space on /data/ [✓] load
                                                           averages: 0.00 0.00 0.00 [✓]
                                                           memory: 0.0s waiting over the
                                                           last 60s [✓] cpu: 0.4s waiting
                                                           over the last 60s [✓] io: 0.0s
                                                           waiting over the last 60s
pg   passing 6b97fa06   iad    SCRIPT 5m36s ago            [✓] leader check:
                                                           connected [✓] replication lag:
                                                           246µs [✓] connections: 7 used, 3
                                                           reserved, 100 max
role passing 6b97fa06   iad    SCRIPT 2021-02-15T22:49:36Z replica
vm   passing da8141e7   iad    SCRIPT 14s ago              [✓] 20.66 GB (84.4%) free
                                                           space on /data/ [✓] load
                                                           averages: 0.31 0.37 0.32 [✓]
                                                           memory: 1.1s waiting over the
                                                           last 60s [✓] cpu: 9.4s waiting
                                                           over the last 60s [✓] io: 0.1s
                                                           waiting over the last 60s
pg   passing da8141e7   iad    SCRIPT 2m53s ago            [✓] replication: currently
                                                           leader [✓] connections: 31
                                                           used, 3 reserved, 100 max
role passing da8141e7   iad    SCRIPT 2021-02-15T22:49:38Z leader


Fly Postgres apps run several processes inside each VM, including postgres, stolon keeper, stolon sentinel, stolon proxy, and postgres_export. Each of those processes redirect STDOUT and STDERR to logs which you can view with flyctl logs.


Fly Postgres apps export metrics to prometheus which can be seen in the Metrics UI or queried from grafana.

The available metrics are



Scaling the Postgres Cluster

Scaling Vertically - Adding More VM Resources

You can change VM resources with the flyctl scale vm command:

$ flyctl scale vm dedicated-cpu-2x
Scaled VM Type to dedicated-cpu-1x
      CPU Cores: 1
         Memory: 2 GB

See Scaling VM Resources for more.

Scaling Horizontally - Adding More Replicas

flyctl volumes create pg_data --region syd --size 10
flyctl scale count 3


You can update a Postgres cluster, installed with flyctl postgres create, to the latest release using flyctl image update.

Check your current image with flyctl image show:

flyctl image show -a <postgres-app-name>

And upgrade with:

flyctl image update -a <postgres-app-name>