Did you ever have a chance to look into this? I’m experiencing the same issues connecting to Supabase on Fly. I’m moving an existing app from another service where it works fine, so this is definitely something Fly or IPv6-specific.

Using :verify_none for demonstration.

Supavisor, IPv4

database_url = "Supavisor DB connection string"

config :foundation, Foundation.Repo,
  ssl: true,
  ssl_opts: [
    verify: :verify_none
  ],
  url: database_url,
  pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10")
  # no ipv6 support enabled

Postgrex.Protocol (#PID<0.2411.0>) timed out because it was handshaking for longer than 15000ms
Postgrex.Protocol (#PID<0.2411.0>) failed to connect: ** (DBConnection.ConnectionError) ssl recv (idle): closed

Supavisor, IPv6

# export ECTO_IPV6="true" in env.sh.eex
database_url = "Supavisor DB connection string"
maybe_ipv6 = if System.get_env("ECTO_IPV6") in ~w(true 1), do: [:inet6], else: []

config :foundation, Foundation.Repo,
  ssl: true,
  ssl_opts: [
    verify: :verify_none
  ],
  url: database_url,
  pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10"),
  socket_options: maybe_ipv6

Postgrex.Protocol (#PID<0.2412.0>) failed to connect: ** (DBConnection.ConnectionError) tcp connect (aws-0-us-west-1.pooler.supabase.com:5432): non-existing domain - :nxdomain

Direct IPv6 connection URL

# export ECTO_IPV6="true" in env.sh.eex
database_url = "IPv6 direct connection string"
maybe_ipv6 = if System.get_env("ECTO_IPV6") in ~w(true 1), do: [:inet6], else: []

config :foundation, Foundation.Repo,
  ssl: true,
  ssl_opts: [
    verify: :verify_none,
  ],
  url: database_url,
  pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10"),
  socket_options: maybe_ipv6

Works, but runs out of connections after a restart or two:

Postgrex.Protocol (#PID<0.2409.0>) failed to connect: ** (Postgrex.Error) FATAL 53300 (too_many_connections) remaining connection slots are reserved for non-replication superuser connections

I reached out to Supabase when this was happening for me, and they suggested I up the pool size for my database in database settings. I bumped it from 15 to 25 and it solved it.