Typesense sink fails on IPv6-only Fly.io .internal hosts due to Finch pool missing inet6 transport opts

[uncvrd]

Hi there, I am absolutely not an Elixir expert but I've been tinkering around trying to get Sequin deployed on Fly.io and I think I ran in to an ipv6 issue? See my exploration below...

Sequin’s Typesense sink fails to connect to a Typesense service on Fly.io private networking when the endpoint is an IPv6-only .internal hostname.

Fly.io .internal hostnames resolve to AAAA/6PN addresses only. In this setup, curl from the same Sequin machine succeeds, and Erlang can resolve/connect over IPv6 when explicitly told to use :inet6, but Sequin’s Req/Finch HTTP path fails with :nxdomain.

For example I see this

curl -sv "http://foundry-staging-typesense-v2.internal:8080/health"

IPv6: fdaa:0:3735:a7b:2ef:d5a3:2347:2
Connected ...
{"ok":true}

And then from erlang?

/home/app/prod/rel/sequin/bin/sequin rpc ':inet_res.lookup(~c"foundry-staging-typesense-v2.internal", :in, :a) |> IO.inspect()'
# []

/home/app/prod/rel/sequin/bin/sequin rpc ':inet_res.lookup(~c"foundry-staging-typesense-v2.internal", :in, :aaaa) |> IO.inspect()'
# [{64938, 0, 14133, 2683, 751, 54691, 9031, 2}]

As a result, when trying to connect I see

[Typesense] Failed to get collection: non-existing domain

Where the sink itself shows

[typesense]: Transport error: non-existing domain

Sequin configures a shared Finch pool via Sequin.Finch. The pool appears to be configured with only pool size/count, without IPv6 transport options.

For AAAA-only hosts, Finch needs IPv6 transport options

inet6: true

or Finch-level:

conn_opts: [transport_opts: [inet6: true]]

Because Sequin globally sets finch: Sequin.Finch, users cannot work around this per request with Req connect_options; Req rejects using both :finch and :connect_options.

So...is it feasible to add the inet6 option to the pool like this? Assuming this is correct?

pools = %{
  default: [
    size: pool_size,
    count: config!(:pool_count),
    conn_opts: [transport_opts: [inet6: true]] <---- this?
  ]
}

---

Edit: obviously this would fail for other sinks it's just the sink I was trying to build with currently