HTTP/2

Use TLS and put Http2 before Http1. Gun then selects HTTP/2 when possible and falls back to HTTP/1.1 when necessary.

How fallback works

TLS ALPN (Application-Layer Protocol Negotiation) selects the protocol. When you list [Http2, Http1], Gun advertises h2 and http/1.1 in the TLS ClientHello. The server selects one. connection.await_up returns the selected protocol. If the server advertises only HTTP/1.1, Gun negotiates HTTP/1.1 and await_up returns Http1.

Plain TCP (connection.Tcp) does not negotiate. Gun uses the first protocol in the list.

import gleam/result
import gluegun/client
import gluegun/connection

pub fn get_over_http2() {
  let options =
    connection.options()
    |> connection.with_transport(transport: connection.Tls)
    |> connection.with_protocols(protocols: [connection.Http2, connection.Http1])

  use conn <- result.try(
    options
    |> connection.open(host: "example.com", port: 443),
  )
  use protocol <- result.try(
    connection.await_up(conn, connection.Milliseconds(5000)),
  )

  case protocol {
    connection.Http2 -> Nil
    connection.Http1 -> Nil
  }

  client.get(conn, "/", [], connection.Milliseconds(5000))
}

WebSocket note

WebSocket works only on HTTP/1.1. Use HTTP/2 for usual HTTP requests. Keep WebSocket connections on HTTP/1.1. If you call websocket.upgrade_with_protocol on an HTTP/2 connection, it returns UnsupportedFeature and does not call Gun.

See the connection reference for the full connection option API.