HTTP/2

Use TLS and put Http2 before Http1 to prefer HTTP/2 while allowing Gun to fall back to HTTP/1.1 when needed.

How fallback works

Protocol selection happens through TLS ALPN (Application-Layer Protocol Negotiation). When you list [Http2, Http1], Gun advertises both h2 and http/1.1 in the TLS ClientHello. The server picks one and the chosen protocol is returned by connection.await_up. If the server only advertises 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 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])

  let assert Ok(conn) =
    options
    |> connection.open(host: "example.com", port: 443)
  let assert Ok(protocol) =
    connection.await_up(conn, connection.Milliseconds(5000))

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

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

WebSocket note

WebSocket support is HTTP/1.1 only. Use HTTP/2 for regular HTTP requests and keep WebSocket connections on HTTP/1.1. Calling websocket.upgrade_with_protocol on an HTTP/2 connection returns UnsupportedFeature before reaching Gun.

See the connection reference for the complete connection option API.