glisten

A Gleam TCP server library. Built on top of gleam_otp, it provides a supervisor over a pool of socket acceptors. Each acceptor will block on accept until a connection is opened. The acceptor will then spawn a handler process and then block again on accept.

Below is a simple example that will echo received messages. You can get this up and running with the following:

$ gleam new <your_project>
$ cd <your_project>
$ gleam add glisten gleam_erlang gleam_otp

Then place this code in src/<your_project>.gleam:

import gleam/bytes_builder
import gleam/erlang/process
import gleam/option.{None}
import gleam/otp/actor
import glisten.{Packet}

pub fn main() {
  let assert Ok(_) =
    glisten.handler(fn(_conn) { #(Nil, None) }, fn(msg, state, conn) {
      let assert Packet(msg) = msg
      let assert Ok(_) = glisten.send(conn, bytes_builder.from_bit_array(msg))
      actor.continue(state)
    })
    // NOTE:  By default, `glisten` will listen on the loopback interface.  If
    // you want to listen on all interfaces, pass the following.  You can also
    // specify other interface values, including IPv6 addresses.
    // |> glisten.bind("0.0.0.0")
    |> glisten.serve(3000)

  process.sleep_forever()
}

SSL is also handled using the glisten.{serve_ssl} method. This requires a certificate and key file path. The rest of the handler flow remains unchanged.

glisten doesn’t provide a public API for connected clients. In order to hook into the socket lifecycle, you can establish some functions which are called for the opening and closing of the socket. An example is provided below.

To serve over SSL:

// ...
import glisten/ssl

pub fn main() {
  let assert Ok(_) =
    glisten.handler(
      // omitted
    )
    |> glisten.serve_ssl(
      // Passing labeled arguments for clarity
      port: 8080,
      certfile: "/path/to/server.crt",
      keyfile: "/path/to/server.key",
    )
  process.sleep_forever()
}

But you can also drop down to the lower level listen/accept flow if you’d prefer to manage connections yourself, or only handle a small number at a time.

import gleam/io
import gleam/result
import glisten/socket/options.{ActiveMode, Passive}
import glisten/tcp

pub fn main() {
  use listener <- result.then(tcp.listen(8000, [ActiveMode(Passive)]))
  use socket <- result.then(tcp.accept(listener))
  use msg <- result.then(tcp.receive(socket, 0))
  io.debug(#("got a msg", msg))

  Ok(Nil)
}

See mist for HTTP support built on top of this library.