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Very lightweight HTTP server using Lwt or Async
OCaml JavaScript Other
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Cohttp is an OCaml library for creating HTTP daemons. It has a portable HTTP parser, and implementations using various asynchronous programming libraries:

  • Cohttp_lwt_unix uses the Lwt library, and specifically the UNIX bindings.
  • Cohttp_async uses the Async library.
  • Cohttp_lwt exposes an OS-independent Lwt interface, which is used by the Mirage interface to generate standalone microkernels (see the mirage-http repository).
  • Cohttp_lwt_xhr compiles to a JavaScript module that maps the Cohttp calls to XMLHTTPRequests. This is used to compile OCaml libraries like the GitHub bindings to JavaScript and still run efficiently.

You can implement other targets using the parser very easily. Look at the IO signature in lib/s.mli and implement that in the desired backend.

You can activate some runtime debugging by setting COHTTP_DEBUG to any value, and all requests and responses will be written to stderr. Further debugging of the connection layer can be obtained by setting CONDUIT_DEBUG to any value.


Latest stable version should be obtained from opam. Make sure to install the specific backends you want as well. E.g.

$ opam install cohttp lwt js_of_ocaml

You can also obtain the development release:

$ opam pin add cohttp --dev-repo

Client Tutorial

Cohttp provides clients for Async, Lwt, and jsoo (Lwt based). In this tutorial, we will use the lwt client but it should be easily translateable to Async.

To create a simple request, use one of the methods in Cohttp_lwt_unix.Client. call is the most general, there are also http method specialized such as get, post, etc.

For example downloading the reddit frontpage:

open Lwt
open Cohttp
open Cohttp_lwt_unix

let body =
  Client.get (Uri.of_string "") >>= fun (resp, body) ->
  let code = resp |> Response.status |> Code.code_of_status in
  Printf.printf "Response code: %d\n" code;
  Printf.printf "Headers: %s\n" (resp |> Response.headers |> Header.to_string);
  body |> Cohttp_lwt_body.to_string >|= fun body ->
  Printf.printf "Body of length: %d\n" (String.length body);

let () =
  let body = body in
  print_endline ("Received body\n" ^ body)

There's a few things to notice:

  • We open 2 modules. Cohttp contains the backend independent stuff and Cohttp_lwt_unix is the lwt + unix specific stuff.

  • Client.get accepts a Uri.t and makes an http request. Client.get also accepts optional arguments for things like header information.

  • The http response is returned in a tuple. The first element of the tuple contains the response's status code, headers, http version, etc. The second element contains the body.
  • The body is then converted to a string and is returned (after the length is printed). Note that Cohttp_lwt_body.to_string hence it's up to us to keep a reference to the result.
  • We must trigger lwt's event loop for the request to run. will run the event loop and return with final value of body which we then print.

Consult the following modules for reference:

Basic Server Tutorial

Implementing a server in cohttp is mostly equivalent to implementing a function of type:

conn -> Cohttp.Request.t -> Cohttp_lwt_body.t -> (Cohttp.Response.t * Cohttp_lwt_body.t) Lwt.t

The parameters are self explanatory but we'll summarize them quickly here:

  • conn - contains connection information
  • Cohttp.Request.t - Request information such as method, uri, headers, etc.
  • Cohttp_lwt_body.t - Contains the request body. You must manually decode the request body into json, form encoded pairs, etc. For cohttp, the body is simply binary data.

Here's an example of a simple cohttp server that outputs back request information.

open Lwt
open Cohttp
open Cohttp_lwt_unix

let server =
  let callback _conn req body =
    let uri = req |> Request.uri |> Uri.to_string in
    let meth = req |> Request.meth |> Code.string_of_method in
    let headers = req |> Request.headers |> Header.to_string in
    body |> Cohttp_lwt_body.to_string >|= (fun body ->
      (Printf.sprintf "Uri: %s\nMethod: %s\nHeaders\nHeaders: %s\nBody: %s"
         uri meth headers body))
    >>= (fun body -> Server.respond_string ~status:`OK ~body ())
  Server.create ~mode:(`TCP (`Port 8000)) (Server.make ~callback ())

let () = ignore ( server)

The following modules are useful references:

Installed Binaries

Cohttp comes with a few simple binaries that are handy, useful testing cohttp itself, and serve as examples of how to use cohttp. The binaries come in two flavours - Async and Lwt based.

  • $ cohttp-curl-{lwt,async}

This is a simple curl utility implemented using cohttp. An example of an invocation is:

$ cohttp-curl-lwt -v -X GET ""
  • $ cohttp-server-{lwt,async}

This binary acts in a similar fashion to the Python SimpleHTTPServer. Just run cohttp-server-async in a directory and it will open up a local port and serve the files over HTTP.

$ cohttp-server-async

Assuming that the server is running in cohttp's source directory:

$ cohttp-curl-lwt ''
Something went wrong with that request. Please try again.