package tiny_httpd

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Tiny Http Server

This library implements a very simple, basic HTTP/1.1 server using blocking IOs and threads. Basic routing based on Scanf is provided for convenience, so that several handlers can be registered.

It is possible to use a thread pool, see create's argument new_thread.

The echo example (see src/examples/ demonstrates some of the features by declaring a few endpoints, including one for uploading files:

module S = Tiny_httpd

let () =
  let server = S.create () in

  (* say hello *)
  S.add_route_handler ~meth:`GET server
    S.Route.(exact "hello" @/ string @/ return)
    (fun name _req -> S.Response.make_string (Ok ("hello " ^name ^"!\n")));

  (* echo request *)
  S.add_route_handler server
    S.Route.(exact "echo" @/ return)
    (fun req -> S.Response.make_string
        (Ok (Format.asprintf "echo:@ %a@." S.Request.pp req)));

  (* file upload *)
  S.add_route_handler ~meth:`PUT server
    S.Route.(exact "upload" @/ string_urlencoded @/ return)
    (fun path req ->
          let oc = open_out @@ "/tmp/" ^ path in
          output_string oc req.S.Request.body;
          flush oc;
          S.Response.make_string (Ok "uploaded file")
        with e ->
 ~code:500 "couldn't upload file: %s"
            (Printexc.to_string e)

  (* run the server *)
  Printf.printf "listening on http://%s:%d\n%!" (S.addr server) (S.port server);
  match server with
  | Ok () -> ()
  | Error e -> raise e

It is then possible to query it using curl:

$ dune exec src/examples/echo.exe &
listening on

# the path "hello/name" greets you.
$ curl -X GET http://localhost:8080/hello/quadrarotaphile
hello quadrarotaphile!

# the path "echo" just prints the request.
$ curl -X GET http://localhost:8080/echo --data "howdy y'all"
 headers=Host: localhost:8080
         User-Agent: curl/7.66.0
         Accept: */*
         Content-Length: 10
         Content-Type: application/x-www-form-urlencoded;
 path="/echo"; body="howdy y'all"}

Tiny buffer implementation

These buffers are used to avoid allocating too many byte arrays when processing streams and parsing requests.

module Buf_ : sig ... end

Generic stream of data

Streams are used to represent a series of bytes that can arrive progressively. For example, an uploaded file will be sent as a series of chunks.

type byte_stream = {
  1. bs_fill_buf : unit -> bytes * int * int;

    See the current slice of the internal buffer as bytes, i, len, where the slice is bytes[i] .. [bytes[i+len-1]]. Can block to refill the buffer if there is currently no content. If len=0 then there is no more data.

  2. bs_consume : int -> unit;

    Consume n bytes from the buffer. This should only be called with n <= len after a call to is_fill_buf that returns a slice of length len.

  3. bs_close : unit -> unit;

    Close the stream.


A buffered stream, with a view into the current buffer (or refill if empty), and a function to consume n bytes. See Byte_stream for more details.

module Byte_stream : sig ... end


module Meth : sig ... end


Headers are metadata associated with a request or response.

module Headers : sig ... end


Requests are sent by a client, e.g. a web browser or cURL.

module Request : sig ... end

Response Codes

module Response_code : sig ... end


Responses are what a http server, such as Tiny_httpd, send back to the client to answer a Request.t

module Response : sig ... end
module Route : sig ... end

Main Server type

type t

A HTTP server. See create for more details.

val create : ?masksigpipe:bool -> ?max_connections:int -> ?new_thread:((unit -> unit) -> unit) -> ?addr:string -> ?port:int -> ?sock:Unix.file_descr -> unit -> t

Create a new webserver.

The server will not do anything until run is called on it. Before starting the server, one can use add_path_handler and set_top_handler to specify how to handle incoming requests.

  • parameter masksigpipe

    if true, block the signal Sys.sigpipe which otherwise tends to kill client threads when they try to write on broken sockets. Default: true.

  • parameter new_thread

    a function used to spawn a new thread to handle a new client connection. By default it is Thread.create but one could use a thread pool instead.

  • parameter max_connections

    maximum number of simultaneous connections.

  • parameter addr

    address (IPv4 or IPv6) to listen on. Default "".

  • parameter port

    to listen on. Default 8080.

  • parameter sock

    an existing socket given to the server to listen on, e.g. by systemd on Linux (or launchd on macOS). If passed in, this socket will be used instead of the addr and port. If not passed in, those will be used. This parameter exists since 0.10.

val addr : t -> string

Address on which the server listens.

val is_ipv6 : t -> bool

is_ipv6 server returns true iff the address of the server is an IPv6 address.

  • since 0.3
val port : t -> int

Port on which the server listens.

val add_decode_request_cb : t -> (unit Request.t -> (unit Request.t * (byte_stream -> byte_stream)) option) -> unit

Add a callback for every request. The callback can provide a stream transformer and a new request (with modified headers, typically). A possible use is to handle decompression by looking for a Transfer-Encoding header and returning a stream transformer that decompresses on the fly.

val add_encode_response_cb : t -> (unit Request.t -> Response.t -> Response.t option) -> unit

Add a callback for every request/response pair. Similarly to add_encode_response_cb the callback can return a new response, for example to compress it. The callback is given the query with only its headers, as well as the current response.

Request handlers

val set_top_handler : t -> (string Request.t -> Response.t) -> unit

Setup a handler called by default.

This handler is called with any request not accepted by any handler installed via add_path_handler. If no top handler is installed, unhandled paths will return a 404 not found.

val add_route_handler : ?accept:(unit Request.t -> (unit, Response_code.t * string) Stdlib.result) -> ?meth:Meth.t -> t -> ('a, string Request.t -> Response.t) Route.t -> 'a -> unit

add_route_handler server Route.(exact "path" @/ string @/ int @/ return) f calls f "foo" 42 request when a request with path "path/foo/42/" is received.

Note that the handlers are called in the reverse order of their addition, so the last registered handler can override previously registered ones.

  • parameter meth

    if provided, only accept requests with the given method. Typically one could react to `GET or `PUT.

  • parameter accept

    should return Ok() if the given request (before its body is read) should be accepted, Error (code,message) if it's to be rejected (e.g. because its content is too big, or for some permission error). See the http_of_dir program for an example of how to use accept to filter uploads that are too large before the upload even starts. The default always returns Ok(), i.e. it accepts all requests.

  • since 0.6
val add_route_handler_stream : ?accept:(unit Request.t -> (unit, Response_code.t * string) Stdlib.result) -> ?meth:Meth.t -> t -> ('a, byte_stream Request.t -> Response.t) Route.t -> 'a -> unit

Similar to add_route_handler, but where the body of the request is a stream of bytes that has not been read yet. This is useful when one wants to stream the body directly into a parser, json decoder (such as Jsonm) or into a file.

  • since 0.6
val add_path_handler : ?accept:(unit Request.t -> (unit, Response_code.t * string) Stdlib.result) -> ?meth:Meth.t -> t -> ('a, Stdlib.Scanf.Scanning.in_channel, 'b, 'c -> string Request.t -> Response.t, 'a -> 'd, 'd) Stdlib.format6 -> 'c -> unit

Similar to add_route_handler but based on scanf.

This uses Scanf's splitting, which has some gotchas (in particular, "%s" is eager, so it's generally necessary to delimit its scope with a "@/" delimiter. The "@" before a character indicates it's a separator.

val add_path_handler_stream : ?accept:(unit Request.t -> (unit, Response_code.t * string) Stdlib.result) -> ?meth:Meth.t -> t -> ('a, Stdlib.Scanf.Scanning.in_channel, 'b, 'c -> byte_stream Request.t -> Response.t, 'a -> 'd, 'd) Stdlib.format6 -> 'c -> unit

Similar to add_path_handler, but where the body of the request is a stream of bytes that has not been read yet. This is useful when one wants to stream the body directly into a parser, json decoder (such as Jsonm) or into a file.

  • since 0.3
  • deprecated use add_route_handler_stream instead

Server-sent events

EXPERIMENTAL: this API is not stable yet.

module type SERVER_SENT_GENERATOR = sig ... end

A server-side function to generate of Server-sent events.

type server_sent_generator = (module SERVER_SENT_GENERATOR)

Server-sent event generator

  • since 0.9
val add_route_server_sent_handler : ?accept:(unit Request.t -> (unit, Response_code.t * string) Stdlib.result) -> t -> ('a, string Request.t -> server_sent_generator -> unit) Route.t -> 'a -> unit

Add a handler on an endpoint, that serves server-sent events.

The callback is given a generator that can be used to send events as it pleases. The connection is always closed by the client, and the accepted method is always GET. This will set the header "content-type" to "text/event-stream" automatically and reply with a 200 immediately. See server_sent_generator for more details.

This handler stays on the original thread (it is synchronous).

  • since 0.9

Run the server

val stop : t -> unit

Ask the server to stop. This might not have an immediate effect as run might currently be waiting on IO.

val run : t -> (unit, exn) Stdlib.result

Run the main loop of the server, listening on a socket described at the server's creation time, using new_thread to start a thread for each new client.

This returns Ok () if the server exits gracefully, or Error e if it exits with an error.


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