Writing service handlers

Many kind of outputs modules

Once the service identification mechanism identified the service responsible for a given URL it executes its service handler. The service handler is a function taking the GET and POST parameters as argument and returning the content to be sent to the client. The return type of the service handler depends on the function used to register it. On the most common case its HTML5 contents build with the TyXML library, but Eliom provides a lot of outputs modules to ease the implementation of common web interaction. See section Predefined output modules for a comprehensive list.

Cooperative service handler

As Ocsigenserver, Eliom is based on cooperative threading library and service handler must be written in a cooperative way. See section Writing cooperative service handlers with Lwt for some examples.

Error handling

See section Error handling.

Predefined output modules

List of predefined output modules

Services can send several types of data, using a variety of predefined modules. It is also possible to create your own output modules. The main predefined output modules are:

Generating content for the browser

Registration of functions that generate html5 pages statically checked using polymorphic variant types. You may use constructor functions from Eliom_content.​Html5.​D or a syntax extension close to the standard html syntax.
Registration of functions that generate a portion of page using Eliom_content.​Html5.​F or the syntax extension (useful for XMLHttpRequest requests for example). Do not use with Eliom applications: use Eliom_registration.​Ocaml instead.
Registration of functions that generate text html pages, without any typechecking of the content. The content type sent by the server is "text/html".
Registration of functions that generate CSS pages, without any typechecking of the content. The content type sent by the server is "text/css".
Registration of functions that generate text pages, without any typechecking of the content. The services return a pair of strings. The first one is the content of the page, the second one is the content type.
Registration of services that send files. See here for an example of use.
Registration of services that send "byte" contents. It is used when big contents (that does not fit in memory) is generated.

Generating content for client-server applications

Functor that generates a module that allows creation of services belonging to a client-server Eliom application (see chapter client-server applications).

Special browser interraction

Registration of actions (functions that do not generate any page, see Action). The page corresponding to the URL (without the coservice parameter that triggered the action) is reloaded after the action by default if possible.
Like Eliom_registration.​Action, but the URL is not reloaded after the action. (Same as Eliom_registration.Action with [`NoReload] option).
Registration of HTTP redirections. The handler returns the service (without parameter) of the page you want to redirect to. The browser will get a 301 or 307 code in answer and redo the request to the new URL. To specify whether you want temporary (307) or permanent (301) redirections, use the ?options parameter of registration functions. For example: register options:`Permanent ... or register options:`Temporary ....
Same but the ouput type is a string. Use with care! Warning: According to the RFC of the HTTP protocol, the URL must be absolute!

Customization of other outputs

Specialization of service registration functions by customizing the page type.

Sending caml values to client side code

Registration of services sending marshalled OCaml values. See the section on communications in the chapter about client-server applications.

Runtime choice of content

Registration of services that can choose what they send, for example an HTML page or a file, depending on some situation (parameter, user logged or not, page present in a cache ...). It is also possible to create your own modules for other types of pages. See here for an example of use.

Specific output modules

Sending files

You may want to register a service that sends files. To do so, use the Eliom_registration.​File module. Example:

let sendfile =
    (fun () () -> return "filename")

Other example, with "suffix" services (see here):

let sendfile2 =
    ~get_params:Eliom_parameter.(suffix (all_suffix "filename"))
    (fun s () ->
      return ("//path//"^(Ocsigen_lib.Url.string_of_url_path ~encode:false s)))

The extension Staticmod is another way to handle static files.

Sending portions of pages

The Eliom_registration.​Flow5 modules allow you to register services that send portions of pages, of any "block" type for Html5.F. It is sometimes useful to create AJAX pages (i.e. pages using the XMLHttpRequest Javascript object). Note that the service returns a list of blocks. For sending HTML to client side Eliom application, Eliom_registration.​Ocaml is better suited.

let divpage =
    (fun () () ->
        [div [h2 [pcdata "Hallo"];
              p [pcdata "Blablablabla"] ]])

The Eliom_registration.​Make_typed_xml_registration module allows the creation of new modules for registering portions of pages of other types. For example, Eliom_registration.​Flow5 is defined by:

module Flow5 = Make_typed_xml_registration(Xml)(Html5.F)(struct
  type content = Html5_types.body_content


Redirections to Eliom services

The Eliom_registration.​Redirection module allows you to register HTTP redirections.
If a request is made for such a service, the server asks the browser to retry with another URL.

Such services return a GET service without parameter at all. Example:

let redir1 = Eliom_registration.Redirection.register_service
   (fun () () -> Lwt.return coucou)

If you want to give parameters to such services, use Eliom_service.​preapply (see also in section about pre-applied services). Example:

let redir = Eliom_registration.Redirection.register_service
   ~get_params:Eliom_parameter.(int "o")
   (fun o () ->
        (Eliom_service.preapply coucou_params (o,(22,"ee"))))

The options parameter may be either `Temporary or `Permanent.

Note that the cost of a redirection is one more request and one more response.

Redirections to generated urls

The Eliom_registration.​String_redirection allows you to register HTTP redirections to generated URLs. Usually, prefer Eliom_registration.​String_redirection, even for external redirections (using Eliom_service.​Http.​external_service). Use Eliom_registration.​String_redirection only when it is not possible to have a service corresponding to an URL.

Notice that the supplied URL must be absolute.


Actions are used to perform side effects before generating the fallback of a service. When an action is called, the service handler is executed, then the service handler of the fallback service is executed.

Eliom references of scope Eliom_common.​request_scope set in an action handler are still available in the service handler of the fallback.

A common use of actions and non-attached coservices working together is the implementation of login/logout forms. An example is given in the chapter about the server-side state of the application (section sec-state-connect). In that example, actions and non-attached coservices make staightforward the implementation of the behaviour you generally want for such features:

  • Connection and disconnection stay on the same page,
  • If you want a connection/disconnection form on each page, no need to create a version with POST parameters of each service.

The implementation of the same behaviour with usual Web programming techniques is usually much more complicated.

Registering services that decide what they want to send

You may want to register a service that will send, for instance, sometimes an HTML page, sometimes a file, sometimes something else. To do that, use the Eliom_registration.​Any module, together with the send function of the module you want to use. Example:

let send_any =
    ~get_params:Eliom_parameter.(string "type")
   (fun s () ->
     if s = "valid"
            (head (title (pcdata "")) [])
            (body [p [pcdata
                        "This page has been statically typechecked.\n                         If you change the parameter in the URL you will\n                         get an unchecked text page"]]))
         "<html><body><p>It is not a valid page. Put type=\"valid\" in the\n          URL to get a typechecked page.</p></body></html>"
Dynamically modifying register options using Any

You may also use Eliom_registration.​Any to dynamically modify the parameters usually set on the register function. You can set the HTTP code, the charset, the content_type, the HTTP headers and the specific option of the output module.

let change_option =
   (fun () () ->
          (head (title (pcdata "forbidden")) [])
          (body [p [pcdata "forbidden"]])))
About kind type and how to serve application and other content with the same service

In Eliom applications, changing the current page does not do always the same thing. When going to a page inside the same application by clicking a link (or calling Eliom_client.​change_page) the client application perform a XmlHttpRequest and modify the displayed page according to the result. When going to content outside the application (an other site, a static file, etc.) the client leaves the application by changing the browser url.

When using Eliom_registration.​Any, there is no way to know before the request wether the content is from the same application or not. To that end there are phantom type annotations to the type of the send functions: Eliom_registration.​kind. The Eliom_registration.​Any.​register takes a service handler that can server only one kind of content: that way it is not possible to inadvertantly mix kinds. The different kinds of content are:

  • Browser content: everything that can't be handled by application directly: ex Html pages, files
  • Block content: subparts of pages sent as XML: ex Flow5, Block.
  • Application content: pages of application.
  • Ocaml content: marshalled OCaml values.
  • Unknown content: content generated as text.

Yet sometimes you may want to mix the kinds of contents a service can return. The function Eliom_registration.​appl_self_redirect allows you to cast browser content to application content. When an application request some content cast through that function the server send some information telling the client to exit to that address instead. You should not use that on POST services: leaving the application sending POST parameters is not always possible and the request will be performed 2 times.

For instance if you want to serve files if they exists and generate some error message with client side code otherwise, you should do something like that.

let file_or_application =
    ~get_params:Eliom_parameter.(string "filename")
   (fun filename () ->
     if Eliom_registration.File.check_file filename
       Eliom_registration.appl_self_redirect Eliom_registration.File.send filename
       My_application.send ~code:404
           (head (title (pcdata "no page")) [])
           (body [p ~a:[a_onclick {{ do some client action }}]
	     [pcdata "the file does not exist"]])))

Unknown content can be cast to browser content using Eliom_registration.​cast_unknown_content_kind.

Creating your own output modules

By customizing an existing registration module

Using Eliom_registration.​Customize you can specialise a registration function to avoid code duplication. You can for instance for each service add parameters before calling the service handler, or modify the answer.

A classical use is to check wether a user is logged in before serving the content of a page. That way, we don't need to do the check in every service handler and we can get directly the user informations.

In this example, we check if we are in a session group telling that the user is connected. If it not the case, we generate a login page. When we are in a group, we retrieve the user informations and pass it to the service handler. It returns a title and the content of the body. That way, it also always generate the same css without code duplication.

module Connected_param =
  type page = user_informations -> Eliom_registration.Html5.page Lwt.t
  let translate page =
    match Eliom_state.get_volatile_data_session_group
      ~scope:Eliom_common.default_session_scope () with
	| None -> login_page ()
	| Some group_name ->
	  lwt user_info = get_user_info group_name in
	  lwt (page_title,content) = page user_info in
	  return (html (head (title (pcdata page_title))
	                 [ Some css and things like that ])
	               (body content))

module Connected =
  Eliom_registration.Customize ( Eliom_registration.Html5 ) ( Connected_param )

By building the HTTP frame

When you need to declare some kind of specific service, you may need to create your own registration module: For instance here we define a module serving integers as text. Notice that in that case you can and should use customize instead.

For that you need to use the Eliom_mkreg.​MakeRegister functor.

module Int_reg_base = struct

  type page = int

  type options = unit

  type return = http_service

  type result = (browser_content, http_service) kind

  let result_of_http_result = Eliom_registration.cast_http_result

  let send_appl_content = Eliom_service.XNever

  let send ?options ?charset ?code ?content_type ?headers content =
    let content = string_of_int content in
    lwt r = Ocsigen_senders.Text_content.result_of_content content in
      { r with
        res_code    = code_of_code_option code;
        res_charset = (match charset with
          | None ->  Some (Eliom_config.get_config_default_charset ())
          | _ -> charset);
        res_content_type = (match content_type with
          | None -> r.Ocsigen_http_frame.res_content_type
          | _ -> content_type
        res_headers = (match headers with
          | None -> r.Ocsigen_http_frame.res_headers
          | Some headers ->
            Http_headers.with_defaults headers r.Ocsigen_http_frame.res_headers


module Int = Eliom_mkreg.MakeRegister(Int_reg_base)

If your page type has parameters you should use Eliom_mkreg.​MakeRegister_AlphaReturn instead.

Writing cooperative service handlers with Lwt

Remember that a Web site written with Eliom is an OCaml application. This application must be able to handle several requests at the same time, in order to prevent a single request from making the whole server hang. To make this possible, Ocsigen uses cooperative threads (implemented in monadic style) which make them really easy to use (see Lwt module).

Below is an example of a page written in a non-cooperative way, that has the effect of stopping the entire server for 5 seconds. No one will be able to query the server during this period:

open Eliom_content.Html5.D
let looong =
    (fun () () ->
       Unix.sleep 5;
                    (body [h1 [pcdata "Ok now, you can read the page."]])))

To solve this problem, use a cooperative version of Unix.sleep: Lwt_unix.​sleep:

open Eliom_content.Html5.D
let looong =
    (fun () () ->
       lwt () = Lwt_unix.sleep 5.0 in

If you want to use, say, a database library that is not written in a cooperative way, but is thread-safe for preemptive threads, use the Lwt_preemptive module to detach the computation. In the following example, we simulate the request by making a call to Unix.sleep:

let looong2 =
    (fun () () ->
       lwt () = Lwt_preemptive.detach Unix.sleep 5 in

Error handling

Exception handling

You can catch the exceptions raised during page generation in two places:

You can use it to catch exception Eliom_common.​Eliom_404 and generate a custom 404 page.

let _ = Eliom_registration.set_exn_handler
   (fun e -> match e with
    | Eliom_common.Eliom_404 ->
        Eliom_registration.Html5.send ~code:404
             (body [h1 [pcdata "Eliom tutorial"];
                    p [pcdata "Page not found"]]))
    | Eliom_common.Eliom_Wrong_parameter ->
             (body [h1 [pcdata "Eliom tutorial"];
                    p [pcdata "Wrong parameters"]]))
    | e -> fail e)

Fallback services

You can check wether a service was directly called or if it was used as a fallback using the Eliom_request_info.​get_link_too_old function. In case of coservices registered with a restricted scope, you can check which state was closed using Eliom_request_info.​get_expired_service_sessions

Error in service handlers of actions

If something wrong happens during an action, it is possible to inform the service generating the page. For instance, if you want to display a "wrong password" message after an aborted connection. To transmit that kind of information, use Eliom references (see module Eliom_reference) created using scope Eliom_common.​request_scope, the value will be available to the service generating the page.

Other examples: creating user accounts using actions. If the creation fails you may want to display some message to the user, like "password too weak" or "name already used".