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Programming client-server applications with Eliom

General principles

What is a client/server Eliom application

An Eliom application is a distributed application that runs partly on the server, partly on a browser. The program is fully written in OCaml, with a syntax extension to distinguish between server and client code. Both parts are extracted during the compilation process, the server part is compiled as usual for an Eliom website, and the client part is compiled to Javascript to be run in the browser.

An intersting feature of Eliom applications is that the client side process does not stop when you click on a link or send a form, and it is possible to keep the traditional Web interaction (with URLs, bookmarks, back button, etc). For example if the page is playing music, it won't stop when the user continue his visit on the Web site.

Client side parts are using Lwt for concurrency, making possible to have concurrent programs in the browser very easily.

As both part are implemented in OCaml, it is very easy to use client side OCaml data on server side, and vice-versa. Eliom handle the communication between client and server automatically in both direction. For example it possible to use a server-side variable in the client program.

Eliom also implements an "HTTP-push" mechanism, allowing the server to send messages to a client.

Client-side parts of the program can use most Eliom features, exactly as usual, for example to create HTML, or links, forms from services.

On server side, it is possible to save data (some state) for each client process (that is, one tab in a browser), simply by using Eliom references with scope `Client_process. You can register services for one client process, or even set cookies for one tab.

How it works

The code of an Eliom application is written in OCaml, with a syntax extension to distinguish between server and client code. The files using this syntax usually have the extension .eliom. As the compling process is quite complex, we provide commands called eliomc and js_of_eliom that does everything for you (separating client and server parts, calling ocamlc, js_of_ocaml, etc).

Services belonging to the application are registered using the module Eliom_registration.App. More precisely, this is a functor that needs to be applied for each application you create. These services just return HTML5 pages as usual (using Tyxml.Html5 – not possible with OcamlDuce or any other HTML module). The client side program (compiled in JS) is added automatically by Eliom, with all its data, and run automatically when the page is loaded.

The module Eliom_client provides some useful functions for client side programming with Eliom: mainly switch to another page or call a service returning some OCaml value.

The module Eliom_comet allows for the server to send notifications to the client (even if the client is not explicitely doing a request). The module Eliom_react use this to make client-server reactive programming (using the React external library).

Structure of a program


Eliom application are written in files with extension .eliom. Some special brackets make possible to distinuish between client and server code:


or no brackets for server side code,


for client side code, and


for some code that is common to client and server parts.

Inside server-side code block, Eliom also provides a custom syntax {{ ... }} for defining client-side event handler. For example:

p ~a:[a_onclick {{ Dom_html.window##alert(Js.string "clicked!") }}]
  [pcdata "I am a clickable paragraph"]

Those client-side code block are classical OCaml values ; their type is Eliom_content.Xml.caml_event_handler. Inside those code-block, an implicit _ev variable represent the javascript event object. On client side, this specific syntax is not required: function like a_onclick expect a function of type #Dom_html.event -> unit as parameter.

Using server side values on client side

Client code inside {{ ... }} quotation can use server side value using the %variable syntax or the %(expr) syntax: variables prefixed by %, and the evaluation of expressions prefixed by %, are sent to the client along the content of the page. For example:

let value = 3 in
p ~a:[a_onclick {{
           ("value = " ^ (string_of_int %value)) )
  [pcdata "I am a clickable paragraph"]

Or the same example using the %(expr) syntax:

let value = ref 3 in
p ~a:[a_onclick {{
           ("value = " ^ (string_of_int %(!value))) )
  [pcdata "I am a clickable paragraph"]

Notice that not all values are possible to send that way: since client and server side representation are not the same, it is impossible to send functions. This means that unforced lazy values, objects, or anything containing functions can't be send. Some eliom types use a specific machanism to circumvent this limitation. This is the case of: services, comet channels and busses. To use this mechanism see chpater Wrapping values.

Those values are typechecked "by name": the most general type of a variable is inferred for server side then use as a type constraint on client side. For instance

let value = [] in
let v = {{ %value }}

can be read as

let value = [] in
let v = {{ (%value: _ list) }}

As client an server code are compiled separately, this means that a code like the following would be incorrect but would typecheck.

type a = A of int
{client{ type a = A of string }}
let value = A 1 in
let v = {{ match %value with A s -> Dom_html.window##alert(Js.string s) }}

Note that for some reason, it is impossible to use the {...{ }} and {{ }} syntax inside a module. For {{ }} you can usually circumvent this limitation by declaring a function at toplevel with all the %variable as parameters.

The App functor

For each Eliom application, you must create a service registration module by applying the App functor:

module My_appl =
  Eliom_registration.App (
      let application_name = "the name of your application"

the application_name parameter is the name of the javascript file containing the application.

Then you can do for example:

let my_service =
    (fun () () -> Lwt.return (html
                               (head (title (pcdata "Hi!")) [])
                               (body [p [pcdata "Hey."]])))

Eliom will add automatically the required headers to send the client side program and all its data.

Application, life and death

When an user enter the page of a service registered by an application module ( created with the App functor ), the application is started. During the life of the application, a single ocaml program will run on the browser: Lwt thread will keep running, global variables will stay available, etc... until the application is closed. The application will keep running when the user clicks on links to pages inside the same application.

This application will be closed when:

  • the user closes the browser tab containing the application
  • the user goes to a page outside of the application
  • the user change the current url by another mean than the application interraction ( reload the page with F5, type an url, ... )
  • the application call the Eliom_client.exit_to function

It is possible to prevent the application from launching when visiting an application page by setting the do_not_launch to true at the service registration:

let no_launch_service =
    ~option:{ Eliom_registration.default_appl_service_options with
              do_not_launch = true }
    (fun () () -> Lwt.return (html
                               (head (title (pcdata "Hi!")) [])
                               (body [p [pcdata "Hey."]])))

That way, you can delay the javascript loading until it is really needed. Visiting a service registered with do_not_launch=true will not stop a running application.

It is possible to force reloading an application when clicking a link by creating the link with the xhr option set to false.

Navigating in and out of the application.

Two function are available on client side to change the current page without interraction of the user. The function Eliom_client.change_page goes to the service taken as parameter. If the service is in another application or not in an application it will stop the current one. The function Eliom_client.window_open opens an Eliom service in a new browser window (cf. Javascript's window.open). Eliom_client.exit_to changes the current page and always leave the application.

Generating HTML for Eliom applications

The TyXML library vs. the DOM API

On client side there are two kinds of HTML representations: one is based on the TyXML library and the other one is the browser DOM tree accessible through Js_of_ocaml modules Dom and Dom_html. The TyXML representation is a OCaml immutable typed tree. The DOM tree is mutable structure manipulated using the browser API which permit the modification of the displayed page. In the DOM represention adding a node as a child to an other node removes it from its previous ancessor.

Since those representation does not behave at all the same way, they are not used for the same thing.

  • It is far easier and safer to describe content using TyXML, but it is not possible to add a TyXML element to the page without explicit conversion to the DOM representation.
  • The TyXML representation has the same interface on client and server side. This allows share code between server and client.
  • Dom manipulation is heavy: to build some part of a tree, one needs to create each node separately then append them to their parents.

For example, here is a div element build with TyXML and then converted to the DOM representation using the module

open Eliom_content.Html5.D
let n = div ~a:[a_id "some div id"]
  [ pcdata "some text";
    br ();
    pcdata "some other text"; ]
let b = Eliom_client.Html5.of_div n

And here the same build using the DOM API:

open Dom
open Dom_html

let d = createDiv document in
let t1 = document##createTextNode( Js.string "some text" ) in
let t2 = document##createTextNode( Js.string "some other text" ) in
let b = createB document in
  appendChild d t1;
  appendChild d b;
  appendChild d t2;
  d##id <- (Js.string "some div id");

To ease the DOM manipulation on the client, the usual DOM manipulation function are also available on TyXML elements. See section HTML5 element manipulation, by value and by reference.

HTML5 element manipulation, by value and by reference

When defining a service that returns an HTML5 page with Eliom we usually use the module Eliom_content.Html5.F. When programming client/server application with Eliom, we usualy prefer to use the module Eliom_content.Html5.D. This is because elements build with Eliom_content.Html5.D are sent to the client by reference while elements build with Eliom_content.Html5.F are sent by value.

Sending elements by reference allows to easily manipulate elements included in the initial html document from event handlers, as the input element in the following example.

let main_service =
  My_appl.register_service ~path:[""] ~get_params:Eliom_parameter.unit
    (fun () () ->
       let open Eliom_content.Html5.D in
       let input = input ~a:[a_input_type `Text] () in
       let onclick_handler =
	 {{ let v =
	      Js.to_string (Eliom_client.Html5.of_input %input)##value
	    Dom_html.window##alert(Js.string ("Input value :" ^ v)) }}
       let button =
         button ~a:[a_onclick onclick_handler] [pcdata "Read value"]
	    (head (title (pcdata "Test")) [])
            (body [input; button]) ) )

In this example, if the input button would have been incorrectly sent by value, two different input fields would have been created: one displayed in the document and one referenced from the event handler. The latter will always contains an empty value.

There is still two situations where sending elements by value is still required:

  • one want to have multiple occurences of the same elements in the document. Indeed, elements sent by reference follow the DOM semantics where an element have only one instance in current document. For example, the following list will contains a single element:

    let li = li [pcdata "Shared item"] in ul [li; li; li;] .
  • one have a large page with a lot elements. Handling elements by references add a small overhead while loading the page, around 50ms per 1000 elements on a not so fast computer.

In every case, it is possible to mix elements sent by references and elements sent by value in the same document.

The module Eliom_dom allows to use the classical DOM manipulation functions (e.g. appendChild, addEventlistener, ...) directly on HTML5 elements that follow the DOM semantics.

By default, a reference on an element is only valid in the current HTTP request: hence, sending an element build with Eliom_content.Html5.D in two different page will produce two distinct nodes. If you want to define a element reference that is preserved accross the different page of an application, you must explicitely name this element with the function Eliom_content.Html5.Id.create_named_elt, that take as parameters an element identifier and a non named element. Element identifiers are created with the function Eliom_content.Html5.Id.new_elt_id. See also section Global elements of an application.

The module Eliom_dom.Named allows to use the classical DOM manipulation functions (e.g. appendChild, addEventlistener, ...) directly on the identifier of an HTML5 elements.

Global elements of an application

Sometimes you may want to modify the content of an HTML element and to keep the element and its modified content when changing page. For examplem a div element which contains a chat box or a music player should be preserved while browsing across the different page of your site. For purpose Eliom provides a notion of global element. Such elements are instantied only once for an application and that unique instance is used in every page that references the element.

You could create a global element with the function Eliom_content.Html5.Id.create_global_elt.

val create_global_elt: 'a elt -> 'a elt

In the following example, the content of global_list will be preserved when you click on the "reload page" link.

open Eliom_content.Html5.D

let global_list = create_global_elt (ul [])
let cpt = ref 0

let main_service =
    ~path:[""] ~get_params:Eliom_parameter.unit

let reload_link =
  a ~service:main_service [pcdata "reload page"] ()

let _ =
  My_appl.register ~service:main_service
    (fun () () ->
       let page_number = incr cpt; string_of_int !cpt in
       let append_item =
	 {{ let item_text = "item inserted in page #" ^ %page_number in
	    let item = Eliom_client.Html5.of_li (li [pcdata item_text]) in
	    Dom.appendChild (Eliom_client.Html5.of_ul %global_list) item }}
       let append_link =
         a ~a:[a_onclick append_item] [pcdata "append item"]
	    (head (title (pcdata "Test")) [])
            (body [h1 [pcdata ("Page #" ^ page_number)];
	           p [append_link];
		   p [reload_link];
		   global_list]) ) )

Another use of global element is for external javascript that should be included in every page but must be executed only once in an application. In the following code snippet, the alert "global script" is displayed only once, while the alert "non global script" is display every time you click on the "reload page" link.

open Eliom_content.Html5.D

let global_script =
    (script (cdata_script "alert(\"global script\")"))
let simple_script =
     script (cdata_script "alert(\"non global script\")")

let main_service =
     ~path:[] ~get_params:Eliom_parameter.unit ()

let reload_link =
  a ~service:main_service [pcdata "reload page"] ()

let _ = My_appl.register ~service:main_service
  (fun () () ->
	   (title (pcdata "Global script example"))
	   [ global_script;
	     simple_script ])
	   [ p [reload_link] ])))

Using Eliom on client side


Leaving application and going back

Usualy, when going to an application page, a new client process is launched on the server, but there are situations where an old client process is used instead: Browsers tend to take the result from their cache when using the back button even if the page was marked ( by HTTP headers ) as non-cacheable.