# How does a client-server app source code look like?


### Eliom client-server applications

Eliom client-server applications are running in your browser for a `certain lifetime` and consist of one or several pages/URLs. An application has its associated js file, which must have the same name (`generated automatically by the default makefile` and added automatically by Eliom in the page).

For example, we define an application called *example*:

```ocaml
module Example =
  Eliom_registration.App
    (struct
      let application_name = "example"
     end)
```

### Eliom Services

Pages are generated by *services*. Here is how to define a service:

```ocaml
let main =
  Eliom_service.create
    ~path:(Eliom_service.Path [])
    ~meth:(Eliom_service.Get unit)
    ()
```
The path is a list of string corresponding to the url. Here, the list is empty because it's the main page.

If, for example, the list is `"hello";"world"`, the page will be accessed using the address:

```ocaml
http://website.com/hello/world/
```
More information about parameters: `How to use GET parameters (parameters in the URL)?`

Now that our service has been declared, we associate to it an OCaml function that will generate the page. We call this *service registration*. If the service belongs to the application, we use the registrations functions from module `Example` defined above.

```ocaml
let _ =
  Example.register
    ~service:main
    (fun () () ->
      Lwt.return ...)
```
The third parameter is the function that will be called to generate the page. This function takes two parameters, corresponding respectively to GET and POST parameters. Here both are `()` because the service does not take any parameter. The function returns an element of type html (using Lwt).


### Page Content

The content of Eliom application pages is made using functions. The **html function** takes two parameters: head and body, which are also functions that takes parameters coresponding to their content.

```ocaml
(html
           (head (title (txt "Page Title")) [])
           (body [p [txt "Hello World!"]])))
```
Most of the elements functions take a list of other elements as a parameter.

<!--wodoc:img src="http://public.db0.fr/dev/ocsigen/html.png" alt="ast"-->
Validity of HTML is checked at compile time, which means that a program that may generate a page that does not respect the recommendations of the W3C will be rejected at compile time.


### Download full code

*Warning: This third party code may be outdated. Please notify the author is something is broken, or do a pull request on github.*

- [Read the full code](https://github.com/db0company/Ocsigen-Quick-Howto/blob/master/page/example.eliom)
- [Download and try this example](https://github.com/db0company/Ocsigen-Quick-Howto)

### Links

- `About services`
- `About html generation`