# Custom configuration options

<!--wodoc:aside class="concepts"-->**Concepts**

Custom configuration options<!--wodoc:end-->

It is not convenient to have to edit the code to change some configurations, like the location where are saved the favorite images in the Graffiti tutorial (see: `Saving favorite pictures`). Fortunately Ocsigen provides a mechanism to extend its configuration file.


## Basic interface

```ocaml
let static_dir =
  match Eliom_config.get_config () with
    | [Simplexmlparser.Element
     ("staticdir", [], [Simplexmlparser.PCData dir])] ->
        dir
    | [] ->
      raise (Ocsigen_extensions.Error_in_config_file
               ("staticdir must be configured"))
    | _ ->
      raise (Ocsigen_extensions.Error_in_config_file
               ("Unexpected content inside config"))
```
This will add a mandatory child to the eliom tag in the configuration file:

```
<eliom module="path/to/your/module.cma">
  <staticdir>/tmp/static</staticdir>
</eliom>
```

## New interface

From Eliom 4\.0 it is much easier to define configuration file extension. Have a look at module `Eliom_config`. For instance, here is how you can add an element "\<ldap\>" in the configuration file to store a list of LDAP servers your application interacts with.

```ocaml

(** An LDAP server is characterized by an host and a port. *)
type ldap_configuration = {
  mutable host : string;
  mutable port : int;
  }


(** We store a list of LDAP servers to interact with. *)
let ldap_servers = ref []

(** The user-defined extension of the configuration file. *)
let ldap_configuration = Ocsigen_extensions.Configuration.(
  (** Default configuration *)
  let config () = {
    host = "";
    port = 339;
  }
  in
  let init () =
    ldap_servers := config () :: !ldap_servers
  in
  let current () =
    List.hd !ldap_servers
  in

  (** Parsing functions. *)
  let req_attr name = attribute ~name ~obligatory:true
  and opt_attr name = attribute ~name ~obligatory:false
  in
  let name = "LDAP"
  and obligatory = false
  and attributes = [
    req_attr "host" (fun h -> (current ()).host <- h);
    opt_attr "port" (fun p -> (current ()).port <- int_of_string p);
    );
  ]
  in
  element ~init ~name ~obligatory ~attributes ()
)

let _ = Eliom_config.parse_config [ldap_configuration]

```