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## Chapter 16. Using cookies

Cookies are a mechanism allowing applications to maintain state on top of the stateless HTTP protocol.

Cookies are a name/value store where the names and values are stored in plain text. They expire either after a delay or when the browser closes. They can be configured on a specific domain name or path, and restricted to secure resources (sent or downloaded over HTTPS), or restricted to the server (disallowing access from client-side scripts).

Cookie names are de facto case sensitive.

Cookies are stored client-side and sent with every subsequent request that matches the domain and path for which they were stored, until they expire. This can create a non-negligible cost.

Cookies should not be considered secure. They are stored on the user’s computer in plain text, and can be read by any program. They can also be read by proxies when using clear connections. Always validate the value before using it, and never store any sensitive information inside it.

Cookies set by the server are only available in requests following the client reception of the response containing them.

Cookies may be sent repeatedly. This is often useful to update the expiration time and avoid losing a cookie.

## 16.1. Setting cookies

By default cookies are defined for the duration of the session:

    SessionID = generate_session_id(),
    Req = cowboy_req:set_resp_cookie(<<"sessionid">>, SessionID, Req0).

They can also be set for a duration in seconds:

    SessionID = generate_session_id(),
    Req = cowboy_req:set_resp_cookie(<<"sessionid">>, SessionID, Req0,
        #{max_age => 3600}).

To delete cookies, set `max_age` to 0:

    SessionID = generate_session_id(),
    Req = cowboy_req:set_resp_cookie(<<"sessionid">>, SessionID, Req0,
        #{max_age => 0}).

To restrict cookies to a specific domain and path, the options of the same name can be used:

    Req = cowboy_req:set_resp_cookie(<<"inaccount">>, <<"1">>, Req0,
        #{domain => "my.example.org", path => "/account"}).

Cookies will be sent with requests to this domain and all its subdomains, and to resources on this path or deeper in the path hierarchy.

To restrict cookies to secure channels (typically resources available over HTTPS):

    SessionID = generate_session_id(),
    Req = cowboy_req:set_resp_cookie(<<"sessionid">>, SessionID, Req0,
        #{secure => true}).

To prevent client-side scripts from accessing a cookie:

    SessionID = generate_session_id(),
    Req = cowboy_req:set_resp_cookie(<<"sessionid">>, SessionID, Req0,
        #{http_only => true}).

Cookies may also be set client-side, for example using Javascript.

## 16.2. Reading cookies

The client only ever sends back the cookie name and value. All other options that can be set are never sent back.

Cowboy provides two functions for reading cookies. Both involve parsing the cookie header(s) and so should not be called repeatedly.

You can get all cookies as a key/value list:

    Cookies = cowboy_req:parse_cookies(Req),
    {_, Lang} = lists:keyfind(<<"lang">>, 1, Cookies).

Or you can perform a match against cookies and retrieve only the ones you need, while at the same time doing any required post processing using constraints. This function returns a map:

    #{id := ID, lang := Lang} = cowboy_req:match_cookies([id, lang], Req).

You can use constraints to validate the values while matching them. The following snippet will crash if the `id` cookie is not an integer number or if the `lang` cookie is empty. Additionally the `id` cookie value will be converted to an integer term:

    CookiesMap = cowboy_req:match_cookies([{id, int}, {lang, nonempty}], Req).

Note that if two cookies share the same name, then the map value will be a list of the two cookie values.

A default value can be provided. The default will be used if the `lang` cookie is not found. It will not be used if the cookie is found but has an empty value:

    #{lang := Lang} = cowboy_req:match_cookies([{lang, [], <<"en-US">>}], Req).

If no default is provided and the value is missing, an exception is thrown.

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