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== The Req object

The Req object is a variable used for obtaining information
about a request, read its body or send a response.

It is not really an object in the object-oriented sense.
It is a simple map that can be directly accessed or
used when calling functions from the `cowboy_req` module.

The Req object is the subject of a few different chapters.
In this chapter we will learn about the Req object and
look at how to retrieve information about the request.

=== Direct access

The Req map contains a number of fields which are documented
and can be accessed directly. They are the fields that have
a direct mapping to HTTP: the request `method`; the HTTP
`version` used; the effective URI components `scheme`,
`host`, `port`, `path` and `qs`; the request `headers`;
the connection `peer` address and port; and the TLS
certificate `cert` when applicable.

Note that the `version` field can be used to determine
whether a connection is using HTTP/2.

To access a field, you can simply match in the function
head. The following example sends a simple "Hello world!"
response when the `method` is GET, and a 405 error

init(Req0=#{method := <<"GET">>}, State) ->
    Req = cowboy_req:reply(200, #{
        <<"content-type">> => <<"text/plain">>
    }, <<"Hello world!">>, Req0),
    {ok, Req, State};
init(Req0, State) ->
    Req = cowboy_req:reply(405, #{
        <<"allow">> => <<"GET">>
    }, Req0),
    {ok, Req, State}.

Any other field is internal and should not be accessed.
They may change in future releases, including maintenance
releases, without notice.

Modifying the Req object, while allowed, is not recommended
unless strictly necessary. If adding new fields, make sure
to namespace the field names so that no conflict can occur
with future Cowboy updates or third party projects.

=== Introduction to the cowboy_req interface

// @todo Link to cowboy_req manual

Functions in the `cowboy_req` module provide access to
the request information but also various operations that
are common when dealing with HTTP requests.

All the functions that begin with a verb indicate an action.
Other functions simply return the corresponding value
(sometimes that value does need to be built, but the
cost of the operation is equivalent to retrieving a value).

Some of the `cowboy_req` functions return an updated Req
object. They are the read, reply, set and delete functions.
While ignoring the returned Req will not cause incorrect
behavior for some of them, it is highly recommended to
always keep and use the last returned Req object. The
manual for `cowboy_req` details these functions and what
modifications are done to the Req object.

Some of the calls to `cowboy_req` have side effects. This
is the case of the read and reply functions. Cowboy reads
the request body or replies immediately when the function
is called.

All functions will crash if something goes wrong. There
is usually no need to catch these errors, Cowboy will
send the appropriate 4xx or 5xx response depending on
where the crash occurred.

=== Request method

The request method can be retrieved directly:

[source, erlang]
#{method := Method} = Req.

Or using a function:

Method = cowboy_req:method(Req).

The method is a case sensitive binary string. Standard
methods include GET, HEAD, OPTIONS, PATCH, POST, PUT

=== HTTP version

The HTTP version is informational. It does not indicate that
the client implements the protocol well or fully.

There is typically no need to change behavior based on the
HTTP version: Cowboy already does it for you.

It can be useful in some cases, though. For example, one may
want to redirect HTTP/1.1 clients to use Websocket, while HTTP/2
clients keep using HTTP/2.

The HTTP version can be retrieved directly:

#{version := Version} = Req.

Or using a function:

Version = cowboy_req:version(Req).

Cowboy defines the `'HTTP/1.0'`, `'HTTP/1.1'` and `'HTTP/2'`
versions. Custom protocols can define their own values as

=== Effective request URI

The scheme, host, port, path and query string components
of the effective request URI can all be retrieved directly:

    scheme := Scheme,
    host := Host,
    port := Port,
    path := Path,
    qs := Qs
} = Req.

Or using the related functions:

Scheme = cowboy_req:scheme(Req),
Host = cowboy_req:host(Req),
Port = cowboy_req:port(Req),
Path = cowboy_req:path(Req).
Qs = cowboy_req:qs(Req).

The scheme and host are lowercased case insensitive binary
strings. The port is an integer representing the port number.
The path and query string are case sensitive binary strings.

Cowboy defines only the `<<"http">>` and `<<"https">>` schemes.
They are chosen so that the scheme will only be `<<"https">>`
for requests on secure HTTP/1.1 or HTTP/2 connections.
// @todo Is that tested well?

The effective request URI itself can be reconstructed with
the `cowboy_req:uri/1,2` function. By default, an absolute
URI is returned:

%% scheme://host[:port]/path[?qs]
URI = cowboy_req:uri(Req).

Options are available to either disable or replace some
or all of the components. Various URIs or URI formats can
be generated this way, including the origin form:

%% /path[?qs]
URI = cowboy_req:uri(Req, #{host => undefined}).

The protocol relative form:

%% //host[:port]/path[?qs]
URI = cowboy_req:uri(Req, #{scheme => undefined}).

The absolute URI without a query string:

URI = cowboy_req:uri(Req, #{qs => undefined}).

A different host:

URI = cowboy_req:uri(Req, #{host => <<"example.org">>}).

And any other combination.

=== Bindings

Bindings are the host and path components that you chose
to extract when defining the routes of your application.
They are only available after the routing.

Cowboy provides functions to retrieve one or all bindings.

To retrieve a single value:

Value = cowboy_req:binding(userid, Req).

When attempting to retrieve a value that was not bound,
`undefined` will be returned. A different default value
can be provided:

Value = cowboy_req:binding(userid, Req, 42).

To retrieve everything that was bound:

Bindings = cowboy_req:bindings(Req).

They are returned as a map, with keys being atoms.

The Cowboy router also allows you to capture many host
or path segments at once using the `...` qualifier.

To retrieve the segments captured from the host name:

HostInfo = cowboy_req:host_info(Req).

And the path segments:

PathInfo = cowboy_req:path_info(Req).

Cowboy will return `undefined` if `...` was not used
in the route.

=== Query parameters

Cowboy provides two functions to access query parameters.
You can use the first to get the entire list of parameters.

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

Cowboy will only parse the query string, and not do any
transformation. This function may therefore return duplicates,
or parameter names without an associated value. The order of
the list returned is undefined.

When a query string is `key=1&key=2`, the list returned will
contain two parameters of name `key`.

The same is true when trying to use the PHP-style suffix `[]`.
When a query string is `key[]=1&key[]=2`, the list returned will
contain two parameters of name `key[]`.

When a query string is simply `key`, Cowboy will return the
list `[{<<"key">>, true}]`, using `true` to indicate that the
parameter `key` was defined, but with no value.

The second function Cowboy provides allows you to match out
only the parameters you are interested in, and at the same
time do any post processing you require using xref:constraints[constraints].
This function returns a map.

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

Constraints can be applied automatically. The following
snippet will crash when the `id` parameter is not an integer,
or when the `lang` parameter is empty. At the same time, the
value for `id` will be converted to an integer term:

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

A default value may also be provided. The default will be used
if the `lang` key is not found. It will not be used if
the key is found but has an empty value.

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

If no default is provided and the value is missing, the
query string is deemed invalid and the process will crash.

When the query string is `key=1&key=2`, the value for `key`
will be the list `[1, 2]`. Parameter names do not need to
include the PHP-style suffix. Constraints may be used to
ensure that only one value was passed through.

=== Headers

Header values can be retrieved either as a binary string
or parsed into a more meaningful representation.

The get the raw value:

HeaderVal = cowboy_req:header(<<"content-type">>, Req).

Cowboy expects all header names to be provided as lowercase
binary strings. This is true for both requests and responses,
regardless of the underlying protocol.

When the header is missing from the request, `undefined`
will be returned. A different default can be provided:

HeaderVal = cowboy_req:header(<<"content-type">>, Req, <<"text/plain">>).

All headers can be retrieved at once, either directly:

#{headers := AllHeaders} = Req.

Or using a function:

AllHeaders = cowboy_req:headers(Req).

Cowboy provides equivalent functions to parse individual
headers. There is no function to parse all headers at once.

To parse a specific header:

ParsedVal = cowboy_req:parse_header(<<"content-type">>, Req).

An exception will be thrown if it doesn't know how to parse the
given header, or if the value is invalid. The list of known headers
and default values can be found in the manual.

When the header is missing, `undefined` is returned. You can
change the default value. Note that it should be the parsed value

ParsedVal = cowboy_req:parse_header(<<"content-type">>, Req,
    {<<"text">>, <<"plain">>, []}).

=== Peer

The peer address and port number for the connection can be
retrieved either directly or using a function.

To retrieve the peer directly:

#{peer := {IP, Port}} = Req.

And using a function:

{IP, Port} = cowboy_req:peer(Req).

Note that the peer corresponds to the remote end of the
connection to the server, which may or may not be the
client itself. It may also be a proxy or a gateway.