cowboy/guide/req.md

Request object

Purpose

The request object is a special variable that can be used to interact with a request, extracting information from it or modifying it, and sending a response.

It's a special variable because it contains both immutable and mutable state. This means that some operations performed on the request object will always return the same result, while others will not. For example, obtaining request headers can be repeated safely. Obtaining the request body can only be done once, as it is read directly from the socket.

With few exceptions, all calls to the cowboy_req module will return an updated request object. You MUST use the new request object instead of the old one for all subsequent operations.

Request

Cowboy allows you to retrieve a lot of information about the request. All these calls return a {Value, Req} tuple, with Value the requested value and Req the updated request object.

The following access functions are defined in cowboy_req:

• method/1: the request method (<<"GET">>, <<"POST">>...)
• version/1: the HTTP version ({1,0} or {1,1})
• peer/1: the peer address and port number
• peer_addr/1: the peer address guessed using the request headers
• host/1: the hostname requested
• host_info/1: the result of the [...] match on the host
• port/1: the port number used for the connection
• path/1: the path requested
• path_info/1: the result of the [...] match on the path
• qs/1: the entire query string unmodified
• qs_val/{2,3}: the value for the requested query string key
• qs_vals/1: all key/values found in the query string
• fragment/1: the fragment part of the URL (e.g. #nav-links)
• host_url/1: the requested URL without the path, qs and fragment
• url/1: the requested URL
• binding/{2,3}: the value for the requested binding found during routing
• bindings/1: all key/values found during routing
• header/{2,3}: the value for the requested header name
• headers/1: all headers name/value
• cookie/{2,3}: the value for the requested cookie name
• cookies/1: all cookies name/value
• meta/{2,3}: the meta information for the requested key

All the functions above that can take two or three arguments take an optional third argument for the default value if none is found. Otherwise it will return undefined.

In addition, Cowboy allows you to parse headers using the parse_header/{2,3} function, which takes a header name as lowercase binary, the request object, and an optional default value. It returns {ok, ParsedValue, Req} if it could be parsed, {undefined, RawValue, Req} if Cowboy doesn't know this header, and {error, badarg} if Cowboy encountered an error while trying to parse it.

Finally, Cowboy allows you to set request meta information using the set_meta/3 function, which takes a name, a value and the request object and returns the latter modified.

Request body

Cowboy will not read the request body until you ask it to. If you don't, then Cowboy will simply discard it. It will not take extra memory space until you start reading it.

Cowboy has a few utility functions for dealing with the request body.

The function has_body/1 will return whether the request contains a body. Note that some clients may not send the right headers while still sending a body, but as Cowboy has no way of detecting it this function will return false.

The function body_length/1 retrieves the size of the request body. If the body is compressed, the value returned here is the compressed size. If a Transfer-Encoding header was passed in the request, then Cowboy will return a size of undefined, as it has no way of knowing it.

If you know the request contains a body, and that it is of appropriate size, then you can read it directly with either body/1 or body_qs/1. Otherwise, you will want to stream it with stream_body/1 and skip_body/1, with the streaming process optionally initialized using init_stream/4.

Multipart request body

Cowboy provides facilities for dealing with multipart bodies. They are typically used for uploading files. You can use two functions to process these bodies, multipart_data/1 and multipart_skip/1.

Response

You can send a response by calling the reply/{2,3,4} function. It takes the status code for the response (usually 200), an optional list of headers, an optional body and the request object.

The following snippet sends a simple response with no headers specified but with a body.

{ok, Req2} = cowboy_req:reply(200, [], "Hello world!", Req).

If this is the only line in your handler then make sure to return the Req2 variable to Cowboy so it can know you replied.

If you want to send HTML you'll need to specify the Content-Type header so the client can properly interpret it.

{ok, Req2} = cowboy_req:reply(200,
    [{<<"content-type">>, <<"text/html">>}],
	"<html><head>Hello world!</head><body><p>Hats off!</p></body></html>",
	Req).

You only need to make sure to follow conventions and to use a lowercase header name.

Chunked response

You can also send chunked responses using chunked_reply/{2,3}. Chunked responses allow you to send the body in chunks of various sizes. It is the recommended way of performing streaming if the client supports it.

You must first initiate the response by calling the aforementioned function, then you can call chunk/2 as many times as needed. The following snippet sends a body in three chunks.

{ok, Req2} = cowboy_req:chunked_reply(200, Req),
ok = cowboy_req:chunk("Hello...", Req2),
ok = cowboy_req:chunk("chunked...", Req2),
ok = cowboy_req:chunk("world!!", Req2).

As you can see the call to chunk/2 does not return a modified request object. It may return an error, however, so you should make sure that you match the return value on ok.

Response preconfiguration

Cowboy allows you to set response cookies, headers or body in advance without having to send the response at the same time. Then, when you decide to send it, all these informations will be built into the resulting response.

Some of the functions available for this purpose also give you additional functionality, like set_resp_cookie/4 which will build the appropriate Set-Cookie header, or set_resp_body_fun/{2,3} which allows you to stream the response body.

Note that any value given directly to reply/{2,3,4} will override all preset values. This means for example that you can set a default body and then override it when you decide to send a reply.

Reducing the memory footprint

When you are done reading information from the request object and know you are not going to access it anymore, for example when using long-polling or Websocket, you can use the compact/1 function to remove most of the data from the request object and free memory.

Req2 = cowboy_req:compact(Req).