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doc/src/guide/req_body.ezdoc

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+::: Reading the request body
+
+The Req object also allows you to read the request body.
+
+Because the request body can be of any size, all body
+reading operations will only work once, as Cowboy will
+not cache the result of these operations.
+
+Cowboy will not attempt to read the body until you do.
+If handler execution ends without reading it, Cowboy
+will simply skip it.
+
+Cowboy provides different ways to read the request body.
+You can read it directly, stream it, but also read and
+parse in a single call for form urlencoded formats or
+multipart. All of these except multipart are covered in
+this chapter. Multipart is covered later on in the guide.
+
+:: Check for request body
+
+You can check whether a body was sent with the request.
+
+``` erlang
+cowboy_req:has_body(Req).
+```
+
+It will return `true` if there is a request body, and
+`false` otherwise.
+
+Note that it is generally safe to assume that a body is
+sent for `POST`, `PUT` and `PATCH` requests, without
+having to explicitly check for it.
+
+:: Request body length
+
+You can obtain the body length if it was sent with the
+request.
+
+``` erlang
+{Length, Req2} = cowboy_req:body_length(Req).
+```
+
+The value returned will be `undefined` if the length
+couldn't be figured out from the request headers. If
+there's a body but no length is given, this means that
+the chunked transfer-encoding was used. You can read
+chunked bodies by using the stream functions.
+
+:: Reading the body
+
+You can read the whole body directly in one call.
+
+``` erlang
+{ok, Body, Req2} = cowboy_req:body(Req).
+```
+
+By default, Cowboy will attempt to read up to a
+size of 8MB. You can override this limit as needed.
+
+``` erlang
+{ok, Body, Req2} = cowboy_req:body(Req, [{length, 100000000}]).
+```
+
+You can also disable it.
+
+``` erlang
+{ok, Body, Req2} = cowboy_req:body(Req, [{length, infinity}]).
+```
+
+It is recommended that you do not disable it for public
+facing websites.
+
+If the body is larger than the limit, then Cowboy will return
+a `more` tuple instead, allowing you to stream it if you
+would like to.
+
+:: Streaming the body
+
+You can stream the request body by chunks.
+
+Cowboy returns a `more` tuple when there is more body to
+be read, and an `ok` tuple for the last chunk. This allows
+you to loop over all chunks.
+
+``` erlang
+body_to_console(Req) ->
+    case cowboy_req:body(Req) of
+        {ok, Data, Req2} ->
+            io:format("~s", [Data]),
+            Req2;
+        {more, Data, Req2} ->
+            io:format("~s", [Data]),
+            body_to_console(Req2)
+    end.
+```
+
+You can of course set the `length` option to configure the
+size of chunks.
+
+:: Rate of data transmission
+
+You can control the rate of data transmission by setting
+options when calling body functions. This applies not only
+to the functions described in this chapter, but also to
+the multipart functions.
+
+The `read_length` option defines the maximum amount of data
+to be received from the socket at once, in bytes.
+
+The `read_timeout` option defines the time Cowboy waits
+before that amount is received, in milliseconds.
+
+:: Transfer and content decoding
+
+Cowboy will by default decode the chunked transfer-encoding
+if any. It will not decode any content-encoding by default.
+
+The first time you call a body function you can set the
+`transfer_decode` and `content_decode` options. If the body
+was already started being read these options are simply
+ignored.
+
+The following example shows how to set both options.
+
+``` erlang
+{ok, Req2} = cowboy_req:body(Req, [
+    {transfer_decode, fun transfer_decode/2, TransferState},
+    {content_decode, fun content_decode/1}
+]).
+```
+
+:: Reading a form urlencoded body
+
+You can directly obtain a list of key/value pairs if the
+body was sent using the application/x-www-form-urlencoded
+content-type.
+
+``` erlang
+{ok, KeyValues, Req2} = cowboy_req:body_qs(Req).
+```
+
+You can then retrieve an individual value from that list.
+
+``` erlang
+{_, Lang} = lists:keyfind(lang, 1, KeyValues).
+```
+
+You should not attempt to match on the list as the order
+of the values is undefined.
+
+By default Cowboy will reject bodies with a size above
+64KB when using this function. You can override this limit
+by setting the `length` option.
+
+``` erlang
+{ok, KeyValues, Req2} = cowboy_req:body_qs(Req,
+    [{length, 2000000}]).
+```