Greatly expand on the Req object

Cut in four different chapters: request, request body,
response and cookies.

1 files changed, 169 insertions, 0 deletions

diff --git a/guide/req_body.md b/guide/req_body.md
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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.
+
+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
+----------------
+
+If a content-length header was sent with the request,
+you can read the whole body directly.
+
+``` erlang
+{ok, Body, Req2} = cowboy_req:body(Req).
+```
+
+If no content-length header is available, Cowboy will
+return the `{error, chunked}` tuple. You will need to
+stream the request body instead.
+
+By default, Cowboy will reject all body sizes above 8MB,
+to prevent an attacker from needlessly filling up memory.
+You can override this limit however.
+
+``` erlang
+{ok, Body, Req2} = cowboy_req:body(100000000, Req).
+```
+
+You can also disable it.
+
+``` erlang
+{ok, Body, Req2} = cowboy_req:body(infinity, Req).
+```
+
+It is recommended that you do not disable it for public
+facing websites.
+
+Reading a body sent from an HTML form
+-------------------------------------
+
+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
+16KB when using this function. You can override this limit.
+
+``` erlang
+{ok, KeyValues, Req2} = cowboy_req:body_qs(500000, Req).
+```
+
+You can also disable it by passing the atom `infinity`,
+although it is not recommended.
+
+Streaming the body
+------------------
+
+You can stream the request body by chunks.
+
+``` erlang
+{ok, Chunk, Req2} = cowboy_req:stream_body(Req).
+```
+
+By default, Cowboy will attempt to read chunks of up to
+1MB in size. The chunks returned by this function will
+often be smaller, however. You can also change this limit.
+
+``` erlang
+{ok, Chunk, Req2} = cowboy_req:stream_body(500000, Req).
+```
+
+When Cowboy finishes reading the body, any subsequent call
+will return `{done, Req2}`. You can thus write a recursive
+function to read the whole body and perform an action on
+all chunks, for example printing them to the console.
+
+``` erlang
+body_to_console(Req) ->
+    case cowboy_req:stream_body(Req) of
+        {ok, Chunk, Req2} ->
+            io:format("~s", [Chunk]),
+            body_to_console(Req2);
+        {done, Req2} ->
+            Req2
+    end.
+```
+
+Advanced streaming
+------------------
+
+Cowboy will by default decode the chunked transfer-encoding
+if any. It will not decode any content-encoding by default.
+
+Before starting to stream, you can configure the functions
+that will be used for decoding both transfer-encoding and
+content-encoding.
+
+``` erlang
+{ok, Req2} = cowboy_req:init_stream(fun transfer_decode/2,
+    TransferStartState, fun content_decode/1, Req).
+```
+
+Note that you do not need to call this function generally,
+as Cowboy will happily initialize the stream on its own.
+
+Skipping the body
+-----------------
+
+If you do not need the body, or if you started streaming
+the body but do not need the rest of it, you can skip it.
+
+``` erlang
+{ok, Req2} = cowboy_req:skip_body(Req).
+```
+
+You do not have to call this function though, as Cowboy will
+do it automatically when handler execution ends.