[[resp]] == Sending a response The response must be sent using the Req object. Cowboy provides two different ways of sending responses: either directly or by streaming the body. Response headers and body may be set in advance. The response is sent as soon as one of the reply or stream reply function is called. Cowboy also provides a simplified interface for sending files. It can also send only specific parts of a file. While only one response is allowed for every request, HTTP/2 introduced a mechanism that allows the server to push additional resources related to the response. This chapter also describes how this feature works in Cowboy. === Reply Cowboy provides three functions for sending the entire reply, depending on whether you need to set headers and body. In all cases, Cowboy will add any headers required by the protocol (for example the date header will always be sent). When you need to set only the status code, use `cowboy_req:reply/2`: [source,erlang] Req = cowboy_req:reply(200, Req0). When you need to set response headers at the same time, use `cowboy_req:reply/3`: [source,erlang] ---- Req = cowboy_req:reply(303, #{ <<"location">> => <<"https://ninenines.eu">> }, Req0). ---- Note that the header name must always be a lowercase binary. When you also need to set the response body, use `cowboy_req:reply/4`: [source,erlang] ---- Req = cowboy_req:reply(200, #{ <<"content-type">> => <<"text/plain">> }, "Hello world!", Req0). ---- You should always set the content-type header when the response has a body. There is however no need to set the content-length header; Cowboy does it automatically. The response body and the header values must be either a binary or an iolist. An iolist is a list containing binaries, characters, strings or other iolists. This allows you to build a response from different parts without having to do any concatenation: [source,erlang] ---- Title = "Hello world!", Body = <<"Hats off!">>, Req = cowboy_req:reply(200, #{ <<"content-type">> => <<"text/html">> }, ["", Title, "", "

", Body, "

"], Req0). ---- This method of building responses is more efficient than concatenating. Behind the scenes, each element of the list is simply a pointer, and those pointers are used directly when writing to the socket. === Stream reply Cowboy provides two functions for initiating a response, and an additional function for streaming the response body. Cowboy will add any required headers to the response. // @todo For HTTP/1.1 Cowboy should probably not use chunked transfer-encoding if the content-length is set. When you need to set only the status code, use `cowboy_req:stream_reply/2`: [source,erlang] ---- Req = cowboy_req:stream_reply(200, Req0), cowboy_req:stream_body("Hello...", nofin, Req), cowboy_req:stream_body("chunked...", nofin, Req), cowboy_req:stream_body("world!!", fin, Req). ---- The second argument to `cowboy_req:stream_body/3` indicates whether this data terminates the body. Use `fin` for the final flag, and `nofin` otherwise. This snippet does not set a content-type header. This is not recommended. All responses with a body should have a content-type. The header can be set beforehand, or using the `cowboy_req:stream_reply/3`: [source,erlang] ---- Req = cowboy_req:stream_reply(200, #{ <<"content-type">> => <<"text/html">> }, Req0), cowboy_req:stream_body("Hello world!", nofin, Req), cowboy_req:stream_body("

Hats off!

", fin, Req). ---- HTTP provides a few different ways to stream response bodies. Cowboy will select the most appropriate one based on the HTTP version and the request and response headers. While not required by any means, it is recommended that you set the content-length header in the response if you know it in advance. This will ensure that the best response method is selected and help clients understand when the response is fully received. // @todo Document trailers here. === Preset response headers Cowboy provides functions to set response headers without immediately sending them. They are stored in the Req object and sent as part of the response when a reply function is called. To set response headers: [source,erlang] Req = cowboy_req:set_resp_header(<<"allow">>, "GET", Req0). Header names must be a lowercase binary. Do not use this function for setting cookies. Refer to the xref:cookies[Cookies] chapter for more information. To check if a response header has already been set: [source,erlang] cowboy_req:has_resp_header(<<"allow">>, Req). It returns `true` if the header was set, `false` otherwise. To delete a response header that was set previously: [source,erlang] Req = cowboy_req:delete_resp_header(<<"allow">>, Req0). === Overriding headers As Cowboy provides different ways of setting response headers and body, clashes may occur, so it's important to understand what happens when a header is set twice. Headers come from five different origins: * Protocol-specific headers (for example HTTP/1.1's connection header) * Other required headers (for example the date header) * Preset headers * Headers given to the reply function * Set-cookie headers Cowboy does not allow overriding protocol-specific headers. Set-cookie headers will always be appended at the end of the list of headers before sending the response. Headers given to the reply function will always override preset headers and required headers. If a header is found in two or three of these, then the one in the reply function is picked and the others are dropped. Similarly, preset headers will always override required headers. To illustrate, look at the following snippet. Cowboy by default sends the server header with the value "Cowboy". We can override it: [source,erlang] ---- Req = cowboy_req:reply(200, #{ <<"server">> => <<"yaws">> }, Req0). ---- === Preset response body Cowboy provides functions to set the response body without immediately sending it. It is stored in the Req object and sent when the reply function is called. To set the response body: [source,erlang] Req = cowboy_req:set_resp_body("Hello world!", Req0). // @todo Yeah we probably should add that function that // also sets the content-type at the same time... To check if a response body has already been set: [source,erlang] cowboy_req:has_resp_body(Req). It returns `true` if the body was set and is non-empty, `false` otherwise. // @todo We probably should also have a function that // properly removes the response body, including any // content-* headers. The preset response body is only sent if the reply function used is `cowboy_req:reply/2` or `cowboy_req:reply/3`. === Sending files Cowboy provides a shortcut for sending files. When using `cowboy_req:reply/4`, or when presetting the response header, you can give a `sendfile` tuple to Cowboy: [source,erlang] {sendfile, Offset, Length, Filename} Depending on the values for `Offset` or `Length`, the entire file may be sent, or just a part of it. The length is required even for sending the entire file. Cowboy sends it in the content-length header. To send a file while replying: [source,erlang] ---- Req = cowboy_req:reply(200, #{ <<"content-type">> => "image/png" }, {sendfile, 0, 12345, "path/to/logo.png"}, Req0). ---- // @todo An example of presetting a file would be useful, // but let's wait for the function that can set the // content-type at the same time. // @todo What about streaming many files? For example // it should be possible to build a tar file on the fly // while still using sendfile. Another example could be // proper support for multipart byte ranges. Yet another // example would be automatic concatenation of CSS or JS // files. === Push The HTTP/2 protocol introduced the ability to push resources related to the one sent in the response. Cowboy provides two functions for that purpose: `cowboy_req:push/3,4`. Push is only available for HTTP/2. Cowboy will automatically ignore push requests if the protocol doesn't support it. The push function must be called before any of the reply functions. Doing otherwise will result in a crash. To push a resource, you need to provide the same information as a client performing a request would. This includes the HTTP method, the URI and any necessary request headers. Cowboy by default only requires you to give the path to the resource and the request headers. The rest of the URI is taken from the current request (excluding the query string, set to empty) and the method is GET by default. The following snippet pushes a CSS file that is linked to in the response: [source,erlang] ---- cowboy_req:push("/static/style.css", #{ <<"accept">> => <<"text/css">> }, Req0), Req = cowboy_req:reply(200, #{ <<"content-type">> => <<"text/html">> }, ["My web page", "", "

Welcome to Erlang!

"], Req0). ---- To override the method, scheme, host, port or query string, simply pass in a fourth argument. The following snippet uses a different host name: [source,erlang] ---- cowboy_req:push("/static/style.css", #{ <<"accept">> => <<"text/css">> }, #{host => <<"cdn.example.org">>}, Req), ---- Pushed resources don't have to be files. As long as the push request is cacheable, safe and does not include a body, the resource can be pushed. Under the hood, Cowboy handles pushed requests the same as normal requests: a different process is created which will ultimately send a response to the client.