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1 files changed, 240 insertions, 123 deletions
diff --git a/doc/src/guide/resp.asciidoc b/doc/src/guide/resp.asciidoc index 1ffdfbd..b8ba7d5 100644 --- a/doc/src/guide/resp.asciidoc +++ b/doc/src/guide/resp.asciidoc @@ -1,201 +1,318 @@ [[resp]] == Sending a response -The Req object also allows you to send a response. +The response must be sent using the Req object. -You can only send one response. Any other attempt will -trigger a crash. The response may be sent in one go or -with its body streamed by chunks of arbitrary size. +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. -You can also set headers or the response body in advance -and Cowboy will use them when you finally do reply. +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 -You can send a reply with no particular headers or body. -Cowboy will make sure to send the mandatory headers with -the response. +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] -Req2 = cowboy_req:reply(200, Req). +Req = cowboy_req:reply(200, Req0). -You can define headers to be sent with the response. Note -that header names must be lowercase. Again, Cowboy will -make sure to send the mandatory headers with the response. +When you need to set response headers at the same time, +use `cowboy_req:reply/3`: [source,erlang] ---- -Req2 = cowboy_req:reply(303, [ - {<<"location">>, <<"http://ninenines.eu">>} -], Req). +Req = cowboy_req:reply(303, #{ + <<"location">> => <<"http://ninenines.eu">> +}, Req0). ---- -You can override headers that Cowboy would send otherwise. -Any header set by the user will be used over the ones set -by Cowboy. For example, you can advertise yourself as a -different server. +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] ---- -Req2 = cowboy_req:reply(200, [ - {<<"server">>, <<"yaws">>} -], Req). +Req = cowboy_req:reply(200, #{ + <<"content-type">> => <<"text/plain">> +}, "Hello world!", Req0). ---- -We also saw earlier how to force close the connection by -overriding the connection header. +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. -Finally, you can also send a body with the response. Cowboy -will automatically set the content-length header if you do. -We recommend that you set the content-type header so the -client may know how to read the body. +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] ---- -Req2 = cowboy_req:reply(200, [ - {<<"content-type">>, <<"text/plain">>} -], "Hello world!", Req). +Title = "Hello world!", +Body = <<"Hats off!">>, +Req = cowboy_req:reply(200, #{ + <<"content-type">> => <<"text/html">> +}, ["<html><head><title>", Title, "</title></head>", + "<body><p>", Body, "</p></body></html>"], Req0). ---- -Here is the same example but sending HTML this time. +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. -[source,erlang] ----- -Req2 = cowboy_req:reply(200, [ - {<<"content-type">>, <<"text/html">>} -], "<html><head>Hello world!</head><body><p>Hats off!</p></body></html>", Req). ----- +=== Stream reply -Note that the reply is sent immediately. +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. -=== Chunked reply +// @todo For HTTP/1.1 Cowboy should probably not use chunked transfer-encoding if the content-length is set. -You can also stream the response body. First, you need to -initiate the reply by sending the response status code. -Then you can send the body in chunks of arbitrary size. +When you need to set only the status code, +use `cowboy_req:stream_reply/2`: [source,erlang] -Req2 = cowboy_req:chunked_reply(200, Req), -cowboy_req:chunk("Hello...", Req2), -cowboy_req:chunk("chunked...", Req2), -cowboy_req:chunk("world!!", Req2). +---- +Req = cowboy_req:stream_reply(200, Req0), -You should make sure to match on `ok` as an error may be -returned. +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. -While it is possible to send a chunked response without -a content-type header, it is still recommended. You can -set this header or any other just like for normal replies. +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] ---- -Req2 = cowboy_req:chunked_reply(200, [ - {<<"content-type">>, <<"text/html">>} -], Req), -cowboy_req:chunk("<html><head>Hello world!</head>", Req2), -cowboy_req:chunk("<body><p>Hats off!</p></body></html>", Req2). +Req = cowboy_req:stream_reply(200, #{ + <<"content-type">> => <<"text/html">> +}, Req0), + +cowboy_req:stream_body("<html><head>Hello world!</head>", nofin, Req), +cowboy_req:stream_body("<body><p>Hats off!</p></body></html>", fin, Req). ---- -Note that the reply and each chunk following it are sent -immediately. +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 -You can define response headers in advance. They will be -merged into the headers given in the reply call. Headers -in the reply call override preset response headers which -override the default Cowboy 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] -Req2 = cowboy_req:set_resp_header(<<"allow">>, "GET", Req). +Req = cowboy_req:set_resp_header(<<"allow">>, "GET", Req0). + +Header names must be a lowercase binary. -You can check if a response header has already been set. -This will only check the response headers that you set, -and not the ones Cowboy will add when actually sending -the reply. +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 will return `true` if the header is defined, and `false` -otherwise. +It returns `true` if the header was set, `false` otherwise. -Finally, you can also delete a preset response header if -needed. If you do, it will not be sent. +To delete a response header that was set previously: [source,erlang] -Req2 = cowboy_req:delete_resp_header(<<"allow">>, Req). +Req = cowboy_req:delete_resp_header(<<"allow">>, Req0). -=== Preset response body +=== Overriding headers -You can set the response body in advance. Note that this -body will be ignored if you then choose to send a chunked -reply, or if you send a reply with an explicit body. +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. -[source,erlang] -Req2 = cowboy_req:set_resp_body("Hello world!", Req). +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. -You can also set a fun that will be called when it is time -to send the body. There are three different ways of doing -that. +Similarly, preset headers will always override required +headers. -If you know the length of the body that needs to be sent, -you should specify it, as it will help clients determine -the remaining download time and allow them to inform the -user. +To illustrate, look at the following snippet. Cowboy by +default sends the server header with the value "Cowboy". +We can override it: [source,erlang] ---- -F = fun (Socket, Transport) -> - Transport:send(Socket, "Hello world!") -end, -Req2 = cowboy_req:set_resp_body_fun(12, F, Req). +Req = cowboy_req:reply(200, #{ + <<"server">> => <<"yaws">> +}, Req0). ---- -If you do not know the length of the body, you should use -a chunked response body fun instead. +=== 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] ----- -F = fun (SendChunk) -> - Body = lists:duplicate(random:uniform(1024, $a)), - SendChunk(Body) -end, -Req2 = cowboy_req:set_resp_body_fun(chunked, F, Req). ----- +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... -Finally, you can also send data on the socket directly, -without knowing the length in advance. Cowboy may be -forced to close the connection at the end of the response -though depending on the protocol capabilities. +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] ---- -F = fun (Socket, Transport) -> - Body = lists:duplicate(random:uniform(1024, $a)), - Transport:send(Socket, Body) -end, -Req2 = cowboy_req:set_resp_body_fun(F, Req). +Req = cowboy_req:reply(200, #{ + <<"content-type">> => "image/png" +}, {sendfile, 0, 12345, "path/to/logo.png"}, Req0). ---- -=== Sending files +// @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. -You can send files directly from disk without having to -read them. Cowboy will use the `sendfile` syscall when -possible, which means that the file is sent to the socket -directly from the kernel, which is a lot more performant -than doing it from userland. +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. -Again, it is recommended to set the size of the file if it -can be known in advance. +The following snippet pushes a CSS file that is linked to +in the response: [source,erlang] ---- -F = fun (Socket, Transport) -> - Transport:sendfile(Socket, "priv/styles.css") -end, -Req2 = cowboy_req:set_resp_body_fun(FileSize, F, Req). +cowboy_req:push("/static/style.css", #{ + <<"accept">> => <<"text/css">> +}, Req0), +Req = cowboy_req:reply(200, #{ + <<"content-type">> => <<"text/html">> +}, ["<html><head><title>My web page</title>", + "<link rel='stylesheet' type='text/css' href='/static/style.css'>", + "<body><p>Welcome to Erlang!</p></body></html>"], Req0). ---- -Please see the Ranch guide for more information about -sending files. +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. |