From 7839f1367194813c70d7e223a476f96a62b298ae Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lo=C3=AFc=20Hoguin?= <essen@ninenines.eu>
Date: Wed, 24 Aug 2016 17:25:33 +0200
Subject: More 2.0 documentation updates

Still incomplete.
---
 doc/src/guide/req.asciidoc | 403 +++++++++++++++++++++++++++++----------------
 1 file changed, 265 insertions(+), 138 deletions(-)

(limited to 'doc/src/guide/req.asciidoc')

diff --git a/doc/src/guide/req.asciidoc b/doc/src/guide/req.asciidoc
index 09d442a..39fcde6 100644
--- a/doc/src/guide/req.asciidoc
+++ b/doc/src/guide/req.asciidoc
@@ -1,155 +1,293 @@
++++
+title = "The Req object"
++++
+
 [[req]]
 == The Req object
 
-The Req object is this variable that you will use to obtain
-information about a request, read the body of the request
-and send a response.
+The Req object is a variable used for obtaining information
+about a request, read its body or send a response.
+
+It is not really an object in the object-oriented sense.
+It is a simple map that can be directly accessed or
+used when calling functions from the `cowboy_req` module.
+
+The Req object is the subject of a few different chapters.
+In this chapter we will learn about the Req object and
+look at how to retrieve information about the request.
+
+=== Direct access
+
+The Req map contains a number of fields which are documented
+and can be accessed directly. They are the fields that have
+a direct mapping to HTTP: the request `method`; the HTTP
+`version` used; the effective URI components `scheme`,
+`host`, `port`, `path` and `qs`; the request `headers`;
+and the connection `peer` address and port.
+
+Note that the `version` field can be used to determine
+whether a connection is using HTTP/2.
+
+To access a field, you can simply match in the function
+head. The following example sends a simple "Hello world!"
+response when the `method` is GET, and a 405 error
+otherwise.
+
+[source,erlang]
+----
+init(Req0=#{method := <<"GET">>}, State) ->
+	Req = cowboy_req:reply(200, #{
+		<<"content-type">> => <<"text/plain">>
+	}, <<"Hello world!">>, Req0),
+	{ok, Req, State};
+init(Req0, State) ->
+	Req = cowboy_req:reply(405, #{
+		<<"allow">> => <<"GET">>
+	}, Req0),
+	{ok, Req, State}.
+----
+
+Any other field is internal and should not be accessed.
+They may change in future releases, including maintenance
+releases, without notice.
+
+Modifying the Req object, while allowed, is not recommended
+unless strictly necessary. If adding new fields, make sure
+to namespace the field names so that no conflict can occur
+with future Cowboy updates or third party projects.
+
+// @todo There are currently no tests for direct access.
 
-=== A special variable
+=== Introduction to the cowboy_req interface
 
-While we call it an "object", it is not an object in the
-OOP sense of the term. In fact it is completely opaque
-to you and the only way you can perform operations using
-it is by calling the functions from the `cowboy_req`
-module.
+// @todo Link to cowboy_req manual
 
-Almost all the calls to the `cowboy_req` module will
-return an updated request object. Just like you would
-keep the updated `State` variable in a gen_server,
-you MUST keep the updated `Req` variable in a Cowboy
-handler. Cowboy will use this object to know whether
-a response has been sent when the handler has finished
-executing.
+Functions in the `cowboy_req` module provide access to
+the request information but also various operations that
+are common when dealing with HTTP requests.
 
-The Req object allows accessing both immutable and
-mutable state. This means that calling some of the
-functions twice will not produce the same result.
-For example, when streaming the request body, the
-function will return the body by chunks, one at a
-time, until there is none left.
+All the functions that begin with a verb indicate an action.
+Other functions simply return the corresponding value
+(sometimes that value does need to be built, but the
+cost of the operation is equivalent to retrieving a value).
 
-=== Overview of the cowboy_req interface
+Some of the `cowboy_req` functions return an updated Req
+object. They are the read, reply, set and delete functions.
+While ignoring the returned Req will not cause incorrect
+behavior for some of them, it is highly recommended to
+always keep and use the last returned Req object. The
+manual for `cowboy_req` details these functions and what
+modifications are done to the Req object.
 
-With the exception of functions manipulating the request
-body, all functions return a single value. Depending on
-the function this can be the requested value (method,
-host, path, ...), a boolean (has_body, has_resp_header...)
-a new Req object (set_resp_body, set_resp_header...), or
-simply the atom `ok` (chunk, continue, ...).
+Some of the calls to `cowboy_req` have side effects. This
+is the case of the read and reply functions. Cowboy reads
+the request body or replies immediately when the function
+is called.
 
-The request body reading functions may return `{Result, Req}`
-or `{Result, Value, Req}`. The functions in this category
-are `body/{1,2}`, `body_qs/{1,2}`, `part/{1,2}`, `part_body/{1,2}`.
+All functions will crash if something goes wrong. There
+is usually no need to catch these errors, Cowboy will
+send the appropriate 4xx or 5xx response depending on
+where the crash occurred.
 
-This chapter covers the access functions mainly. Cookies,
-request body and response functions are covered in their
-own chapters.
+=== Request method
 
-=== Request
+The request method can be retrieved directly:
 
-When a client performs a request, it first sends a few required
-values. They are sent differently depending on the protocol
-being used, but the intent is the same. They indicate to the
-server the type of action it wants to do and how to locate
-the resource to perform it on.
+[source, erlang]
+#{method := Method} = Req.
 
-The method identifies the action. Standard methods include
-GET, HEAD, OPTIONS, PATCH, POST, PUT, DELETE. Method names
-are case sensitive.
+Or using a function:
 
 [source,erlang]
 Method = cowboy_req:method(Req).
 
-The host, port and path parts of the URL identify the resource
-being accessed. The host and port information may not be
-available if the client uses HTTP/1.0.
+The method is a case sensitive binary string. Standard
+methods include GET, HEAD, OPTIONS, PATCH, POST, PUT
+or DELETE.
+
+=== HTTP version
+
+The HTTP version is informational. It does not indicate that
+the client implements the protocol well or fully.
+
+There is typically no need to change behavior based on the
+HTTP version: Cowboy already does it for you.
+
+It can be useful in some cases, though. For example, one may
+want to redirect HTTP/1.1 clients to use Websocket, while HTTP/2
+clients keep using HTTP/2.
+
+The HTTP version can be retrieved directly:
+
+[source,erlang]
+#{version := Version} = Req.
+
+Or using a function:
 
 [source,erlang]
+Version = cowboy_req:version(Req).
+
+Cowboy defines the `'HTTP/1.0'`, `'HTTP/1.1'` and `'HTTP/2'`
+versions. Custom protocols can define their own values as
+atoms.
+
+=== Effective request URI
+
+The scheme, host, port, path and query string components
+of the effective request URI can all be retrieved directly:
+
+[source,erlang]
+----
+#{
+	scheme := Scheme,
+	host := Host,
+	port := Port,
+	path := Path,
+	qs := Qs
+} = Req.
+----
+
+Or using the related functions:
+
+[source,erlang]
+Scheme = cowboy_req:scheme(Req),
 Host = cowboy_req:host(Req),
 Port = cowboy_req:port(Req),
 Path = cowboy_req:path(Req).
+Qs = cowboy_req:qs(Req).
+
+The scheme and host are lowercased case insensitive binary
+strings. The port is an integer representing the port number.
+The path and query string are case sensitive binary strings.
+
+Cowboy defines only the <<"http">> and <<"https">> schemes.
+They are chosen so that the scheme will only be <<"https">>
+for requests on secure HTTP/1.1 or HTTP/2 connections.
+// @todo Is that tested well?
 
-The version used by the client can of course also be obtained.
+The effective request URI itself can be reconstructed with
+the `cowboy_req:uri/1,2` function. By default, an absolute
+URI is returned:
 
 [source,erlang]
-Version = cowboy_req:version(Req).
+%% scheme://host[:port]/path[?qs]
+URI = cowboy_req:uri(Req).
+
+Options are available to either disable or replace some
+or all of the components. Various URIs or URI formats can
+be generated this way, including the origin form:
+
+[source,erlang]
+%% /path[?qs]
+URI = cowboy_req:uri(Req, #{host => undefined}).
+
+The protocol relative form:
+
+[source,erlang]
+%% //host[:port]/path[?qs]
+URI = cowboy_req:uri(Req, #{scheme => undefined}).
 
-Do note however that clients claiming to implement one version
-of the protocol does not mean they implement it fully, or even
-properly.
+The absolute URI without a query string:
+
+[source,erlang]
+URI = cowboy_req:uri(Req, #{qs => undefined}).
+
+A different host:
+
+[source,erlang]
+URI = cowboy_req:uri(Req, #{host => <<"example.org">>}).
+
+And any other combination.
 
 === Bindings
 
-After routing the request, bindings are available. Bindings
-are these parts of the host or path that you chose to extract
-when defining the routes of your application.
+// @todo Bindings should probably be a map themselves.
 
-You can fetch a single binding. The value will be `undefined`
-if the binding doesn't exist.
+Bindings are the host and path components that you chose
+to extract when defining the routes of your application.
+They are only available after the routing.
+
+Cowboy provides functions to retrieve one or all bindings.
+
+To retrieve a single value:
 
 [source,erlang]
-Binding = cowboy_req:binding(my_binding, Req).
+Value = cowboy_req:binding(userid, Req).
 
-If you need a different value when the binding doesn't exist,
-you can change the default.
+When attempting to retrieve a value that was not bound,
+`undefined` will be returned. A different default value
+can be provided:
 
 [source,erlang]
-Binding = cowboy_req:binding(my_binding, Req, 42).
+Value = cowboy_req:binding(userid, Req, 42).
 
-You can also obtain all bindings in one call. They will be
-returned as a list of key/value tuples.
+To retrieve everything that was bound:
 
 [source,erlang]
-AllBindings = cowboy_req:bindings(Req).
+Bindings = cowboy_req:bindings(Req).
+
+They are returned as a list of key/value pairs, with
+keys being atoms.
+
+// ...
+
+The Cowboy router also allows you to capture many host
+or path segments at once using the `...` qualifier.
 
-If you used `...` at the beginning of the route's pattern
-for the host, you can retrieve the matched part of the host.
-The value will be `undefined` otherwise.
+To retrieve the segments captured from the host name:
 
 [source,erlang]
 HostInfo = cowboy_req:host_info(Req).
 
-Similarly, if you used `...` at the end of the route's
-pattern for the path, you can retrieve the matched part,
-or get `undefined` otherwise.
+And the path segments:
 
 [source,erlang]
 PathInfo = cowboy_req:path_info(Req).
 
-=== Query string
+Cowboy will return `undefined` if `...` was not used
+in the route.
 
-The raw query string can be obtained directly.
+=== Query parameters
 
-[source,erlang]
-Qs = cowboy_req:qs(Req).
-
-You can parse the query string and then use standard library
-functions to access individual values.
+Cowboy provides two functions to access query parameters.
+You can use the first to get the entire list of parameters.
 
 [source,erlang]
 QsVals = cowboy_req:parse_qs(Req),
 {_, Lang} = lists:keyfind(<<"lang">>, 1, QsVals).
 
-You can match the query string into a map.
+Cowboy will only parse the query string, and not do any
+transformation. This function may therefore return duplicates,
+or parameter names without an associated value.
+
+When a query string is `key=1&key=2`, the list returned will
+contain two parameters of name `key`.
+
+The same is true when trying to use the PHP-style suffix `[]`.
+When a query string is `key[]=1&key[]=2`, the list returned will
+contain two parameters of name `key[]`.
+
+When a query string is simply `key`, Cowboy will return the
+list `[{<<"key">>, true}]`, using `true` to indicate that the
+parameter `key` was defined, but with no value.
+
+The second function Cowboy provides allows you to match out
+only the parameters you are interested in, and at the same
+time do any post processing you require using ^constraints^.
+This function returns a map.
 
 [source,erlang]
 #{id := ID, lang := Lang} = cowboy_req:match_qs([id, lang], Req).
 
-You can use constraints to validate the values while matching
-them. The following snippet will crash if the `id` value is
-not an integer number or if the `lang` value is empty. Additionally
-the `id` value will be converted to an integer term, saving
-you a conversion step.
+Constraints can be applied automatically. The following
+snippet will crash when the `id` parameter is not an integer,
+or when the `lang` parameter is empty. At the same time, the
+value for `id` will be converted to an integer term:
 
 [source,erlang]
 QsMap = cowboy_req:match_qs([{id, int}, {lang, nonempty}], Req).
 
-Note that in the case of duplicate query string keys, the map
-value will become a list of the different values.
-
-Read more about ^constraints^.
-
-A default value can be provided. The default will be used
+A default value may also be provided. The default will be used
 if the `lang` key is not found. It will not be used if
 the key is found but has an empty value.
 
@@ -159,51 +297,56 @@ the key is found but has an empty value.
 If no default is provided and the value is missing, the
 query string is deemed invalid and the process will crash.
 
-=== Request URL
-
-You can reconstruct the full URL of the resource.
-
-[source,erlang]
-URL = cowboy_req:url(Req).
-
-You can also obtain only the base of the URL, excluding the
-path and query string.
-
-[source,erlang]
-BaseURL = cowboy_req:host_url(Req).
+When the query string is `key=1&key=2`, the value for `key`
+will be the list `[1, 2]`. Parameter names do not need to
+include the PHP-style suffix. Constraints may be used to
+ensure that only one value was passed through.
 
 === Headers
 
-Cowboy allows you to obtain the header values as string,
+Header values can be retrieved either as a binary string
 or parsed into a more meaningful representation.
 
-This will get the string value of a header.
+The get the raw value:
 
 [source,erlang]
 HeaderVal = cowboy_req:header(<<"content-type">>, Req).
 
-You can of course set a default in case the header is missing.
+Cowboy expects all header names to be provided as lowercase
+binary strings. This is true for both requests and responses,
+regardless of the underlying protocol.
+
+When the header is missing from the request, `undefined`
+will be returned. A different default can be provided:
 
 [source,erlang]
 HeaderVal = cowboy_req:header(<<"content-type">>, Req, <<"text/plain">>).
 
-And also obtain all headers.
+All headers can be retrieved at once, either directly:
+
+[source,erlang]
+#{headers := AllHeaders} = Req.
+
+Or using a function:
 
 [source,erlang]
 AllHeaders = cowboy_req:headers(Req).
 
-To parse the previous header, simply call `parse_header/{2,3}`
-where you would call `header/{2,3}` otherwise.
+Cowboy provides equivalent functions to parse individual
+headers. There is no function to parse all headers at once.
+
+To parse a specific header:
 
 [source,erlang]
 ParsedVal = cowboy_req:parse_header(<<"content-type">>, Req).
 
-Cowboy will crash if it doesn't know how to parse the given
-header, or if the value is invalid.
+An exception will be thrown if it doesn't know how to parse the
+given header, or if the value is invalid. The list of known headers
+and default values can be found in the manual.
 
-You can of course define a default value. Note that the default
-value you specify here is the parsed value you'd like to get
-by default.
+When the header is missing, `undefined` is returned. You can
+change the default value. Note that it should be the parsed value
+directly:
 
 [source,erlang]
 ----
@@ -211,37 +354,21 @@ ParsedVal = cowboy_req:parse_header(<<"content-type">>, Req,
 	{<<"text">>, <<"plain">>, []}).
 ----
 
-The list of known headers and default values is defined in the
-manual.
-
-=== Meta
-
-Cowboy will sometimes associate some meta information with
-the request. Built-in meta values are listed in the manual
-for their respective modules.
-
-This will get a meta value. The returned value will be `undefined`
-if it isn't defined.
-
-[source,erlang]
-MetaVal = cowboy_req:meta(websocket_version, Req).
-
-You can change the default value if needed.
+=== Peer
 
-[source,erlang]
-MetaVal = cowboy_req:meta(websocket_version, Req, 13).
+The peer address and port number for the connection can be
+retrieved either directly or using a function.
 
-You can also define your own meta values. The name must be
-an `atom()`.
+To retrieve the peer directly:
 
 [source,erlang]
-Req2 = cowboy_req:set_meta(the_answer, 42, Req).
+#{peer := {IP, Port}} = Req.
 
-=== Peer
-
-You can obtain the peer address and port number. This is
-not necessarily the actual IP and port of the client, but
-rather the one of the machine that connected to the server.
+And using a function:
 
 [source,erlang]
 {IP, Port} = cowboy_req:peer(Req).
+
+Note that the peer corresponds to the remote end of the
+connection to the server, which may or may not be the
+client itself. It may also be a proxy or a gateway.
-- 
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