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doc/src/guide/req.ezdoc

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+::: 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.
+
+:: A special variable
+
+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.
+
+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.
+
+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.
+
+It also caches the result of operations performed
+on the immutable state. That means that some calls
+will give a result much faster when called many times.
+
+:: Overview of the cowboy_req interface
+
+The `cowboy_req` interface is divided in four groups
+of functions, each having a well defined return type
+signature common to the entire group.
+
+The first group, access functions, will always return
+`{Value, Req}`. The group includes all the following
+functions: `binding/{2,3}`, `bindings/1`, `body_length/1`,
+`cookie/{2,3}`, `cookies/1`, `header/{2,3}`, `headers/1`,
+`host/1`, `host_info/1`, `host_url/1`, `meta/{2,3}`,
+`method/1`, `path/1`, `path_info/1`, `peer/1`, `port/1`,
+`qs/1`, `qs_val/{2,3}`, `qs_vals/1`, `url/1`, `version/1`.
+
+The second group, question functions, will always return
+a `boolean()`. The group includes the following three
+functions: `has_body/1`, `has_resp_body/1`, `has_resp_header/2`.
+
+The third group contains the functions that manipulate
+the socket or perform operations that may legitimately fail.
+They may return `{Result, Req}`, `{Result, Value, Req}`
+or `{error, atom()}`. This includes the following functions:
+`body/{1,2}`, `body_qs/{1,2}`, `chunked_reply/{2,3}`,
+`parse_header/{2,3}`, `part/{1,2}`, `part_body/{1,2}`
+and `reply/{2,3,4}`. Finally, the group also includes the
+`chunk/2` and `continue/1` functions which always return `ok`.
+
+The final group modifies the Req object state without
+performing any immediate operations. As these functions
+can't fail, they always return a new `Req` directly.
+This includes the following functions: `compact/1`,
+`delete_resp_header/2`, `set_meta/3`, `set_resp_body/2`,
+`set_resp_body_fun/{2,3}`, `set_resp_cookie/4`, `set_resp_header/3`.
+
+This chapter covers most of the first group, plus a few other
+functions. The next few chapters cover cookies handling, reading
+the request body and sending a response.
+
+:: Request
+
+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.
+
+The method identifies the action. Standard methods include
+GET, HEAD, OPTIONS, PATCH, POST, PUT, DELETE. Method names
+are case sensitive.
+
+``` erlang
+{Method, Req2} = 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.
+
+``` erlang
+{Host, Req2} = cowboy_req:host(Req),
+{Port, Req3} = cowboy_req:port(Req2),
+{Path, Req4} = cowboy_req:path(Req3).
+```
+
+The version used by the client can of course also be obtained.
+
+``` erlang
+{Version, Req2} = cowboy_req:version(Req).
+```
+
+Do note however that clients claiming to implement one version
+of the protocol does not mean they implement it fully, or even
+properly.
+
+:: 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.
+
+You can fetch a single binding. The value will be `undefined`
+if the binding doesn't exist.
+
+``` erlang
+{Binding, Req2} = cowboy_req:binding(my_binding, Req).
+```
+
+If you need a different value when the binding doesn't exist,
+you can change the default.
+
+``` erlang
+{Binding, Req2} = cowboy_req:binding(my_binding, Req, 42).
+```
+
+You can also obtain all bindings in one call. They will be
+returned as a list of key/value tuples.
+
+``` erlang
+{AllBindings, Req2} = cowboy_req:bindings(Req).
+```
+
+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.
+
+``` erlang
+{HostInfo, Req2} = 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.
+
+``` erlang
+{PathInfo, Req2} = cowboy_req:path_info(Req).
+```
+
+:: Query string
+
+The query string can be obtained directly.
+
+``` erlang
+{Qs, Req2} = cowboy_req:qs(Req).
+```
+
+You can also requests only one value.
+
+``` erlang
+{QsVal, Req2} = cowboy_req:qs_val(<<"lang">>, Req).
+```
+
+If that value is optional, you can define a default to simplify
+your task.
+
+``` erlang
+{QsVal, Req2} = cowboy_req:qs_val(<<"lang">>, Req, <<"en">>).
+```
+
+Finally, you can obtain all query string values.
+
+``` erlang
+{AllValues, Req2} = cowboy_req:qs_vals(Req).
+```
+
+:: Request URL
+
+You can reconstruct the full URL of the resource.
+
+``` erlang
+{URL, Req2} = cowboy_req:url(Req).
+```
+
+You can also obtain only the base of the URL, excluding the
+path and query string.
+
+``` erlang
+{BaseURL, Req2} = cowboy_req:host_url(Req).
+```
+
+:: Headers
+
+Cowboy allows you to obtain the header values as string,
+or parsed into a more meaningful representation.
+
+This will get the string value of a header.
+
+``` erlang
+{HeaderVal, Req2} = cowboy_req:header(<<"content-type">>, Req).
+```
+
+You can of course set a default in case the header is missing.
+
+``` erlang
+{HeaderVal, Req2}
+    = cowboy_req:header(<<"content-type">>, Req, <<"text/plain">>).
+```
+
+And also obtain all headers.
+
+``` erlang
+{AllHeaders, Req2} = cowboy_req:headers(Req).
+```
+
+To parse the previous header, simply call `parse_header/{2,3}`
+where you would call `header/{2,3}` otherwise. Note that the
+return value changes and includes the result of the operation
+as the first element of the returned tuple. A successful parse
+returns `ok`.
+
+``` erlang
+{ok, ParsedVal, Req2} = cowboy_req:parse_header(<<"content-type">>, Req).
+```
+
+When Cowboy doesn't know how to parse the given header, the
+result of the operation will be `undefined` and the string value
+will be returned instead.
+
+``` erlang
+{undefined, HeaderVal, Req2}
+    = cowboy_req:parse_header(<<"unicorn-header">>, Req).
+```
+
+When parsing fails, `{error, Reason}` is returned instead.
+
+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.
+
+``` erlang
+{ok, ParsedVal, Req2}
+    = cowboy_req:parse_header(<<"content-type">>, Req,
+    {<<"text">>, <<"plain">>, []}).
+```
+
+The list of known headers and default values is defined in the
+manual. Also note that the result of parsing is cached, so
+calling this function multiple times for the same values will
+not have a significant performance impact.
+
+:: 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.
+
+``` erlang
+{MetaVal, Req2} = cowboy_req:meta(websocket_version, Req).
+```
+
+You can change the default value if needed.
+
+``` erlang
+{MetaVal, Req2} = cowboy_req:meta(websocket_version, Req, 13).
+```
+
+You can also define your own meta values. The name must be
+an `atom()`.
+
+``` erlang
+Req2 = cowboy_req:set_meta(the_answer, 42, 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.
+
+``` erlang
+{{IP, Port}, Req2} = cowboy_req:peer(Req).
+```
+
+:: Reducing the memory footprint
+
+When you are done reading information from the request object
+and know you are not going to access it anymore, for example
+when using long-polling or Websocket, you can use the `compact/1`
+function to remove most of the data from the request object and
+free memory.
+
+``` erlang
+Req2 = cowboy_req:compact(Req).
+```
+
+You will still be able to send a reply if needed.