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