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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. |