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diff --git a/guide/http.md b/guide/http.md new file mode 100644 index 0000000..1034a5d --- /dev/null +++ b/guide/http.md @@ -0,0 +1,208 @@ +Using HTTP +========== + +This chapter describes how to use the Gun client for +communicating with an HTTP or SPDY server. + +Streams +------- + +Every time a request is initiated, either by the client or the +server, Gun creates a "stream". The stream controls whether +the endpoints are still sending any data, and allows you to +identify incoming messages. + +Streams are references in Gun, and are therefore always unique. + +Streams can be canceled at any time. This will stop any further +messages to be sent to the controlling process. Depending on +its capabilities, the server will also be instructed to drop +the request. + +Canceling a stream may result in Gun dropping the connection +temporarily, to avoid uploading or downloading data that will +not be used. This situation can only occur with HTTP, as SPDY +features stream canceling as part of its protocol. + +To cancel a stream, the `gun:cancel/2` function can be used. + +``` erlang +gun:cancel(Pid, StreamRef}. +``` + +Sending requests +---------------- + +Gun provides many convenient functions for performing common +operations, like GET, POST or DELETE. It also provides a +general purpose function in case you need other methods. + +The availability of these methods on the server can vary +depending on the software used but also on a per-resource +basis. + +To retrieve a resource, `gun:get/{2,3}` can be used. If you +don't need the response body, `gun:head/{2,3}` is available. +As this type of requests can't have a body, only the path +and optionally the headers can be specified. + +``` erlang +%% Without headers. +StreamRef = gun:get(Pid, "/organizations/extend"). +%% With headers. +StreamRef = gun:get(Pid, "/organizations/extend", [ + {"accept", "application/json"}, + {"user-agent", "revolver/1.0"}]). +``` + +To create or update a resource, the functions `gun:patch/{3,4}`, +`gun:post/{3,4}` and `gun:put/{3,4}` can be used. As this type +of request is meant to come with a body, headers are not optional, +because you must specify at least the content-type of the body, +and if possible also the content-length. The body is however +optional, because there might not be any at all, or because it +will be subsequently streamed. If a body is set here it is assumed +to be the full body. + +``` erlang +%% Without body. +StreamRef = gun:put(Pid, "/organizations/extend", [ + {"content-length", 23}, + {"content-type", "application/json"}]). +%% With body. +StreamRef = gun:put(Pid, "/organizations/extend", [ + {"content-length", 23}, + {"content-type", "application/json"}], + "{\"msg\": \"Hello world!\"}"). +``` + +To delete a resource, the `gun:delete/{2,3}` function can be +used. It works similarly to the GET and HEAD functions. + +``` erlang +%% Without headers. +StreamRef = gun:delete(Pid, "/organizations/extend"). +%% With headers. +StreamRef = gun:delete(Pid, "/organizations/extend", [ + {"accept", "application/json"}, + {"user-agent", "revolver/1.0"}]). +``` + +To obtain the functionality available for a given resource, +the `gun:options/{2,3}` can be used. It also works like the +GET and HEAD functions. + +``` erlang +%% Without headers. +StreamRef = gun:options(Pid, "/organizations/extend"). +%% With headers. +StreamRef = gun:options(Pid, "/organizations/extend", [ + {"accept", "application/json"}, + {"user-agent", "revolver/1.0"}]). +``` + +You can obtain information about the server as a whole by +using the special path `"*"`. + +``` erlang +StreamRef = gun:options(Pid, "*"). +``` + +Streaming data +-------------- + +When a PATCH, POST or PUT operation is performed, and a +content-type is specified but no body is given, Gun will +expect data to be streamed to the connection using the +`gun:data/4` function. + +This function can be called as many times as needed until +all data is sent. The third argument needs to be `nofin` +when there is remaining data to be sent, and `fin` for the +last chunk. The last chunk may be empty if needed. + +For example, with an `IoDevice` opened like follow: + +``` erlang +{ok, IoDevice} = file:open(Filepath, [read, binary, raw]). +``` + +The following function will stream all data until the end +of the file: + +``` erlang +sendfile(Pid, StreamRef, IoDevice) -> + case file:read(IoDevice, 8000) of + eof -> + gun:data(Pid, StreamRef, fin, <<>>), + file:close(IoDevice); + {ok, Bin} -> + gun:data(Pid, StreamRef, nofin, Bin), + sendfile(Pid, StreamRef, IoDevice) + end. +``` + +Receiving responses +------------------- + +All data received from the server is sent to the controlling +process as a message. First a response message is sent, then +zero or more data messages. If something goes wrong, error +messages are sent instead. + +You can find out whether data will be sent by checking the +presence of the content-type or content-length header and +making sure the length isn't 0. If you already know a body is +going to be sent you can skip this check, however do make +sure you have a timeout just in case something goes wrong. + +``` erlang +StreamRef = gun:get(Pid, "/"), +receive + {'DOWN', Tag, _, _, Reason} -> + error_logger:error_msg("Oops!"), + exit(Reason); + {gun_response, Pid, StreamRef, Status, Headers} -> + receive_data(Pid, StreamRef) +after 1000 -> + exit(timeout) +end. +``` + +The `receive_data/2` function could look like this: + +``` erlang +receive_data(Pid, Tag, StreamRef) -> + receive + {'DOWN', Tag, _, _, Reason} -> + {error, incomplete}; + {gun_data, Pid, StreamRef, nofin, Data} -> + io:format("~s~n", [Data]), + receive_data(Pid, Tag, StreamRef); + {gun_data, Pid, StreamRef, fin, Data} -> + io:format("~s~n", [Data]) + after 1000 -> + {error, timeout} + end. +``` + +While it may seem verbose, using messages like this has the +advantage of never locking your process, allowing you to +easily debug your code. It also allows you to start more than +one connection and concurrently perform queries on all of them +at the same time. + +You may also use Gun in a synchronous manner by writing your +own functions that perform a receive like demonstrated above. + +Dealing with server-pushed streams +---------------------------------- + +When using SPDY the server may decide to push extra resources +after a request is performed. It will send a `gun_push` message +which contains two references, one for the pushed stream, and +one for the request this stream is associated with. + +Pushed streams typically feature a body. Replying to a pushed +stream is forbidden and Gun will send an error message if +attempted. |