= cowboy_stream(3) == Name cowboy_handler - Stream handlers == Description The module `cowboy_stream` defines a callback interface and a protocol for handling HTTP streams. An HTTP request and its associated response is called a stream. A connection may have many streams. In HTTP/1.1 they are executed sequentially, while in HTTP/2 they are executed concurrently. Cowboy calls the stream handler for nearly all events related to a stream. Exceptions vary depending on the protocol. Extra care must be taken when implementing stream handlers to ensure compatibility. While some modification of the events and commands is allowed, it is generally not a good idea to completely omit them. == Callbacks Stream handlers must implement the following interface: [source,erlang] ---- init(StreamID, Req, Opts) -> {Commands, State} data(StreamID, IsFin, Data, State) -> {Commands, State} info(StreamID, Info, State) -> {Commands, State} terminate(StreamID, Reason, State) -> any() early_error(StreamID, Reason, PartialReq, Resp, Opts) -> Resp StreamID :: cowboy_stream:streamid() Req :: cowboy_req:req() Opts :: cowboy:opts() Commands :: cowboy_stream:commands() State :: any() IsFin :: cowboy_stream:fin() Data :: binary() Info :: any() Reason :: cowboy_stream:reason() PartialReq - cowboy_req:req(), except all fields are optional Resp :: cowboy_stream:resp_command() ---- HTTP/1.1 will initialize a stream only when the request-line and all headers have been received. When errors occur before that point Cowboy will call the callback `early_error/5` with a partial request, the error reason and the response Cowboy intends to send. All other events go throuh the stream handler using the normal callbacks. HTTP/2 will initialize the stream when the `HEADERS` block has been fully received and decoded. Any protocol error occuring before that will not result in a response being sent and will therefore not go through the stream handler. In addition Cowboy may terminate streams without sending an HTTP response back. The stream is initialized by calling `init/3`. All streams that are initialized will eventually be terminated by calling `terminate/3`. When Cowboy receives data for the stream it will call `data/4`. The data given is the request body after any transfer decoding has been applied. When Cowboy receives a message addressed to a stream, or when Cowboy needs to inform the stream handler that an internal event has occurred, it will call `info/3`. [[commands]] == Commands Stream handlers can return a list of commands to be executed from the `init/3`, `data/4` and `info/3` callbacks. In addition, the `early_error/5` callback must return a response command. // @todo We need a 'log' command that would call error_logger. // It's better than doing in the handlers directly because // then we can have other stream handlers manipulate those logs. // @todo We need a command to send a message so that other // stream handlers can manipulate these messages if necessary. The following commands are defined: [[inform_command]] === inform Send an informational response to the client. [source,erlang] ---- {inform, cowboy:http_status(), cowboy:http_headers()} ---- Any number of informational responses may be sent, but only until the final response is sent. [[response_command]] === response Send a response to the client. [source,erlang] ---- {response, cowboy:http_status(), cowboy:http_headers(), cowboy_req:resp_body()} ---- No more data can be sent after this command. [[headers_command]] === headers Initiate a response to the client. [source,erlang] ---- {headers, cowboy:http_status(), cowboy:http_headers()} ---- This initiates a response to the client. The stream will end when a data command with the `fin` flag or a trailer command is returned. [[data_command]] === data Send data to the client. [source,erlang] ---- {data, fin(), iodata()} ---- [[trailers_command]] === trailers Send response trailers to the client. [source,erlang] ---- {trailers, cowboy:http_headers()} ---- [[push_command]] === push Push a resource to the client. [source,erlang] ---- {push, Method, Scheme, Host, inet:port_number(), Path, Qs, cowboy:http_headers()} Method = Scheme = Host = Path = Qs = binary() ---- The command will be ignored if the protocol does not provide any server push mechanism. === flow [source,erlang] ---- {flow, pos_integer()} ---- Request more data to be read from the request body. The exact behavior depends on the protocol. === spawn Inform Cowboy that a process was spawned and should be supervised. [source,erlang] ---- {spawn, pid(), timeout()} ---- === error_response Send an error response if no response was sent previously. [source,erlang] ---- {error_response, cowboy:http_status(), cowboy:http_headers(), iodata()} ---- [[switch_protocol_command]] === switch_protocol Switch to a different protocol. [source,erlang] ---- {switch_protocol, cowboy:http_headers(), module(), state()} ---- Contains the headers that will be sent in the 101 response, along with the module implementing the protocol we are switching to and its initial state. === stop Stop the stream. [source,erlang] ---- stop ---- While no more data can be sent after the `fin` flag was set, the stream is still tracked by Cowboy until it is stopped by the handler. The behavior when stopping a stream for which no response has been sent will vary depending on the protocol. The stream will end successfully as far as the client is concerned. To indicate that an error occurred, either use `error_response` before stopping, or use `internal_error`. === internal_error Stop the stream with an error. [source,erlang] ---- {internal_error, Reason, HumanReadable} Reason = any() HumanReadable = atom() ---- This command should be used when the stream cannot continue because of an internal error. An `error_response` command may be sent before that to advertise to the client why the stream is dropped. == Predefined events Cowboy will forward all messages sent to the stream to the `info/3` callback. To send a message to a stream, send a message to the connection process with the form `{{Pid, StreamID}, Msg}`. The connection process will then forward `Msg` to the stream handlers. Cowboy will also forward the exit signals for the processes that the stream spawned. === EXIT //info(_StreamID, {'EXIT', Pid, normal}, State=#state{pid=Pid}) -> //info(_StreamID, {'EXIT', Pid, {_Reason, [_, {cow_http_hd, _, _, _}|_]}}, State=#state{pid=Pid}) -> //info(StreamID, Exit = {'EXIT', Pid, {Reason, Stacktrace}}, State=#state{ref=Ref, pid=Pid}) -> A process spawned by this stream has exited. [source,erlang] ---- {'EXIT', pid(), any()} ---- This is the raw exit message without any modification. // === read_body // // //info(_StreamID, {read_body, Ref, Length, _}, // //info(StreamID, {read_body, Ref, Length, Period}, State) -> // // TODO yeah I am not actually sure this one should be public just yet // TODO if it is, then we probably shouldn't send a message directly, // TODO but rather return a command that will end up sending the message // // TODO The problem being that no stream handler has access to that // TODO message if we send it directly. So we should have a command // TODO send_message or something that can be seen from all handlers. // // TODO The thing is that stream handlers can have 0 to N processes // TODO so we have to make it easy to say which process should // TODO receive the message, and perhaps *identify* which process // TODO gets it? === inform Same as the xref:inform_command[inform command]. Sent when the request process reads the body and an expect: 100-continue header was present in the request, or when the request process sends an informational response on its own. === response Same as the xref:response_command[response command]. Usually sent when the request process replies to the client. May also be sent by Cowboy internally. === headers Same as the xref:headers_command[headers command]. Sent when the request process starts replying to the client. === data Same as the xref:data_command[data command]. Sent when the request process streams data to the client. === trailers Same as the xref:trailers_command[trailers command]. Sent when the request process sends the trailer field values to the client. === push Same as the xref:push_command[push command]. Sent when the request process pushes a resource to the client. === switch_protocol Same as the xref:switch_protocol_command[switch_protocol command]. Sent when switching to the HTTP/2 or Websocket protocol. == Exports The following function should be called by modules implementing stream handlers to execute the next stream handler in the list: * link:man:cowboy_stream:init(3)[cowboy_stream:init(3)] - Initialize a stream * link:man:cowboy_stream:data(3)[cowboy_stream:data(3)] - Handle data for a stream * link:man:cowboy_stream:info(3)[cowboy_stream:info(3)] - Handle a message for a stream * link:man:cowboy_stream:terminate(3)[cowboy_stream:terminate(3)] - Terminate a stream * link:man:cowboy_stream:early_error(3)[cowboy_stream:early_error(3)] - Handle an early error for a stream == Types === commands() [source,erlang] ---- commands() :: [Command] ---- See the xref:commands[list of commands] for details. === fin() [source,erlang] ---- fin() :: fin | nofin ---- Used in commands and events to indicate that this is the end of the stream. === partial_req() [source,erlang] ---- req() :: #{ method => binary(), %% case sensitive version => cowboy:http_version() | atom(), scheme => binary(), %% lowercase; case insensitive host => binary(), %% lowercase; case insensitive port => inet:port_number(), path => binary(), %% case sensitive qs => binary(), %% case sensitive headers => cowboy:http_headers(), peer => {inet:ip_address(), inet:port_number()} } ---- Partial request information received when an early error is detected. === reason() [source,erlang] ---- reason() :: normal | switch_protocol | {internal_error, timeout | {error | exit | throw, any()}, HumanReadable} | {socket_error, closed | atom(), HumanReadable} | {stream_error, Error, HumanReadable} | {connection_error, Error, HumanReadable} | {stop, cow_http2:frame(), HumanReadable} Error = atom() HumanReadable = atom() ---- Reason for the stream termination. === resp_command() [source,erlang] ---- resp_command() :: {response, cowboy:http_status(), cowboy:http_headers(), cowboy_req:resp_body()} ---- See the xref:response_command[response command] for details. === streamid() [source,erlang] ---- streamid() :: any() ---- The identifier for this stream. The identifier is unique over the connection process. It is possible to form a unique identifier node-wide and cluster-wide by wrapping it in a `{self(), StreamID}` tuple. == Changelog * *2.2*: The trailers command was introduced. * *2.0*: Module introduced. == See also link:man:cowboy(7)[cowboy(7)], link:man:cowboy_http(3)[cowboy_http(3)], link:man:cowboy_http2(3)[cowboy_http2(3)]