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= 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:

[[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 is
returned.

[[data_command]]
=== data

Send data to the client.

[source,erlang]
----
{data, fin(), iodata()}
----

[[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?

=== 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.

=== 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].

// @todo Not done for HTTP/2 yet.
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

* *2.0*: 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)]