= cowboy_stream(3)

== Name

cowboy_stream - 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 discard 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 through 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 occurring
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 The logger option and the {log, Level, Format, Args}
// options need to be documented and tested.

The order in which the commands are given matters. For example,
when sending a response and at the same time creating a new child
process, the first command should be the `spawn` and the second the
`response`. The reason for that is that the sending of the response
may result in a socket error which leads to the termination of
the connection before the rest of the commands are executed.

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.

Note that in Cowboy it is the `cowboy_req` module that
sets the date and server headers. When using the command
directly those headers will not be added.

[[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.

Note that in Cowboy it is the `cowboy_req` module that
sets the date and server headers. When using the command
directly those headers will not be added.

[[data_command]]
=== data

Send data to the client.

[source,erlang]
----
{data, fin(), cowboy_req:resp_body()}
----

[[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.

Note that the 101 informational response will not be sent
after a final response.

=== 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`.

No other command can be executed after the `stop` command.

=== 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.

=== log

Log a message.

[source,erlang]
----
{log, logger:level(), io:format(), list()}
----

This command can be used to log a message using the
configured `logger` module.

=== set_options

Set protocol options.

[source,erlang]
----
{set_options, map()}
----

This can also be used to override stream handler
options. For example this is supported by
link:man:cowboy_compress_h(3)[cowboy_compress_h(3)].

Not all options can be overridden. Please consult the
relevant option's documentation for details.

== Predefined events

Cowboy will forward all messages sent to the stream to
the `info/3` callback. To send a message to a stream,
the function link:man:cowboy_req:cast(3)[cowboy_req:cast(3)]
can be used.

Cowboy will also forward the exit signals for the
processes that the stream spawned.

When Cowboy needs to send a response it will trigger
an event that looks exactly like the corresponding
command. This event must be returned to be processed
by Cowboy (which is done automatically when using
link:man:cowboy_stream_h(3)[cowboy_stream_h(3)]).

Cowboy may trigger the following events on its own,
regardless of the stream handlers configured:
xref:inform_command[inform] (to send a 101
informational response when upgrading to HTTP/2 or
Websocket), xref:response_command[response],
xref:headers_command[headers], xref:data_command[data]
and xref:switch_protocol_command[switch_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 a direction of a 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() | {exit, any()}, 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

* *2.7*: The `log` and `set_options` commands were introduced.
* *2.6*: The `data` command can now contain a sendfile tuple.
* *2.6*: The `{stop, {exit, any()}, HumanReadable}` terminate reason was added.
* *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)],
link:man:cowboy_req:cast(3)[cowboy_req:cast(3)]
