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= cowboy_req:stream_reply(3)

== Name

cowboy_req:stream_reply - Send the response headers

== Description

[source,erlang]
----
stream_reply(Status, Req :: cowboy_req:req())
    -> stream_reply(StatusCode, #{}, Req)

stream_reply(Status, Headers, Req :: cowboy_req:req())
    -> Req

Status  :: cowboy:http_status()
Headers :: cowboy:http_headers()
----

Send the response headers.

The header names must be given as lowercase binary strings.
While header names are case insensitive, Cowboy requires them
to be given as lowercase to function properly.

Cowboy does not allow duplicate header names. Headers set
by this function may overwrite those set by `set_resp_header/3`.

Use link:man:cowboy_req:set_resp_cookie(3)[cowboy_req:set_resp_cookie(3)]
instead of this function to set cookies.

If a response body was set before calling this function,
it will not be sent.

Use link:man:cowboy_req:stream_body(3)[cowboy_req:stream_body(3)]
to stream the response body and optionally
link:man:cowboy_req:stream_trailers(3)[cowboy_req:stream_trailers(3)]
to send response trailer field values.

You may want to set the content-length header when using
this function, if it is known in advance. This will allow
clients using HTTP/2 and HTTP/1.0 to process the response
more efficiently.

The streaming method varies depending on the protocol being
used. HTTP/2 will use the usual DATA frames. HTTP/1.1 will
use chunked transfer-encoding, if the content-length
response header is set the body will be sent without chunked
chunked transfer-encoding. HTTP/1.0 will send the body
unmodified and close the connection at the end if no
content-length was set.

It is not possible to push resources after this function
returns. Any attempt will result in an error.

== Arguments

Status::

The status code for the response.

Headers::

The response headers.
+
Header names must be given as lowercase binary strings.

Req::

The Req object.

== Return value

A new Req object is returned.

The returned Req object must be used from that point onward
in order to be able to stream the response body.

== Changelog

* *2.0*: Only the Req is returned, it is no longer wrapped in a tuple.
* *2.0*: Function introduced. Replaces `chunked_reply/1,2`.

== Examples

.Initiate the response
[source,erlang]
----
Req = cowboy_req:stream_reply(200, Req0).
----

.Stream the response with custom headers
[source,erlang]
----
Req = cowboy_req:stream_reply(200, #{
    <<"content-type">> => <<"text/plain">>
}, Req0),
cowboy_req:stream_body(<<"Hello\n">>, nofin, Req),
timer:sleep(1000),
cowboy_req:stream_body(<<"World!\n">>, fin, Req).
----

== See also

link:man:cowboy_req(3)[cowboy_req(3)],
link:man:cowboy_req:set_resp_cookie(3)[cowboy_req:set_resp_cookie(3)],
link:man:cowboy_req:set_resp_header(3)[cowboy_req:set_resp_header(3)],
link:man:cowboy_req:set_resp_headers(3)[cowboy_req:set_resp_headers(3)],
link:man:cowboy_req:inform(3)[cowboy_req:inform(3)],
link:man:cowboy_req:reply(3)[cowboy_req:reply(3)],
link:man:cowboy_req:stream_body(3)[cowboy_req:stream_body(3)],
link:man:cowboy_req:stream_events(3)[cowboy_req:stream_events(3)],
link:man:cowboy_req:stream_trailers(3)[cowboy_req:stream_trailers(3)],
link:man:cowboy_req:push(3)[cowboy_req:push(3)]