= gun(3)

== Name

gun - Asynchronous HTTP client

== Description

The `gun` module provides an asynchronous interface for
connecting and communicating with Web servers over HTTP,
HTTP/2 or Websocket.

== Exports

Connection:

* link:man:gun:open(3)[gun:open(3)] - Open a connection to the given host and port
* link:man:gun:open_unix(3)[gun:open_unix(3)] - Open a connection to the given Unix domain socket
// @todo * link:man:gun:shutdown(3)[gun:shutdown(3)] - Gracefully close the connection
* link:man:gun:close(3)[gun:close(3)] - Brutally close the connection
* link:man:gun:info(3)[gun:info(3)] - Obtain information about the connection

Requests:

* link:man:gun:get(3)[gun:get(3)] - Get a resource representation
* link:man:gun:head(3)[gun:head(3)] - Get headers of a resource representation
* link:man:gun:options(3)[gun:options(3)] - Query the capabilities of the server or a resource
* link:man:gun:patch(3)[gun:patch(3)] - Apply a set of changes to a resource
* link:man:gun:post(3)[gun:post(3)] - Process the enclosed representation according to a resource's own semantics
* link:man:gun:put(3)[gun:put(3)] - Create or replace a resource
* link:man:gun:delete(3)[gun:delete(3)] - Delete a resource
* link:man:gun:headers(3)[gun:headers(3)] - Initiate the given request
* link:man:gun:request(3)[gun:request(3)] - Perform the given request
* link:man:gun:data(3)[gun:data(3)] - Stream the body of a request

Proxies:

* link:man:gun:connect(3)[gun:connect(3)] - Establish a tunnel to the origin server

Messages:

* link:man:gun:await(3)[gun:await(3)] - Wait for a response
* link:man:gun:await_body(3)[gun:await_body(3)] - Wait for the complete response body
* link:man:gun:await_up(3)[gun:await_up(3)] - Wait for the connection to be up
* link:man:gun:flush(3)[gun:flush(3)] - Flush all messages related to a connection or a stream

Streams:

* link:man:gun:cancel(3)[gun:cancel(3)] - Cancel the given stream

Websocket:

* link:man:gun:ws_upgrade(3)[gun:ws_upgrade(3)] - Upgrade to Websocket
* link:man:gun:ws_send(3)[gun:ws_send(3)] - Send Websocket frames

== Messages

Gun will inform the calling process of events asynchronously
by sending any of the following messages:

Connection:

* link:man:gun_up(3)[gun_up(3)] - The connection is up
* link:man:gun_down(3)[gun_down(3)] - The connection is down
* link:man:gun_upgrade(3)[gun_upgrade(3)] - Successful protocol upgrade
* link:man:gun_error(3)[gun_error(3)] - Stream or connection-wide error

Responses:

* link:man:gun_push(3)[gun_push(3)] - Server-initiated push
* link:man:gun_inform(3)[gun_inform(3)] - Informational response
* link:man:gun_response(3)[gun_response(3)] - Response
* link:man:gun_data(3)[gun_data(3)] - Response body
* link:man:gun_trailers(3)[gun_trailers(3)] - Response trailers

Websocket:

* link:man:gun_ws(3)[gun_ws(3)] - Websocket frame

The response messages will be sent to the process that opened
the connection by default. The `reply_to` request option can
be used to redirect request-specific messages to a different
process.

== Types

=== connect_destination()

[source,erlang]
----
connect_destination() :: #{
    host      := inet:hostname() | inet:ip_address(),
    port      := inet:port_number(),

    username  => iodata(),
    password  => iodata(),
    protocols => [http | http2],
    transport => tcp | tls,

    tls_opts              => [ssl:connect_option()],
    tls_handshake_timeout => timeout()
}
----

Destination of a CONNECT request.

The default value, if any, is given next to the option name:

host, port::

Destination hostname and port number. Mandatory.
+
Upon successful completion of the CONNECT request, Gun will
begin using these as the host and port of the origin server
for subsequent requests.

username, password::

Proxy authorization credentials. They are only sent when
both options are provided.

protocol (http)::

Protocol that will be used for tunneled requests.

transport (tcp)::

Transport that will be used for tunneled requests. Note that
due to Erlang/OTP limitations it is not possible to tunnel
a TLS connection inside a TLS tunnel.

tls_opts ([])::

Options to use for tunneled TLS connections.

tls_handshake_timeout (infinity)::

Handshake timeout for tunneled TLS connections.

=== http_opts()

[source,erlang]
----
http_opts() :: #{
    keepalive             => timeout(),
    transform_header_name => fun((binary()) -> binary()),
    version               => 'HTTP/1.1' | 'HTTP/1.0'
}
----

Configuration for the HTTP protocol.

The default value is given next to the option name:

// @todo Document content_handlers and gun_sse_h.

keepalive (5000)::

Time between pings in milliseconds. Since the HTTP protocol has
no standardized way to ping the server, Gun will simply send an
empty line when the connection is idle. Gun only makes a best
effort here as servers usually have configurable limits to drop
idle connections. Use `infinity` to disable.

transform_header_name - see below::

A function that will be applied to all header names before they
are sent to the server. Gun assumes that all header names are in
lower case. This function is useful if you, for example, need to
re-case header names in the event that the server incorrectly
considers the case of header names to be significant.

version (`'HTTP/1.1'`)::

HTTP version to use.

=== http2_opts()

[source,erlang]
----
http2_opts() :: #{
    keepalive => timeout()
}
----

Configuration for the HTTP/2 protocol.

The default value is given next to the option name:

// @todo Document content_handlers and gun_sse_h.

keepalive (5000)::

Time between pings in milliseconds.

// @todo Allow and document max_frame_size_sent.

=== opts()

[source,erlang]
----
opts() :: #{
    connect_timeout => timeout(),
    http_opts       => http_opts(),
    http2_opts      => http2_opts(),
    protocols       => [http | http2],
    retry           => non_neg_integer(),
    retry_timeout   => pos_integer(),
    trace           => boolean(),
    transport       => tcp | tls,
    transport_opts  => [gen_tcp:connect_option()] | [ssl:connect_option()],
    ws_opts         => ws_opts()
}
----

Configuration for the connection.

The default value is given next to the option name:

connect_timeout (infinity)::

Connection timeout.

http_opts (#{})::

Options specific to the HTTP protocol.

http2_opts (#{})::

Options specific to the HTTP/2 protocol.

protocols - see below::

Ordered list of preferred protocols. When the transport is `tcp`,
this list must contain exactly one protocol. When the transport
is `tls`, this list must contain at least one protocol and will be
used to negotiate a protocol via ALPN. When the server does not
support ALPN then `http` will always be used. Defaults to
`[http]` when the transport is `tcp`, and `[http2, http]` when the
transport is `tls`.

retry (5)::

Number of times Gun will try to reconnect on failure before giving up.

retry_timeout (5000)::

Time between retries in milliseconds.

trace (false)::

Whether to enable `dbg` tracing of the connection process. Should
only be used during debugging.

transport - see below::

Whether to use TLS or plain TCP. The default varies depending on the
port used. Port 443 defaults to `tls`. All other ports default to `tcp`.

transport_opts ([])::

Transport options. They are TCP options or TLS options depending on
the selected transport.

ws_opts (#{})::

Options specific to the Websocket protocol.

=== req_opts()

[source,erlang]
----
req_opts() :: #{
    reply_to => pid()
}
----

Configuration for a particular request.

The default value is given next to the option name:

reply_to (`self()`)::

The pid of the process that will receive the response messages.

=== ws_opts()

[source,erlang]
----
ws_opts() :: #{
    compress  => boolean(),
    protocols => [{binary(), module()}]
}
----

Configuration for the Websocket protocol.

The default value is given next to the option name:

compress (false)::

Whether to enable permessage-deflate compression. This does
not guarantee that compression will be used as it is the
server that ultimately decides. Defaults to false.

protocols ([])::

A non-empty list enables Websocket protocol negotiation. The
list of protocols will be sent in the sec-websocket-protocol
request header. The handler module interface is currently
undocumented and must be set to `gun_ws_h`.

// @todo Document default_protocol and user_opts.

== Changelog

* *2.0*: Function `gun:headers/4,5` introduced.
* *1.3*: Add the CONNECT destination's `protocols` option and
         deprecate the previously introduced `protocol` option.
* *1.2*: Introduce the type `connect_destination()`.

== See also

link:man:gun(7)[gun(7)]
