{ok, ConnPid} = gun:open("example.org", 443).This chapter describes how to open, monitor and close a connection using the Gun client.
Gun is designed with the SPDY and Websocket protocols in mind. They are built for long-running connections that allow concurrent exchange of data, either in the form of request/responses for SPDY or in the form of messages for Websocket.
A Gun connection is an Erlang process that manages a socket to
a remote endpoint. This Gun connection is owned by a user
process that is called the owner of the connection, and is
managed by the supervision tree of the gun application.
The owner process communicates with the Gun connection
by calling functions from the module gun. All functions
perform their respective operations asynchronously. The Gun
connection will send Erlang messages to the owner process
whenever needed.
When the remote endpoint closes the connection, Gun attempts to reconnect automatically.
The gun:open/{2,3} function must be used to open a connection.
{ok, ConnPid} = gun:open("example.org", 443).If the port given is 443, Gun will attempt to connect using SSL. The protocol will be selected automatically using the NPN extension for TLS. By default Gun supports SPDY/3.1, SPDY/3 and HTTP/1.1 when connecting using SSL.
For any other port, Gun will attempt to connect using TCP and will use the HTTP/1.1 protocol.
The transport and protocol used can be overriden using options. The manual documents all available options.
Options can be provided as a third argument, and take the form of a map.
{ok, ConnPid} = gun:open("example.org", 8443, #{transport=>ssl}).When Gun successfully connects to the server, it sends a
gun_up message with the protocol that has been selected
for the connection.
Gun provides the functions gun:await_up/{1,2,3} that wait
for the gun_up message. They can optionally take a monitor
reference and/or timeout value. If no monitor is provided,
one will be created for the duration of the function call.
{ok, ConnPid} = gun:open("example.org", 443),
{ok, Protocol} = gun:await_up(ConnPid).When the connection is lost, Gun will send a gun_down
message indicating the current protocol, the reason the
connection was lost and two list of stream references.
The first list indicates open streams that may have been processed by the server. The second list indicates open streams that the server did not process.
@todo Gun should detect the owner process being killed
Because software errors are unavoidable, it is important to detect when the Gun process crashes. It is also important to detect when it exits normally. Erlang provides two ways to do that: links and monitors.
Gun leaves you the choice as to which one will be used.
However, if you use the gun:await/{2,3} or gun:await_body/{2,3}
functions, a monitor may be used for you to avoid getting
stuck waiting for a message that will never come.
If you choose to monitor yourself you can do it on a permanent
basis rather than on every message you will receive, saving
resources. Indeed, the gun:await/{3,4} and gun:await_body/{3,4}
functions both accept a monitor argument if you have one already.
{ok, ConnPid} = gun:open("example.org", 443).
MRef = monitor(process, ConnPid).This monitor reference can be kept and used until the connection process exits.
DOWN messagesreceive %% Receive Gun messages here... {'DOWN', Mref, process, ConnPid, Reason} -> error_logger:error_msg("Oops!"), exit(Reason); end.
What to do when you receive a DOWN message is entirely up to you.
The connection can be stopped abruptly at any time by calling
the gun:close/1 function.
gun:close(ConnPid).
The process is stopped immediately without having a chance to perform the protocol’s closing handshake, if any.
The connection can also be stopped gracefully by calling the
gun:shutdown/1 function.
gun:shutdown(ConnPid).
Gun will refuse any new requests or messages after you call this function. It will however continue to send you messages for existing streams until they are all completed.
For example if you performed a GET request just before calling
gun:shutdown/1, you will still receive the response before
Gun closes the connection.
If you set a monitor beforehand, you will receive a message when the connection has been closed.