Connection

This chapter describes how to open, monitor and close a connection using the Gun client.

Gun connections

Gun is designed with the HTTP/2 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 HTTP/2 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.

Opening a new connection

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 TLS. The protocol will be selected automatically using the ALPN extension for TLS. By default Gun supports HTTP/2 and HTTP/1.1 when connecting using TLS.

For any other port, Gun will attempt to connect using plain TCP and will use the HTTP/1.1 protocol.

The transport and protocol used can be overriden via 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 => tls}).

Waiting for the connection to be established

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).

Handling connection loss

When the connection is lost, Gun will send a gun_down message indicating the current protocol, the reason the connection was lost and two lists 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.

Monitoring the connection process

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.

receive
    %% 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.

Closing the connection abruptly

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.