HTTP/1.1 is a text request-response protocol. The client sends a request, the server sends back a response.

Gun provides convenience functions for performing GET, HEAD, OPTIONS, POST, PATCH, PUT, and DELETE requests. All these functions are aliases of gun:request/{4,5,6} for each respective methods. Gun also provides a gun:data/4 function for streaming the request body.

Gun will send a gun_response message for every response received, followed by zero or more gun_data messages for the response body. If something goes wrong, a gun_error will be sent instead.

Gun provides convenience functions for dealing with messages. The gun:await/{2,3,4} function waits for a response to the given request, and the gun:await_body/{2,3,4} function for the response’s body. The gun:flush/1 function can be used to clear all messages related to a request or a connection from the mailbox of the process.

The function gun:cancel/2 can be used to silence the response to a request previously sent if it is no longer needed. When using HTTP/1.1 there is no multiplexing so Gun will have to receive the response fully before any other response can be received.

Finally, Gun can upgrade an HTTP/1.1 connection to Websocket. It provides the gun:ws_upgrade/{2,3,4} function for that purpose. A gun_ws_upgrade message will be sent on success; a gun_response message otherwise.

SPDY is a binary protocol based on HTTP, compatible with the HTTP semantics, that reduces the complexity of parsing requests and responses, compresses the HTTP headers and allows the server to push multiple responses to a single request.

The SPDY interface is very similar to HTTP/1.1, so this section instead focuses on the differences in the interface for the two protocols.

Because a SPDY server can push multiple responses to a single request, Gun might send gun_push messages for every push received. They can be ignored safely if they are not needed.

The gun:cancel/2 function will use the SPDY stream cancellation mechanism which allows Gun to inform the server to stop sending a response for this particular request, saving resources.

It is not possible to upgrade a SPDY connection to Websocket due to protocol limitations.

Websocket is a binary protocol built on top of HTTP that allows asynchronous concurrent communication between the client and the server. A Websocket server can push data to the client at any time.

Websocket is only available as a connection upgrade over an HTTP/1.1 connection.

Once the Websocket connection is established, the only operation available on this connection is sending Websocket frames using gun:ws_send/2.

Gun will send a gun_ws message for every frame received.

The two following tables summarize the supported operations and the messages Gun sends depending on the connection’s current protocol.