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Supported protocols

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This chapter describes the protocols supported and the operations available to them.

HTTP/1.1

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 the respective methods. Gun also provides a gun:data/4 function for streaming the request body.

Gun will send a gun_inform message for every intermediate informational responses received. They will always be sent before the gun_response message.

Gun will send a gun_response message for every response received, followed by zero or more gun_data messages for the response body, which is optionally terminated by a gun_trailers message. 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 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 calling 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 responses 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_upgrade message will be sent on success; a gun_response message otherwise.

HTTP/2

HTTP/2 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 additional resources along with the normal response to the original request.

The HTTP/2 interface is very similar to HTTP/1.1, so this section instead focuses on the differences in the interface for the two protocols.

Gun will send gun_push messages for every push received. They will always be sent before the gun_response message. They can be ignored safely if they are not needed, or they can be canceled.

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

It is not currently possible to upgrade an HTTP/2 connection to Websocket. Support for this will be added in a future release.

Websocket

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.

Summary

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

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Supported operations per protocol
Operation	HTTP/1.1	HTTP/2	Websocket
delete	yes	yes	no
get	yes	yes	no
head	yes	yes	no
options	yes	yes	no
patch	yes	yes	no
post	yes	yes	no
put	yes	yes	no
request	yes	yes	no
data	yes	yes	no
await	yes	yes	no
await_body	yes	yes	no
flush	yes	yes	no
cancel	yes	yes	no
ws_upgrade	yes	no	no
ws_send	no	no	yes

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Messages sent per protocol
Message	HTTP/1.1	HTTP/2	Websocket
gun_push	no	yes	no
gun_inform	yes	yes	no
gun_response	yes	yes	no
gun_data	yes	yes	no
gun_trailers	yes	yes	no
gun_error	yes	yes	yes
gun_upgrade	yes	no	no
gun_ws	no	no	yes

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