cowboy_websocket - Websocket


The module cowboy_websocket implements Websocket as a Ranch protocol. It also defines a callback interface for handling Websocket connections.


Websocket handlers must implement the following callback interface:

init(Req, State)
    -> {cowboy_websocket, Req, State}
     | {cowboy_websocket, Req, State, Opts}

websocket_init(State)            -> CallResult  %% optional
websocket_handle(InFrame, State) -> CallResult
websocket_info(Info, State)      -> CallResult

terminate(Reason, PartialReq, State) -> ok      %% optional

Req        :: cowboy_req:req()
PartialReq :: map()
State      :: any()
Opts       :: cowboy_websocket:opts()
InFrame    :: ping | pong | {text | binary | ping | pong, binary()}
OutFrame   :: cow_ws:frame()                    %% see types below
Info       :: any()

CallResult :: {ok, State}
            | {ok, State, hibernate}
            | {reply, OutFrame | [OutFrame], State}
            | {reply, OutFrame | [OutFrame], State, hibernate}
            | {stop, State}

Reason     :: normal | stop | timeout
            | remote | {remote, cow_ws:close_code(), binary()}
            | {error, badencoding | badframe | closed | atom()}
            | {crash, error | exit | throw, any()}

The init/2 callback is common to all handlers. To upgrade the connection to Websocket, it must return cowboy_websocket as the first element of the tuple.

Any operation requiring the HTTP request must be done in the init/2 function, as the Req object will not be available after it returns. Websocket sub-protocol selection should therefore be done in this function.

The optional websocket_init/1 callback will be called once the connection has been upgraded to Websocket. It can be used to perform any required initialization of the handler.

Note that the init/2 function does not run in the same process as the Websocket callbacks. Any Websocket-specific initialization must be done in websocket_init/1.

The websocket_handle/2 callback will be called for every frame received. The websocket_info/2 callback will be called for every Erlang message received.

All three Websocket callbacks may send one or more frames back to the client (by returning a reply tuple) or terminate the connection (by sending a close frame or returning a stop tuple).

The optional terminate/3 callback will ultimately be called with the reason for the termination of the connection. This callback is common to all handlers. Note that Websocket will not provide the full Req object by default, to save memory.

Cowboy will terminate the process right after closing the Websocket connection. This means that there is no need to perform any cleanup in the terminate/3 callback.

The following terminate reasons are defined for Websocket connections:


The connection was closed normally before establishing a Websocket connection. This typically happens if an ok tuple is returned from the init/2 callback.


The remote endpoint closed the connection without giving any further details.

{remote, Code, Payload}

The remote endpoint closed the connection with the given Code and Payload as the reason.


The handler requested to close the connection, either by returning a stop tuple or by sending a close frame.


The connection has been closed due to inactivity. The timeout value can be configured from init/2.

{crash, Class, Reason}

A crash occurred in the handler. Class and Reason can be used to obtain more information about the crash. The function erlang:get_stacktrace/0 can also be called to obtain the stacktrace of the process when the crash occurred.

{error, badencoding}

A text frame was sent by the client with invalid encoding. All text frames must be valid UTF-8.

{error, badframe}

A protocol error has been detected.

{error, closed}

The socket has been closed brutally without a close frame being received first.

{error, Reason}

A socket error ocurred.



frame() :: {text, iodata()}
    | {binary, iodata()}
    | ping | {ping, iodata()}
    | pong | {pong, iodata()}
    | close | {close, iodata()} | {close, close_code(), iodata()}

close_code() :: 1000..1003 | 1006..1011 | 3000..4999

Websocket frames that can be sent as a response.

Note that there is no need to send pong frames back as Cowboy does it automatically for you.


opts() :: #{
    compress => boolean(),
    idle_timeout => timeout(),
    max_frame_size => non_neg_integer() | infinity,
    req_filter => fun((cowboy_req:req()) -> map())

Websocket handler options.

This configuration is passed to Cowboy from the init/2 function:

init(Req, State) ->
    Opts = #{compress => true},
    {cowboy_websocket, Req, State, Opts}.

The default value is given next to the option name:

compress (false)

Whether to enable the Websocket frame compression extension. Frames will only be compressed for the clients that support this extension.

idle_timeout (60000)

Time in milliseconds that Cowboy will keep the connection open without receiving anything from the client.

max_frame_size (infinity)

Maximum frame size allowed by this Websocket handler. Cowboy will close the connection when a client attempts to send a frame that goes over this limit. For fragmented frames this applies to the size of the reconstituted frame.


A function applied to the Req to compact it and only keep required information. The Req is only given back in the terminate/3 callback. By default it keeps the method, version, URI components and peer information.


  • 2.0: The Req object is no longer passed to Websocket callbacks.
  • 2.0: The callback websocket_terminate/3 was removed in favor of terminate/3.
  • 1.0: Protocol introduced.

See also

cowboy(7), cowboy_handler(3), cowboy_http(3), cowboy_http2(3)

Cowboy 2.5 Function Reference


Version select

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