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()}
Info       :: any()

CallResult :: {commands(), State}
            | {commands(), State, hibernate}
            | Deprecated

Deprecated :: {ok, State}
            | {ok, State, hibernate}
            | {reply, OutFrame | [OutFrame], State}
            | {reply, OutFrame | [OutFrame], State, hibernate}
            | {stop, State}
OutFrame   :: cow_ws:frame()                    %% see types below

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, including close frames to terminate the connection; enable/disable active mode; enable/disable compression for subsequent frames; or change Websocket options.

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.

{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.



commands() :: [Command]

Command :: {active, boolean()}
         | {deflate, boolean()}
         | {set_options, #{idle_timeout => timeout()}}
         | {shutdown_reason, any()}
         | Frame :: cow_ws:frame()

Commands that may be returned from Websocket callbacks.

The following commands are defined:


Whether to disable or enable reading from the socket. This can be used to apply flow control to a Websocket connection.


Whether the subsequent frames should be compressed. Has no effect on connections that did not negotiate compression.


Set Websocket options. Currently only the option idle_timeout may be updated from a Websocket handler.


Change the shutdown reason. The Websocket process will exit with reason normal by default. This command can be used to exit with reason {shutdown, ShutdownReason} under normal conditions. This command has no effect when the Websocket process exits abnormally, for example following a crash in a handler callback.


Send the corresponding Websocket frame.


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() :: #{
    active_n       => pos_integer(),
    compress       => boolean(),
    deflate_opts   => cow_ws:deflate_opts()
    idle_timeout   => timeout(),
    max_frame_size => non_neg_integer() | infinity,
    req_filter     => fun((cowboy_req:req()) -> map()),
    validate_utf8  => boolean()

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:

active_n (100)

The number of packets Cowboy will request from the socket at once. This can be used to tweak the performance of the server. Higher values reduce the number of times Cowboy need to request more packets from the port driver at the expense of potentially higher memory being used.

This option does not apply to Websocket over HTTP/2.

compress (false)

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

deflate_opts (#{})

Configuration for the permessage-deflate Websocket extension. Allows configuring both the negotiated options and the zlib compression options. The defaults optimize the compression at the expense of some memory and CPU.

idle_timeout (60000)

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

This option can be updated at any time using the set_options command.

max_frame_size (infinity)

Maximum frame size in bytes 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.

validate_utf8 (true)

Whether Cowboy should verify that the payload of text and close frames is valid UTF-8. This is required by the protocol specification but in some cases it may be more interesting to disable it in order to save resources.

Note that binary frames do not have this UTF-8 requirement and are what should be used under normal circumstances if necessary.


  • 2.8: The active_n option was added.
  • 2.7: The commands based interface has been documented. The old interface is now deprecated.
  • 2.7: The command shutdown_reason was introduced.
  • 2.7: The option validate_utf8 has been added.
  • 2.6: Deflate options can now be configured via deflate_opts.
  • 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 2.10 Function Reference


Version select

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