cowboy_handler - Plain HTTP handlers


The cowboy_handler middleware executes the handler selected by the router or any other preceding middleware.

This middleware takes the handler module and initial state from the handler and handler_opts environment values, respectively. On completion, it adds a result value to the middleware environment, containing the return value of the terminate/3 callback (if defined) and ok otherwise.

This module also defines a callback interface for handling HTTP requests.


Plain HTTP handlers implement the following interface:

init(Req, State) -> {ok, Req, State}

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

Req    :: cowboy_req:req()
State  :: any()
Reason :: normal
        | {crash, error | exit | throw, any()}

These two callbacks are common to all handlers.

Plain HTTP handlers do all their work in the init/2 callback. Returning ok terminates the handler. If no response is sent, Cowboy will send a 204 No Content.

The optional terminate/3 callback will ultimately be called with the reason for the termination of the handler. Cowboy will terminate the process right after this. There is no need to perform any cleanup in this callback.

The following terminate reasons are defined for plain HTTP handlers:


The connection was closed normally.

{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.


The following function should be called by modules implementing custom handlers to execute the optional terminate callback:

See also


Cowboy 2.7 Function Reference


Version select

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