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diff --git a/guide/middlewares.md b/guide/middlewares.md new file mode 100644 index 0000000..0ab6dc2 --- /dev/null +++ b/guide/middlewares.md @@ -0,0 +1,72 @@ +Middlewares +=========== + +Purpose +------- + +Cowboy delegates the request processing to middleware components. +By default, two middlewares are defined, for the routing and handling +of the request, as is detailed in most of this guide. + +Middlewares give you complete control over how requests are to be +processed. You can add your own middlewares to the mix or completely +change the chain of middlewares as needed. + +Cowboy will execute all middlewares in the given order, unless one +of them decides to stop processing. + +Usage +----- + +Middlewares only need to implement a single callback: `execute/2`. +It is defined in the `cowboy_middleware` behavior. + +This callback has two arguments. The first is the `Req` object. +The second is the environment. + +Middlewares can return one of four different values: + * `{ok, Req, Env}` to continue the request processing + * `{suspend, Module, Function, Args}` to hibernate + * `{halt, Req}` to stop processing and move on to the next request + * `{error, StatusCode, Req}` to reply an error and close the socket + +Of note is that when hibernating, processing will resume on the given +MFA, discarding all previous stacktrace. Make sure you keep the `Req` +and `Env` in the arguments of this MFA for later use. + +If an error happens during middleware processing, Cowboy will not try +to send an error back to the socket, the process will just crash. It +is up to the middleware to make sure that a reply is sent if something +goes wrong. + +Configuration +------------- + +The middleware environment is defined as the `env` protocol option. +In the previous chapters we saw it briefly when we needed to pass +the routing information. It is a list of tuples with the first +element being an atom and the second any Erlang term. + +Two values in the environment are reserved: + * `listener` contains the pid of the listener process + * `result` contains the result of the processing + +The `listener` value is always defined. The `result` value can be +set by any middleware. If set to anything other than `ok`, Cowboy +will not process any subsequent requests on this connection. + +The middlewares that come with Cowboy may define or require other +environment values to perform. + +Routing middleware +------------------ + +The routing middleware requires the `dispatch` value. If routing +succeeds, it will put the handler name and options in the `handler` +and `handler_opts` values of the environment, respectively. + +Handler middleware +------------------ + +The handler middleware requires the `handler` and `handler_opts` +values. It puts the result of the request handling into `result`. |