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diff --git a/doc/src/guide/loop_handlers.asciidoc b/doc/src/guide/loop_handlers.asciidoc new file mode 100644 index 0000000..58c4223 --- /dev/null +++ b/doc/src/guide/loop_handlers.asciidoc @@ -0,0 +1,146 @@ +[[loop_handlers]] +== Loop handlers + +Loop handlers are a special kind of HTTP handlers used when the +response can not be sent right away. The handler enters instead +a receive loop waiting for the right message before it can send +a response. + +Loop handlers are used for requests where a response might not +be immediately available, but where you would like to keep the +connection open for a while in case the response arrives. The +most known example of such practice is known as long polling. + +Loop handlers can also be used for requests where a response is +partially available and you need to stream the response body +while the connection is open. The most known example of such +practice is known as server-sent events. + +While the same can be accomplished using plain HTTP handlers, +it is recommended to use loop handlers because they are well-tested +and allow using built-in features like hibernation and timeouts. + +Loop handlers essentially wait for one or more Erlang messages +and feed these messages to the `info/3` callback. It also features +the `init/2` and `terminate/3` callbacks which work the same as +for plain HTTP handlers. + +=== Initialization + +The `init/2` function must return a `cowboy_loop` tuple to enable +loop handler behavior. This tuple may optionally contain +a timeout value and/or the atom `hibernate` to make the +process enter hibernation until a message is received. + +This snippet enables the loop handler. + +[source,erlang] +---- +init(Req, _Opts) -> + {cowboy_loop, Req, #state{}}. +---- + +However it is largely recommended that you set a timeout +value. The next example sets a timeout value of 30s and +also makes the process hibernate. + +[source,erlang] +---- +init(Req, _Opts) -> + {cowboy_loop, Req, #state{}, 30000, hibernate}. +---- + +=== Receive loop + +Once initialized, Cowboy will wait for messages to arrive +in the process' mailbox. When a message arrives, Cowboy +calls the `info/3` function with the message, the Req object +and the handler's state. + +The following snippet sends a reply when it receives a +`reply` message from another process, or waits for another +message otherwise. + +[source,erlang] +---- +info({reply, Body}, Req, State) -> + Req2 = cowboy_req:reply(200, [], Body, Req), + {stop, Req2, State}; +info(_Msg, Req, State) -> + {ok, Req, State, hibernate}. +---- + +Do note that the `reply` tuple here may be any message +and is simply an example. + +This callback may perform any necessary operation including +sending all or parts of a reply, and will subsequently +return a tuple indicating if more messages are to be expected. + +The callback may also choose to do nothing at all and just +skip the message received. + +If a reply is sent, then the `stop` tuple should be returned. +This will instruct Cowboy to end the request. + +Otherwise an `ok` tuple should be returned. + +=== Streaming loop + +Another common case well suited for loop handlers is +streaming data received in the form of Erlang messages. +This can be done by initiating a chunked reply in the +`init/2` callback and then using `cowboy_req:chunk/2` +every time a message is received. + +The following snippet does exactly that. As you can see +a chunk is sent every time a `chunk` message is received, +and the loop is stopped by sending an `eof` message. + +[source,erlang] +---- +init(Req, _Opts) -> + Req2 = cowboy_req:chunked_reply(200, [], Req), + {cowboy_loop, Req2, #state{}}. + +info(eof, Req, State) -> + {stop, Req, State}; +info({chunk, Chunk}, Req, State) -> + cowboy_req:chunk(Chunk, Req), + {ok, Req, State}; +info(_Msg, Req, State) -> + {ok, Req, State}. +---- + +==== Cleaning up + +It is recommended that you set the connection header to +`close` when replying, as this process may be reused for +a subsequent request. + +Please refer to the xref:handlers[Handlers chapter] +for general instructions about cleaning up. + +=== Timeout + +By default Cowboy will not attempt to close the connection +if there is no activity from the client. This is not always +desirable, which is why you can set a timeout. Cowboy will +close the connection if no data was received from the client +after the configured time. The timeout only needs to be set +once and can't be modified afterwards. + +Because the request may have had a body, or may be followed +by another request, Cowboy is forced to buffer all data it +receives. This data may grow to become too large though, +so there is a configurable limit for it. The default buffer +size is of 5000 bytes, but it may be changed by setting the +`loop_max_buffer` middleware environment value. + +=== Hibernate + +To save memory, you may hibernate the process in between +messages received. This is done by returning the atom +`hibernate` as part of the `loop` tuple callbacks normally +return. Just add the atom at the end and Cowboy will hibernate +accordingly. |