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diff --git a/guide/http_req_life.md b/guide/http_req_life.md new file mode 100644 index 0000000..1462b59 --- /dev/null +++ b/guide/http_req_life.md @@ -0,0 +1,153 @@ +The life of a request +===================== + +This chapter explains the different steps a request +goes through until a response is sent, along with +details of the Cowboy implementation. + +Request/response +---------------- + +As you already know, HTTP clients connect to the server and +send a request for a resource; the server then sends a +response containing the resource if it could obtain it. + +Before the server can send the resource, however, it +needs to perform many different operations to read the +request, find the resource, prepare the response being +sent and often other related operations the user can +add like writing logs. + +Requests take the following route in Cowboy: + + + +This shows the default middlewares, but they may be +configured differently in your setup. The dark green +indicates the points where you can hook your own code, +the light green is the Cowboy code that you can of +course configure as needed. + +The `acceptor` is the part of the server that accepts +the connection and create an Erlang process to handle +it. The `parser` then starts reading from the socket +and handling requests as they come until the socket +is closed. + +A response may be sent at many different points in the +life of the request. If Cowboy can't parse the request, +it gives up with an error response. If the router can't +find the resource, it sends a not found error. Your +own code can of course send a response at any time. + +When a response is sent, you can optionally modify it +or act upon it by enabling the `onresponse` hook. By +default the response is sent directly to the client. + +And then? +--------- + +Behavior depends on what protocol is in use. + +HTTP/1.0 can only process one request per connection, +so Cowboy will close the connection immediately after +it sends the response. + +HTTP/1.1 allows the client to request that the server +keeps the connection alive. This mechanism is described +in the next section. + +SPDY is designed to allow sending multiple requests +asynchronously on the same connection. Details on what +this means for your application is described in this +chapter. + +Keep-alive (HTTP/1.1) +--------------------- + +With HTTP/1.1, the connection may be left open for +subsequent requests to come. This mechanism is called +`keep-alive`. + +When the client sends a request to the server, it includes +a header indicating whether it would like to leave the +socket open. The server may or may not accept, indicating +its choice by sending the same header in the response. + +Cowboy will include this header automatically in all +responses to HTTP/1.1 requests. You can however force +the closing of the socket if you want. When Cowboy sees +you want to send a `connection: close` header, it will +not override it and will close the connection as soon +as the reply is sent. + +This snippet will force Cowboy to close the connection. + +``` erlang +{ok, Req2} = cowboy_req:reply(200, [ + {<<"connection">>, <<"close">>}, +], <<"Closing the socket in 3.. 2.. 1..">>, Req). +``` + +Cowboy will only accept a certain number of new requests +on the same connection. By default it will run up to 100 +requests. This number can be changed by setting the +`max_keepalive` configuration value when starting an +HTTP listener. + +``` erlang +cowboy:start_http(my_http_listener, 100, [{port, 8080}], [ + {env, [{dispatch, Dispatch}]}, + {max_keepalive, 5} +]). +``` + +Cowboy implements the keep-alive mechanism by reusing +the same process for all requests. This allows Cowboy +to save memory. This works well because most code will +not have any side effect impacting subsequent requests. +But it also means you need to clean up if you do have +code with side effects. The `terminate/3` function can +be used for this purpose. + +Pipelining (HTTP/1.1) +--------------------- + +While HTTP is designed as a sequential protocol, with +the client sending a request and then waiting for the +response from the server, nothing prevents the client +from sending more requests to the server without waiting +for the response, due to how sockets work. The server +still handles the requests sequentially and sends the +responses in the same order. + +This mechanism is called pipelining. It allows reducing +latency when a client needs to request many resources +at the same time. This is used by browsers when requesting +static files for example. + +This is handled automatically by the server. + +Asynchronous requests (SPDY) +---------------------------- + +In SPDY, the client can send a request at any time. +And the server can send a response at any time too. + +This means for example that the client does not need +to wait for a request to be fully sent to send another, +it is possible to interleave a request with the request +body of another request. The same is true with responses. +Responses may also be sent in a different order. + +Because requests and responses are fully asynchronous, +Cowboy creates a new process for each request, and these +processes are managed by another process that handles the +connection itself. + +SPDY servers may also decide to send resources to the +client before the client requests them. This is especially +useful for sending static files associated with the HTML +page requested, as this reduces the latency of the overall +response. Cowboy does not support this particular mechanism +at this point, however. |