From 0556fb027c44cf80d3f77fb9c3fac4c504d6720c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lo=C3=AFc=20Hoguin?= <essen@ninenines.eu>
Date: Sun, 23 Jul 2017 09:48:25 +0200
Subject: Update the flowchart chapter

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-[[overview]]
-== Request overview
-
-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:
-
-image::http_req_resp.png[HTTP request/response flowchart]
-
-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.
-
-HTTP/2 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.
-
-[source,erlang]
-----
-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.
-
-[source,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 (HTTP/2)
-
-In HTTP/2, 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.
-
-HTTP/2 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.
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