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Added a chapter about life of HTTP requests

Reworked the table of contents also.
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1 parent 40b8754 commit 7319526de6b983e153151d64e19aec5f7199095b @essen essen committed Sep 16, 2013
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  1. +153 −0 guide/http_req_life.md
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  4. +68 −74 guide/toc.md
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+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:
+
+![HTTP request/response flowchart](http_req_resp.png)
+
+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.
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