From 48f39402181d959cad88cb3f460210c007169f50 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lo=C3=AFc=20Hoguin?= <essen@ninenines.eu>
Date: Wed, 12 May 2021 11:13:47 +0200
Subject: Cowboy 2.9.0

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-[[flow_diagram]]
-== Flow diagram
-
-Cowboy is a lightweight HTTP server with support for HTTP/1.1,
-HTTP/2 and Websocket.
-
-It is built on top of Ranch. Please see the Ranch guide for more
-information about how the network connections are handled.
-
-=== Overview
-
-image::http_req_resp.png[HTTP request/response flowchart]
-
-As you can see on the diagram, the client
-begins by connecting to the server. This step is handled
-by a Ranch acceptor, which is a process dedicated to
-accepting new connections.
-
-After Ranch accepts a new connection, whether it is an
-HTTP/1.1 or HTTP/2 connection, Cowboy starts receiving
-requests and handling them.
-
-In HTTP/1.1 all requests come sequentially. In HTTP/2
-the requests may arrive and be processed concurrently.
-
-When a request comes in, Cowboy creates a stream, which
-is a set of request/response and all the events associated
-with them. The protocol code in Cowboy defers the handling
-of these streams to stream handler modules. When you
-configure Cowboy you may define one or more module that
-will receive all events associated with a stream, including
-the request, response, bodies, Erlang messages and more.
-
-By default Cowboy comes configured with a stream handler
-called `cowboy_stream_h`. This stream handler will create
-a new process for every request coming in, and then
-communicate with this process to read the body or send
-a response back. The request process executes middlewares
-which, by default, including the router and then the
-execution of handlers. Like stream handlers, middlewares
-may also be customized.
-
-A response may be sent at almost any point in this
-diagram. If the response must be sent before the stream
-is initialized (because an error occurred early, for
-example) then stream handlers receive a special event
-indicating this error.
-
-=== Protocol-specific headers
-
-Cowboy takes care of protocol-specific headers and prevents
-you from sending them manually. For HTTP/1.1 this includes
-the `transfer-encoding` and `connection` headers. For HTTP/2
-this includes the colon headers like `:status`.
-
-Cowboy will also remove protocol-specific headers from
-requests before passing them to stream handlers. Cowboy
-tries to hide the implementation details of all protocols
-as well as possible.
-
-=== Number of processes per connection
-
-By default, Cowboy will use one process per connection,
-plus one process per set of request/response (called a
-stream, internally).
-
-The reason it creates a new process for every request is due
-to the requirements of HTTP/2 where requests are executed
-concurrently and independently from the connection. The
-frames from the different requests end up interleaved on
-the single TCP connection.
-
-The request processes are never reused. There is therefore
-no need to perform any cleanup after the response has been
-sent. The process will terminate and Erlang/OTP will reclaim
-all memory at once.
-
-Cowboy ultimately does not require more than one process
-per connection. It is possible to interact with the connection
-directly from a stream handler, a low level interface to Cowboy.
-They are executed from within the connection process, and can
-handle the incoming requests and send responses. This is however
-not recommended in normal circumstances, as a stream handler
-taking too long to execute could have a negative impact on
-concurrent requests or the state of the connection itself.
-
-=== Date header
-
-Because querying for the current date and time can be expensive,
-Cowboy generates one 'Date' header value every second, shares it
-to all other processes, which then simply copy it in the response.
-This allows compliance with HTTP/1.1 with no actual performance loss.
-
-=== Binaries
-
-Cowboy makes extensive use of binaries.
-
-Binaries are more efficient than lists for representing
-strings because they take less memory space. Processing
-performance can vary depending on the operation. Binaries
-are known for generally getting a great boost if the code
-is compiled natively. Please see the HiPE documentation
-for more details.
-
-Binaries may end up being shared between processes. This
-can lead to some large memory usage when one process keeps
-the binary data around forever without freeing it. If you
-see some weird memory usage in your application, this might
-be the cause.
-- 
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