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Update the flowchart chapter
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Loïc Hoguin
parent
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5 changed files with 337 additions and 429 deletions
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@ -80,13 +80,3 @@ include::migrating_from_1.0.asciidoc[Migrating from Cowboy 1.0 to 2.0]
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// @todo Maybe history? Could take info from architecture also.
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// @todo Maybe history? Could take info from architecture also.
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include::specs.asciidoc[HTTP and other specifications]
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include::specs.asciidoc[HTTP and other specifications]
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= Deprecated chapters
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== About the deprecated chapters
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The following chapters were relevant in Cowboy 1.0. They have
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not been updated for Cowboy 2.0 yet. The information in these
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chapters may or may not be useful.
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include::overview.asciidoc[Overview]
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@ -9,9 +9,54 @@ information about how the network connections are handled.
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=== Overview
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=== Overview
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Placeholder section.
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image::http_req_resp.png[HTTP request/response flowchart]
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// @todo Make the diagram.
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As you can see on the diagram, the client
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begins by connecting to the server. This step is handled
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by a Ranch acceptor, which is a process dedicated to
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accepting new connections.
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After Ranch accepts a new connection, whether it is an
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HTTP/1.1 or HTTP/2 connection, Cowboy starts receiving
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requests and handling them.
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In HTTP/1.1 all requests come sequentially. In HTTP/2
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the requests may arrive and be processed concurrently.
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When a request comes in, Cowboy creates a stream, which
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is a set of request/response and all the events associated
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with them. The protocol code in Cowboy defers the handling
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of these streams to stream handler modules. When you
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configure Cowboy you may define one or more module that
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will receive all events associated with a stream, including
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the request, response, bodies, Erlang messages and more.
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By default Cowboy comes configured with a stream handler
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called `cowboy_stream_h`. This stream handler will create
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a new process for every request coming in, and then
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communicate with this process to read the body or send
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a response back. The request process executes middlewares
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which, by default, including the router and then the
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execution of handlers. Like stream handlers, middlewares
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may also be customized.
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A response may be sent at almost any point in this
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diagram. If the response must be sent before the stream
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is initialized (because an error occurred early, for
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example) then stream handlers receive a special event
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indicating this error.
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=== Protocol-specific headers
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Cowboy takes care of protocol-specific headers and prevents
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you from sending them manually. For HTTP/1.1 this includes
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the `transfer-encoding` and `connection` headers. For HTTP/2
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this includes the colon headers like `:status`.
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Cowboy will also remove protocol-specific headers from
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requests before passing them to stream handlers. Cowboy
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tries to hide the implementation details of all protocols
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as well as possible.
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=== Number of processes per connection
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=== Number of processes per connection
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Before Width: | Height: | Size: 20 KiB After Width: | Height: | Size: 22 KiB |
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@ -1,150 +0,0 @@
|
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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.
|
|
||||||
Loading…
Add table
Add a link
Reference in a new issue