From d7580dbfdbdcf3ecf085479b6550284a47745138 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lo=C3=AFc=20Hoguin?= <essen@ninenines.eu>
Date: Fri, 28 Apr 2023 13:34:22 +0200
Subject: Cowboy 2.10.0

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-[[rest_flowcharts]]
-== REST flowcharts
-
-This chapter will explain the REST handler state machine through
-a number of different diagrams.
-
-There are four main paths that requests may follow. One for the
-method OPTIONS; one for the methods GET and HEAD; one for the
-methods PUT, POST and PATCH; and one for the method DELETE.
-
-All paths start with the "Start" diagram, and all paths excluding
-the OPTIONS path go through the "Content negotiation" diagram
-and optionally the "Conditional requests" diagram if the resource
-exists.
-
-The red squares refer to another diagram. The light green squares
-indicate a response. Other squares may be either a callback or a
-question answered by Cowboy itself. Green arrows tend to indicate
-the default behavior if the callback is undefined.
-
-=== Start
-
-All requests start from here.
-
-image::rest_start.png[REST starting flowchart]
-
-A series of callbacks are called in succession to perform
-a general checkup of the service, the request line and
-request headers.
-
-The request body, if any, is not expected to have been
-received for any of these steps. It is only processed
-at the end of the "PUT, POST and PATCH methods" diagram,
-when all conditions have been met.
-
-The `known_methods` and `allowed_methods` callbacks
-return a list of methods. Cowboy then checks if the request
-method is in the list, and stops otherwise.
-
-The `is_authorized` callback may be used to check that
-access to the resource is authorized. Authentication
-may also be performed as needed. When authorization is
-denied, the return value from the callback must include
-a challenge applicable to the requested resource, which
-will be sent back to the client in the www-authenticate
-header.
-
-This diagram is immediately followed by either the
-"OPTIONS method" diagram when the request method is
-OPTIONS, or the "Content negotiation" diagram otherwise.
-
-=== OPTIONS method
-
-This diagram only applies to OPTIONS requests.
-
-image::rest_options.png[REST OPTIONS method flowchart]
-
-The `options` callback may be used to add information
-about the resource, such as media types or languages
-provided; allowed methods; any extra information. A
-response body may also be set, although clients should
-not be expected to read it.
-
-If the `options` callback is not defined, Cowboy will
-send a response containing the list of allowed methods
-by default.
-
-=== Content negotiation
-
-This diagram applies to all request methods other than
-OPTIONS. It is executed right after the "Start" diagram
-is completed.
-
-image::rest_conneg.png[REST content negotiation flowchart]
-
-The purpose of these steps is to determine an appropriate
-representation to be sent back to the client.
-
-The request may contain any of the accept header; the
-accept-language header; or the accept-charset header.
-When present, Cowboy will parse the headers and then
-call the corresponding callback to obtain the list
-of provided content-type, language or charset for this
-resource. It then automatically select the best match
-based on the request.
-
-If a callback is not defined, Cowboy will select the
-content-type, language or charset that the client
-prefers.
-
-The `content_types_provided` also returns the name of
-a callback for every content-type it accepts. This
-callback will only be called at the end of the
-"GET and HEAD methods" diagram, when all conditions
-have been met.
-
-The selected content-type, language and charset are
-saved as meta values in the Req object. You *should*
-use the appropriate representation if you set a
-response body manually (alongside an error code,
-for example).
-
-This diagram is immediately followed by
-the "GET and HEAD methods" diagram,
-the "PUT, POST and PATCH methods" diagram,
-or the "DELETE method" diagram, depending on the
-method.
-
-=== GET and HEAD methods
-
-This diagram only applies to GET and HEAD requests.
-
-For a description of the `cond` step, please see
-the "Conditional requests" diagram.
-
-image::rest_get_head.png[REST GET/HEAD methods flowchart]
-
-When the resource exists, and the conditional steps
-succeed, the resource can be retrieved.
-
-Cowboy prepares the response by first retrieving
-metadata about the representation, then by calling
-the `ProvideResource` callback. This is the callback
-you defined for each content-types you returned from
-`content_types_provided`. This callback returns the body
-that will be sent back to the client, or a fun if the
-body must be streamed.
-
-When the resource does not exist, Cowboy will figure out
-whether the resource existed previously, and if so whether
-it was moved elsewhere in order to redirect the client to
-the new URI.
-
-The `moved_permanently` and `moved_temporarily` callbacks
-must return the new location of the resource if it was in
-fact moved.
-
-=== PUT, POST and PATCH methods
-
-This diagram only applies to PUT, POST and PATCH requests.
-
-For a description of the `cond` step, please see
-the "Conditional requests" diagram.
-
-image::rest_put_post_patch.png[REST PUT/POST/PATCH methods flowchart]
-
-When the resource exists, first the conditional steps
-are executed. When that succeeds, and the method is PUT,
-Cowboy will call the `is_conflict` callback. This function
-can be used to prevent potential race conditions, by locking
-the resource for example.
-
-Then all three methods reach the `content_types_accepted`
-step that we will describe in a few paragraphs.
-
-When the resource does not exist, and the method is PUT,
-Cowboy will check for conflicts and then move on to the
-`content_types_accepted` step. For other methods, Cowboy
-will figure out whether the resource existed previously,
-and if so whether it was moved elsewhere. If the resource
-is truly non-existent, the method is POST and the call
-for `allow_missing_post` returns `true`, then Cowboy will
-move on to the `content_types_accepted` step. Otherwise
-the request processing ends there.
-
-The `moved_permanently` and `moved_temporarily` callbacks
-must return the new location of the resource if it was in
-fact moved.
-
-The `content_types_accepted` returns a list of
-content-types it accepts, but also the name of a callback
-for each of them. Cowboy will select the appropriate
-callback for processing the request body and call it.
-
-This callback may return one of three different return
-values.
-
-If an error occurred while processing the request body,
-it must return `false` and Cowboy will send an
-appropriate error response.
-
-If the method is POST, then you may return `true` with
-an URI of where the resource has been created. This is
-especially useful for writing handlers for collections.
-
-Otherwise, return `true` to indicate success. Cowboy
-will select the appropriate response to be sent depending
-on whether a resource has been created, rather than
-modified, and on the availability of a location header
-or a body in the response.
-
-=== DELETE method
-
-This diagram only applies to DELETE requests.
-
-For a description of the `cond` step, please see
-the "Conditional requests" diagram.
-
-image::rest_delete.png[REST DELETE method flowchart]
-
-When the resource exists, and the conditional steps
-succeed, the resource can be deleted.
-
-Deleting the resource is a two steps process. First
-the callback `delete_resource` is executed. Use this
-callback to delete the resource.
-
-Because the resource may be cached, you must also
-delete all cached representations of this resource
-in the system. This operation may take a while though,
-so you may return before it finished.
-
-Cowboy will then call the `delete_completed` callback.
-If you know that the resource has been completely
-deleted from your system, including from caches, then
-you can return `true`. If any doubts persist, return
-`false`. Cowboy will assume `true` by default.
-
-To finish, Cowboy checks if you set a response body,
-and depending on that, sends the appropriate response.
-
-When the resource does not exist, Cowboy will figure out
-whether the resource existed previously, and if so whether
-it was moved elsewhere in order to redirect the client to
-the new URI.
-
-The `moved_permanently` and `moved_temporarily` callbacks
-must return the new location of the resource if it was in
-fact moved.
-
-=== Conditional requests
-
-This diagram applies to all request methods other than
-OPTIONS. It is executed right after the `resource_exists`
-callback, when the resource exists.
-
-image::rest_cond.png[REST conditional requests flowchart]
-
-A request becomes conditional when it includes either of
-the if-match header; the if-unmodified-since header; the
-if-none-match header; or the if-modified-since header.
-
-If the condition fails, the request ends immediately
-without any retrieval or modification of the resource.
-
-The `generate_etag` and `last_modified` are called as
-needed. Cowboy will only call them once and then cache
-the results for subsequent use.
-- 
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