+doc #20192 explain need of draining entities in server/client HTTP (#20852)

* +doc #20192 explain need of draining entities in server/client HTTP

* missing javadsl for Connection header

* Update HttpClientExampleDocTest.java

akka-docs/rst/scala/http/client-side/connection-level.rst

@@ -7,6 +7,10 @@ The connection-level API is the lowest-level client-side API Akka HTTP provides.
 HTTP connections are opened and closed and how requests are to be send across which connection. As such it offers the
 highest flexibility at the cost of providing the least convenience.
 
+.. note::
+  It is recommended to first read the :ref:`implications-of-streaming-http-entities` section, 
+  as it explains the underlying full-stack streaming concepts, which may be unexpected when coming
+  from a background with non-"streaming first" HTTP Clients.
 
 Opening HTTP Connections
 ------------------------
@@ -90,4 +94,4 @@ On the client-side the stand-alone HTTP layer forms a ``BidiStage`` that is defi
    :snippet: client-layer
 
 You create an instance of ``Http.ClientLayer`` by calling one of the two overloads of the ``Http().clientLayer`` method,
-which also allows for varying degrees of configuration.
+which also allows for varying degrees of configuration.

akka-docs/rst/scala/http/client-side/host-level.rst

@@ -7,6 +7,10 @@ As opposed to the :ref:`connection-level-api` the host-level API relieves you fr
 connections. It autonomously manages a configurable pool of connections to *one particular target endpoint* (i.e.
 host/port combination).
 
+.. note::
+  It is recommended to first read the :ref:`implications-of-streaming-http-entities` section, 
+  as it explains the underlying full-stack streaming concepts, which may be unexpected when coming
+  from a background with non-"streaming first" HTTP Clients.
 
 Requesting a Host Connection Pool
 ---------------------------------
@@ -153,4 +157,4 @@ Example
 -------
 
 .. includecode:: ../../code/docs/http/scaladsl/HttpClientExampleSpec.scala
-   :include: host-level-example
+   :include: host-level-example

akka-docs/rst/scala/http/client-side/index.rst

@@ -6,6 +6,10 @@ Consuming HTTP-based Services (Client-Side)
 All client-side functionality of Akka HTTP, for consuming HTTP-based services offered by other endpoints, is currently
 provided by the ``akka-http-core`` module.
 
+It is recommended to first read the :ref:`implications-of-streaming-http-entities` section, 
+as it explains the underlying full-stack streaming concepts, which may be unexpected when coming
+from a background with non-"streaming first" HTTP Clients.
+
 Depending on your application's specific needs you can choose from three different API levels:
 
 :ref:`connection-level-api`
@@ -28,4 +32,4 @@ Akka HTTP will happily handle many thousand concurrent connections to a single o
    host-level
    request-level
    client-https-support
-   websocket-support
+   websocket-support

akka-docs/rst/scala/http/client-side/request-level.rst

@@ -7,6 +7,11 @@ The request-level API is the most convenient way of using Akka HTTP's client-sid
 :ref:`host-level-api` to provide you with a simple and easy-to-use way of retrieving HTTP responses from remote servers.
 Depending on your preference you can pick the flow-based or the future-based variant.
 
+.. note::
+  It is recommended to first read the :ref:`implications-of-streaming-http-entities` section, 
+  as it explains the underlying full-stack streaming concepts, which may be unexpected when coming
+  from a background with non-"streaming first" HTTP Clients.
+
 .. note::
   The request-level API is implemented on top of a connection pool that is shared inside the ActorSystem. A consequence of
   using a pool is that long-running requests block a connection while running and starve other requests. Make sure not to use

akka-docs/rst/scala/http/implications-of-streaming-http-entity.rst

@@ -0,0 +1,129 @@
+.. _implications-of-streaming-http-entities:
+
+Implications of the streaming nature of Request/Response Entities
+-----------------------------------------------------------------
+
+Akka HTTP is streaming *all the way through*, which means that the back-pressure mechanisms enabled by Akka Streams
+are exposed through all layers–from the TCP layer, through the HTTP server, all the way up to the user-facing ``HttpRequest`` 
+and ``HttpResponse`` and their ``HttpEntity`` APIs.
+
+This has suprising implications if you are used to non-streaming / not-reactive HTTP clients.
+Specifically it means that: "*lack of consumption of the HTTP Entity, is signaled as back-pressure to the other 
+side of the connection*". This is a feature, as it allows one only to consume the entity, and back-pressure servers/clients
+from overwhelming our application, possibly causing un-necessary buffering of the entity in memory.
+
+.. warning::
+  Consuming (or discarding) the Entity of a request is mandatory!
+  If *accidentally* left neither consumed or discarded Akka HTTP will 
+  asume the incoming data should remain back-pressured, and will stall the incoming data via TCP back-pressure mechanisms.
+
+Client-Side handling of streaming HTTP Entities
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Consuming the HTTP Response Entity (Client)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The most commong use-case of course is consuming the response entity, which can be done via
+running the underlying ``dataBytes`` Source. This is as simple as running the dataBytes source,
+(or on the server-side using directives such as 
+
+It is encouraged to use various streaming techniques to utilise the underlying infrastructure to its fullest,
+for example by framing the incoming chunks, parsing them line-by-line and the connecting the flow into another 
+destination Sink, such as a File or other Akka Streams connector:
+
+.. includecode:: ../code/docs/http/scaladsl/HttpClientExampleSpec.scala
+   :include: manual-entity-consume-example-1
+
+however sometimes the need may arise to consume the entire entity as ``Strict`` entity (which means that it is
+completely loaded into memory). Akka HTTP provides a special ``toStrict(timeout)`` method which can be used to 
+eagerly consume the entity and make it available in memory:
+
+.. includecode:: ../code/docs/http/scaladsl/HttpClientExampleSpec.scala
+   :include: manual-entity-consume-example-2
+
+     
+Discarding the HTTP Response Entity (Client)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Sometimes when calling HTTP services we do not care about their response payload (e.g. all we care about is the response code),
+yet as explained above entity still has to be consumed in some way, otherwise we'll be exherting back-pressure on the 
+underlying TCP connection.
+
+The ``discardEntityBytes`` convenience method serves the purpose of easily discarding the entity if it has no purpose for us.
+It does so by piping the incoming bytes directly into an ``Sink.ignore``.
+
+The two snippets below are equivalent, and work the same way on the server-side for incoming HTTP Requests:
+
+.. includecode:: ../code/docs/http/scaladsl/HttpClientExampleSpec.scala
+   :include: manual-entity-discard-example-1
+
+Or the equivalent low-level code achieving the same result:
+
+.. includecode:: ../code/docs/http/scaladsl/HttpClientExampleSpec.scala
+     :include: manual-entity-discard-example-2
+
+Server-Side handling of streaming HTTP Entities
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Similarily as with the Client-side, HTTP Entities are directly linked to Streams which are fed by the underlying
+TCP connection. Thus, if request entities remain not consumed, the server will back-pressure the connection, expecting
+that the user-code will eventually decide what to do with the incoming data.
+
+Note that some directives force an implicit ``toStrict`` operation, such as ``entity(as[String])`` and similar ones.
+
+Consuming the HTTP Request Entity (Server)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The simplest way of consuming the incoming request entity is to simply transform it into an actual domain object,
+for example by using the :ref:`-entity-` directive:
+
+.. includecode:: ../code/docs/http/scaladsl/HttpServerExampleSpec.scala
+   :include: consume-entity-directive
+
+Of course you can access the raw dataBytes as well and run the underlying stream, for example piping it into an
+FileIO Sink, that signals completion via a ``Future[IoResult]`` once all the data has been written into the file:
+
+.. includecode:: ../code/docs/http/scaladsl/HttpServerExampleSpec.scala
+   :include: consume-raw-dataBytes
+
+Discarding the HTTP Request Entity (Server)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Sometimes, depending on some validation (e.g. checking if given user is allowed to perform uploads or not)
+you may want to decide to discard the uploaded entity. 
+
+Please note that discarding means that the entire upload will proceed, even though you are not interested in the data 
+being streamed to the server - this may be useful if you are simply not interested in the given entity, however
+you don't want to abort the entire connection (which we'll demonstrate as well), since there may be more requests
+pending on the same connection still. 
+
+In order to discard the databytes explicitly you can invoke the ``discardEntityBytes`` bytes of the incoming ``HTTPRequest``:
+
+.. includecode:: ../code/docs/http/scaladsl/HttpServerExampleSpec.scala
+   :include: discard-discardEntityBytes
+
+A related concept is *cancelling* the incoming ``entity.dataBytes`` stream, which results in Akka HTTP 
+*abruptly closing the connection from the Client*. This may be useful when you detect that the given user should not be allowed to make any
+uploads at all, and you want to drop the connection (instead of reading and ignoring the incoming data).
+This can be done by attaching the incoming ``entity.dataBytes`` to a ``Sink.cancelled`` which will cancel 
+the entity stream, which in turn will cause the underlying connection to be shut-down by the server – 
+effectively hard-aborting the incoming request:
+
+.. includecode:: ../code/docs/http/scaladsl/HttpServerExampleSpec.scala
+   :include: discard-close-connections
+
+Closing connections is also explained in depth in the :ref:`http-closing-connection-low-level` section of the docs.
+
+Pending: Automatic discarding of not used entities
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Under certin conditions is is possible to detect an entity is very unlikely to be used by the user for a given request,
+and issue warnings or discard the entity automatically. This advanced feature has not been implemented yet, see the below
+note and issues for further discussion and ideas.
+
+.. note:: 
+  An advanced feature code named "auto draining" has been discussed and proposed for Akka HTTP, and we're hoping 
+  to implement or help the community implement it.
+   
+  You can read more about it in `issue #18716 <https://github.com/akka/akka/issues/18716>`_ 
+  as well as `issue #18540 <https://github.com/akka/akka/issues/18540>`_ ; as always, contributions are very welcome!
+

akka-docs/rst/scala/http/index.rst

@@ -9,6 +9,7 @@ Akka HTTP
    introduction
    configuration
    common/index
+   implications-of-streaming-http-entity
    low-level-server-side-api
    routing-dsl/index
    client-side/index

akka-docs/rst/scala/http/low-level-server-side-api.rst

@@ -40,6 +40,10 @@ Depending on your needs you can either use the low-level API directly or rely on
 :ref:`Routing DSL <http-high-level-server-side-api>` which can make the definition of more complex service logic much
 easier.
 
+.. note::
+  It is recommended to read the :ref:`implications-of-streaming-http-entities` section, 
+  as it explains the underlying full-stack streaming concepts, which may be unexpected when coming
+  from a background with non-"streaming first" HTTP Servers.
 
 Streams and HTTP
 ----------------
@@ -123,6 +127,7 @@ See :ref:`HttpEntity-scala` for a description of the alternatives.
 If you rely on the :ref:`http-marshalling-scala` and/or :ref:`http-unmarshalling-scala` facilities provided by
 Akka HTTP then the conversion of custom types to and from streamed entities can be quite convenient.
 
+.. _http-closing-connection-low-level:
 
 Closing a connection
 ~~~~~~~~~~~~~~~~~~~~

akka-docs/rst/scala/http/routing-dsl/index.rst

@@ -8,6 +8,11 @@ defining RESTful web services. It picks up where the low-level API leaves off an
 functionality of typical web servers or frameworks, like deconstruction of URIs, content negotiation or
 static content serving.
 
+.. note::
+  It is recommended to read the :ref:`implications-of-streaming-http-entities` section, 
+  as it explains the underlying full-stack streaming concepts, which may be unexpected when coming
+  from a background with non-"streaming first" HTTP Servers.
+
 .. toctree::
    :maxdepth: 1
 
@@ -100,4 +105,4 @@ and split each line before we send it to an actor for further processing:
 Configuring Server-side HTTPS
 -----------------------------
 
-For detailed documentation about configuring and using HTTPS on the server-side refer to :ref:`serverSideHTTPS-scala`.
+For detailed documentation about configuring and using HTTPS on the server-side refer to :ref:`serverSideHTTPS-scala`.