.. _topics-architecture:

=====================
Architecture overview
=====================

This document describes the architecture of Scrapy and how its components
interact.

Overview
========

The following diagram shows an overview of the Scrapy architecture with its
components and an outline of the data flow that takes place inside the system
(shown by the red arrows). A brief description of the components is included
below with links for more detailed information about them. The data flow is
also described below.

.. _data-flow:

Data flow
=========

.. image:: _images/scrapy_architecture_02.png
   :width: 700
   :height: 470
   :alt: Scrapy architecture

The data flow in Scrapy is controlled by the execution engine, and goes like
this:

1. The :ref:`Engine <component-engine>` gets the initial Requests to crawl from the
   :ref:`Spider <component-spiders>`.

2. The :ref:`Engine <component-engine>` schedules the Requests in the
   :ref:`Scheduler <component-scheduler>` and asks for the
   next Requests to crawl.

3. The :ref:`Scheduler <component-scheduler>` returns the next Requests
   to the :ref:`Engine <component-engine>`.

4. The :ref:`Engine <component-engine>` sends the Requests to the
   :ref:`Downloader <component-downloader>`, passing through the
   :ref:`Downloader Middlewares <component-downloader-middleware>` (see
   :meth:`~scrapy.downloadermiddlewares.DownloaderMiddleware.process_request`).

5. Once the page finishes downloading the
   :ref:`Downloader <component-downloader>` generates a Response (with
   that page) and sends it to the Engine, passing through the
   :ref:`Downloader Middlewares <component-downloader-middleware>` (see
   :meth:`~scrapy.downloadermiddlewares.DownloaderMiddleware.process_response`).

6. The :ref:`Engine <component-engine>` receives the Response from the
   :ref:`Downloader <component-downloader>` and sends it to the
   :ref:`Spider <component-spiders>` for processing, passing
   through the :ref:`Spider Middleware <component-spider-middleware>` (see
   :meth:`~scrapy.spidermiddlewares.SpiderMiddleware.process_spider_input`).

7. The :ref:`Spider <component-spiders>` processes the Response and returns
   scraped items and new Requests (to follow) to the
   :ref:`Engine <component-engine>`, passing through the
   :ref:`Spider Middleware <component-spider-middleware>` (see
   :meth:`~scrapy.spidermiddlewares.SpiderMiddleware.process_spider_output`).

8. The :ref:`Engine <component-engine>` sends processed items to
   :ref:`Item Pipelines <component-pipelines>`, then send processed Requests to
   the :ref:`Scheduler <component-scheduler>` and asks for possible next Requests
   to crawl.

9. The process repeats (from step 1) until there are no more requests from the
   :ref:`Scheduler <component-scheduler>`.

Components
==========

.. _component-engine:

Scrapy Engine
-------------

The engine is responsible for controlling the data flow between all components
of the system, and triggering events when certain actions occur. See the
:ref:`Data Flow <data-flow>` section above for more details.

.. _component-scheduler:

Scheduler
---------

The :ref:`scheduler <topics-scheduler>` receives requests from the engine and 
enqueues them for feeding them later (also to the engine) when the engine 
requests them.

.. _component-downloader:

Downloader
----------

The Downloader is responsible for fetching web pages and feeding them to the
engine which, in turn, feeds them to the spiders.

.. _component-spiders:

Spiders
-------

Spiders are custom classes written by Scrapy users to parse responses and
extract :ref:`items <topics-items>` from them or additional requests to
follow. For more information see :ref:`topics-spiders`.

.. _component-pipelines:

Item Pipeline
-------------

The Item Pipeline is responsible for processing the items once they have been
extracted (or scraped) by the spiders. Typical tasks include cleansing,
validation and persistence (like storing the item in a database). For more
information see :ref:`topics-item-pipeline`.

.. _component-downloader-middleware:

Downloader middlewares
----------------------

Downloader middlewares are specific hooks that sit between the Engine and the
Downloader and process requests when they pass from the Engine to the
Downloader, and responses that pass from Downloader to the Engine.

Use a Downloader middleware if you need to do one of the following:

* process a request just before it is sent to the Downloader
  (i.e. right before Scrapy sends the request to the website);
* change received response before passing it to a spider;
* send a new Request instead of passing received response to a spider;
* pass response to a spider without fetching a web page;
* silently drop some requests.

For more information see :ref:`topics-downloader-middleware`.

.. _component-spider-middleware:

Spider middlewares
------------------

Spider middlewares are specific hooks that sit between the Engine and the
Spiders and are able to process spider input (responses) and output (items and
requests).

Use a Spider middleware if you need to

* post-process output of spider callbacks - change/add/remove requests or items;
* post-process start_requests;
* handle spider exceptions;
* call errback instead of callback for some of the requests based on response
  content.

For more information see :ref:`topics-spider-middleware`.

Event-driven networking
=======================

Scrapy is written with `Twisted`_, a popular event-driven networking framework
for Python. Thus, it's implemented using a non-blocking (aka asynchronous) code
for concurrency.

For more information about asynchronous programming and Twisted see these
links:

* :doc:`twisted:core/howto/defer-intro`
* `Twisted - hello, asynchronous programming`_
* `Twisted Introduction - Krondo`_

.. _Twisted: https://twistedmatrix.com/trac/
.. _Twisted - hello, asynchronous programming: http://jessenoller.com/blog/2009/02/11/twisted-hello-asynchronous-programming/
.. _Twisted Introduction - Krondo: http://krondo.com/an-introduction-to-asynchronous-programming-and-twisted/
