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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
The data flow in Scrapy is controlled by the execution engine, and goes like this:
The :ref:`Engine <component-engine>` gets the initial Requests to crawl from the :ref:`Spider <component-spiders>`.
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The :ref:`Engine <component-engine>` schedules the Requests in the :ref:`Scheduler <component-scheduler>` and asks for the next Requests to crawl.
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The :ref:`Scheduler <component-scheduler>` returns the next Requests to the :ref:`Engine <component-engine>`.
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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`).
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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`).
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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`).
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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`).
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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.
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The process repeats (from step 3) until there are no more requests from the :ref:`Scheduler <component-scheduler>`.
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Components
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.
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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.
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The Downloader is responsible for fetching web pages and feeding them to the engine which, in turn, feeds them to the 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`.
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Unknown interpreted text role "ref".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`.
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Unknown interpreted text role "ref".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`.
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Unknown interpreted text role "ref".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`.
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Unknown interpreted text role "ref".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`
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