2020-04-12 20:27:32 +03:00
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# Overview
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2020-04-12 20:41:10 +03:00
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**aiogram** provides powerful mechanism for customizing event handlers via middlewares.
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2020-04-12 20:27:32 +03:00
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2020-04-12 20:41:10 +03:00
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Middlewares in bot framework seems like Middlewares mechanism in web-frameworks
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2020-04-12 20:27:32 +03:00
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(like [aiohttp](https://docs.aiohttp.org/en/stable/web_advanced.html#aiohttp-web-middlewares),
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[fastapi](https://fastapi.tiangolo.com/tutorial/middleware/),
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[Django](https://docs.djangoproject.com/en/3.0/topics/http/middleware/) or etc.)
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with small difference - here is implemented many layers of processing
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(named as [pipeline](#event-pipeline)).
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!!! info
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Middleware is function that triggered on every event received from
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Telegram Bot API in many points on processing pipeline.
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## Base theory
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As many books and other literature in internet says:
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> Middleware is reusable software that leverages patterns and frameworks to bridge
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>the gap between the functional requirements of applications and the underlying operating systems,
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> network protocol stacks, and databases.
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Middleware can modify, extend or reject processing event before-,
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on- or after- processing of that event.
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[](../../assets/images/basics_middleware.png)
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_(Click on image to zoom it)_
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## Event pipeline
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As described below middleware an interact with event in many stages of pipeline.
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Simple workflow:
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1. Dispatcher receive an [Update](../../api/types/update.md)
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1. Call **pre-process** update middleware in all routers tree
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1. Filter Update over handlers
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1. Call **process** update middleware in all routers tree
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1. Router detects event type (Message, Callback query, etc.)
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1. Router triggers **pre-process** <event> middleware of specific type
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1. Pass event over [filters](../filters/index.md) to detect specific handler
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1. Call **process** <event> middleware for specific type (only when handler for this event exists)
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1. *Do magick*. Call handler (Read more [Event observers](../router.md#event-observers))
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1. Call **post-process** <event> middleware
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1. Call **post-process** update middleware in all routers tree
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1. Emit response into webhook (when it needed)
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### Pipeline in pictures:
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#### Simple pipeline
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[](../../assets/images/middleware_pipeline.png)
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_(Click on image to zoom it)_
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#### Nested routers pipeline
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[](../../assets/images/middleware_pipeline_nested.png)
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_(Click on image to zoom it)_
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## Read more
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- [Middleware Basics](basics.md)
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