> For the complete documentation index, see [llms.txt](https://docs.activejs.dev/activejs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.activejs.dev/activejs/fundamentals/systems.md).

# Systems

Systems are nothing but a specialized combination of a few Units, a System creates a few predefined relationships among its member Units to make them part of a whole.

While the Units are storage mediums, Systems are task helpers.

A System tries to accomplish a specific type of task (*that has multiple aspects to it, and hence requiring multiple Units to work in cohesion*) in a reactive and shared fashion, such as data fetching using `XHR` API, or `fetch` API. Both of these APIs or any other asynchronous API more or less involve a **query**, a successful **response**, an **error**, and **pending-status** of the task. 

The [AsyncSystem](/activejs/fundamentals/systems/asyncsystem.md) is the System that is built to help with these kinds of asynchronous APIs.

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[AsyncSystem](/activejs/fundamentals/systems/asyncsystem.md)
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