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# Controller

> See also: [How to manage controllers](https://documentation.ubuntu.com/juju/2.9/howto/manage-controllers.md#manage-controllers)

In software design, a **controller** is an architectural component responsible for managing the flow of data and interactions within a system, and for mediating between different parts of the system. In Juju, it is defined in the same way, with the mention that:

- It is set up via the boostrap process.
- It refers to the initial controller [unit](https://documentation.ubuntu.com/juju/2.9/reference/unit.md#unit) as well as any units added later on (for machine clouds, for the purpose of [high-availability](https://documentation.ubuntu.com/juju/2.9/reference/high-availability.md#high-availability)).

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- It is responsible for implementing all the changes defined by a Juju [user](https://documentation.ubuntu.com/juju/2.9/reference/user.md#user) via a Juju client post-bootstrap.
- It stores state in an internal MongoDB database.