Controller

See also: How to 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 as well as any units added later on (for machine clouds, for the purpose of high-availability) – each of which includes

  • a unit agent,

  • juju-controller charm code, and

  • a controller agent running, among other things, the Juju API server and an in-process embedded Dqlite database.

• It is responsible for implementing all the changes defined by a Juju user via a Juju client post-bootstrap.

• It stores state in the internal Dqlite database.

Controller storage

A Juju controller has two basic persistent storage needs: database access and blob storage. Prior to Juju 4, both these needs were satisfied by a MongoDB database; however, with the switch to a Dqlite database in Juju 4, only the database access need is satisfied – for blob storage the Juju controller will require an object store. By default, Juju will use the filesystem of the controller’s supporting infrastructure.